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Kotlin
#kotlin
Table of Contents
About
Chapter 1: Getting started with Kotlin
Remarks
Compiling Kotlin
Versions
Examples
Hello World
Hello World using an Object Declaration
Hello World using a Companion Object
Main methods using varargs
Compile and Run Kotlin Code in Command Line
Reading input from Command Line
Chapter 2: Annotations
Examples
Declaring an annotation
Meta-annotations
Chapter 3: Arrays
Examples
Generic Arrays
Arrays of Primitives
Extensions
Iterate Array
Create an array
Create an array using a closure
Create an uninitialized array
Chapter 4: Basic Lambdas
Syntax
Remarks
Examples
Lambda as parameter to filter function
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Lambda passed as a variable
Lambda for benchmarking a function call
Chapter 5: Basics of Kotlin
Introduction
Remarks
Examples
Basic examples
Chapter 6: Class Delegation
Introduction
Examples
Delegate a method to another class
Chapter 7: Class Inheritance
Introduction
Syntax
Parameters
Examples
Basics: the 'open' keyword
Inheriting fields from a class
Defining the base class:
Defining the derived class:
Using the subclass:
Inheriting methods from a class
Defining the base class:
Defining the derived class:
The Ninja has access to all of the methods in Person
Overriding properties and methods
Overriding properties (both read-only and mutable):
Overriding methods:
Chapter 8: Collections
Introduction
Syntax
Examples
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20
Using list
Using map
Using set
Chapter 9: Conditional Statements
Remarks
Examples
Standard if-statement
If-statement as an expression
When-statement instead of if-else-if chains
When-statement argument matching
When-statement as expression
When-statement with enums
Chapter 10: Configuring Kotlin build
Examples
Gradle configuration
Targeting JVM
Targeting Android
Targeting JS
Using Android Studio
Install the plugin
Configure a project
Converting Java
Migrating from Gradle using Groovy script to Kotlin script
Chapter 11: coroutines
Introduction
Examples
Simple coroutine which delay's 1 second but not blocks
Chapter 12: Delegated properties
Introduction
Examples
Lazy initialization
Observable properties
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Map-backed properties
Custom delegation
Delegate Can be used as a layer to reduce boilerplate
Chapter 13: DSL Building
Introduction
Examples
Infix approach to build DSL
Overriding invoke method to build DSL
Using operators with lambdas
Using extensions with lambdas
Chapter 14: Enum
Remarks
Examples
Initialization
Functions and Properties in enums
Simple enum
Mutability
Chapter 15: Exceptions
Examples
Catching exception with try-catch-finally
Chapter 16: Extension Methods
Syntax
Remarks
Examples
Top-Level Extensions
Potential Pitfall: Extensions are Resolved Statically
Sample extending long to render a human readable string
Sample extending Java 7+ Path class
Using extension functions to improve readability
Sample extending Java 8 Temporal classes to render an ISO formatted string
Extension functions to Companion Objects (appearance of Static functions)
Lazy extension property workaround
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Extensions for easier reference View from code
Extensions
Usage
Chapter 17: Functions
Syntax
Parameters
Examples
Functions Taking Other Functions
Lambda Functions
Function References
Basic Functions
Shorthand Functions
Inline Functions
Operator functions
Chapter 18: Generics
Introduction
Syntax
Parameters
Remarks
Implied Upper Bound is Nullable
Examples
Declaration-site variance
Use-site variance
Chapter 19: Idioms
Examples
Creating DTOs (POJOs/POCOs)
Filtering a list
Delegate to a class without providing it in the public constructor
Serializable and serialVersionUid in Kotlin
Fluent methods in Kotlin
Use let or also to simplify working with nullable objects
Use apply to initialize objects or to achieve method chaining
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Chapter 20: Interfaces
Remarks
Examples
Basic Interface
Interface with default implementations
Properties
Multiple implementations
Properties in Interfaces
Conflicts when Implementing Multiple Interfaces with Default Implementations
super keyword
Chapter 21: Java 8 Stream Equivalents
Introduction
Remarks
About laziness
Why are there no Types?!?
Reusing Streams
See also:
Examples
Accumulate names in a List
Convert elements to strings and concatenate them, separated by commas
Compute sum of salaries of employee
Group employees by department
Compute sum of salaries by department
Partition students into passing and failing
Names of male members
Group names of members in roster by gender
Filter a list to another list
Finding shortest string a list
Different Kinds of Streams #2 - lazily using first item if exists
Different Kinds of Streams #3 - iterate a range of Integers
Different Kinds of Streams #4 - iterate an array, map the values, calculate the average
Different Kinds of Streams #5 - lazily iterate a list of strings, map the values, convert
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Different Kinds of Streams #6 - lazily iterate a stream of Ints, map the values, print res
Different Kinds of Streams #7 - lazily iterate Doubles, map to Int, map to String, print e
Counting items in a list after filter is applied
How streams work - filter, upper case, then sort a list
Different Kinds of Streams #1 - eager using first item if it exists
Collect example #5 - find people of legal age, output formatted string
Collect example #6 - group people by age, print age and names together
Collect example #7a - Map names, join together with delimiter
Collect example #7b - Collect with SummarizingInt
Chapter 22: JUnit
Examples
Rules
Chapter 23: Kotlin Android Extensions
Introduction
Examples
Configuration
Using Views
Product flavors
Painfull listener for getting notice, when the view is completely drawn now is so simple a
Chapter 24: Kotlin Caveats
Examples
Calling a toString() on a nullable type
Chapter 25: Kotlin for Java Developers
Introduction
Examples
Declaring Variables
Quick Facts
Equality & Identity
IF, TRY and others are expressions, not statements
Chapter 26: logging in kotlin
Remarks
Examples
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kotlin.logging
Chapter 27: Loops in Kotlin
Remarks
Examples
Repeat an action x times
Looping over iterables
While Loops
Break and continue
Iterating over a Map in kotlin
Recursion
Functional constructs for iteration
Chapter 28: Null Safety
Examples
Nullable and Non-Nullable types
Safe call operator
Idiom: calling multiple methods on the same, null-checked object
Smart casts
Eliminate nulls from an Iterable and array
Null Coalescing / Elvis Operator
Assertion
Elvis Operator (?:)
Chapter 29: Ranges
Introduction
Examples
Integral Type Ranges
downTo() function
step() function
until function
Chapter 30: RecyclerView in Kotlin
Introduction
Examples
Main class and Adapter
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Chapter 31: Reflection
Introduction
Remarks
Examples
Referencing a class
Referencing a function
Inter-operating with Java reflection
Getting values of all properties of a class
Setting values of all properties of a class
Chapter 32: Regex
Examples
Idioms for Regex Matching in When Expression
Using immutable locals:
Using anonymous temporaries:
Using the visitor pattern:
Introduction to regular expressions in Kotlin
The RegEx class
Null safety with regular expressions
Raw strings in regex patterns
find(input: CharSequence, startIndex: Int): MatchResult?
findAll(input: CharSequence, startIndex: Int): Sequence
matchEntire(input: CharSequence): MatchResult?
matches(input: CharSequence): Boolean
containsMatchIn(input: CharSequence): Boolean
split(input: CharSequence, limit: Int): List
replace(input: CharSequence, replacement: String): String
Chapter 33: Singleton objects
Introduction
Examples
Use as repalcement of static methods/fields of java
Use as a singleton
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Chapter 34: Strings
Examples
Elements of String
String Literals
String Templates
String Equality
Chapter 35: Type aliases
Introduction
Syntax
Remarks
Examples
Function type
Generic type
Chapter 36: Type-Safe Builders
Remarks
A typical structure of a type-safe builder
Type-safe builders in Kotlin libraries
Examples
Type-safe tree structure builder
Chapter 37: Vararg Parameters in Functions
Syntax
Examples
Basics: Using the vararg keyword
Spread Operator: Passing arrays into vararg functions
Chapter 38: Visibility Modifiers
Introduction
Syntax
Examples
Code Sample
Credits
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About
You can share this PDF with anyone you feel could benefit from it, downloaded the latest version
from: kotlin
It is an unofficial and free Kotlin ebook created for educational purposes. All the content is
extracted from Stack Overflow Documentation, which is written by many hardworking individuals at
Stack Overflow. It is neither affiliated with Stack Overflow nor official Kotlin.
The content is released under Creative Commons BY-SA, and the list of contributors to each
chapter are provided in the credits section at the end of this book. Images may be copyright of
their respective owners unless otherwise specified. All trademarks and registered trademarks are
the property of their respective company owners.
Use the content presented in this book at your own risk; it is not guaranteed to be correct nor
accurate, please send your feedback and corrections to info@zzzprojects.com
https://riptutorial.com/
1
Chapter 1: Getting started with Kotlin
Remarks
Kotlin is a statically-typed object-oriented programming language developed by JetBrains primarily
targeting the JVM. Kotlin is developed with the goals of being quick to compile, backwards-
compatible, very type safe, and 100% interoperable with Java. Kotlin is also developed with the
goal of providing many of the features wanted by Java developers. Kotlin's standard compiler
allows it to be compiled both into Java bytecode for the JVM and into JavaScript.
Compiling Kotlin
Kotlin has a standard IDE plugin for Eclipse and IntelliJ. Kotlin can also be compiled using Maven,
using Ant, and using Gradle, or through the command line.
It is worth noting in $ kotlinc Main.kt will output a java class file, in this case MainKt.class (Note
the Kt appended to the class name). However if one was to run the class file using $ java MainKt
java will throw the following exception:
Exception in thread "main" java.lang.NoClassDefFoundError: kotlin/jvm/internal/Intrinsics
at MainKt.main(Main.kt)
Caused by: java.lang.ClassNotFoundException: kotlin.jvm.internal.Intrinsics
at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:335)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
... 1 more
In order to run the resulting class file using Java, one must include the Kotlin runt-time jar file to
the current class path.
java -cp .:/path/to/kotlin/runtime/jar/kotlin-runtime.jar MainKt
Versions
Version Release Date
1.0.0
2016-02-15
1.0.1
2016-03-16
1.0.2
2016-05-13
1.0.3
2016-06-30
1.0.4
2016-09-22
https://riptutorial.com/
2
Version Release Date
1.0.5
2016-11-08
1.0.6
2016-12-27
1.1.0
2017-03-01
1.1.1
2017-03-14
1.1.2
2017-04-25
1.1.3
2017-06-23
Examples
Hello World
All Kotlin programs start at the main function. Here is an example of a simple Kotlin "Hello World"
program:
package my.program
fun main(args: Array<String>) {
println("Hello, world!")
}
Place the above code into a file named Main.kt (this filename is entirely arbitrary)
When targeting the JVM, the function will be compiled as a static method in a class with a name
derived from the filename. In the above example, the main class to run would be my.program.MainKt
.
To change the name of the class that contains top-level functions for a particular file, place the
following annotation at the top of the file above the package statement:
@file:JvmName("MyApp")
In this example, the main class to run would now be my.program.MyApp.
See also:
•
•
Package level functions including @JvmName annotation.
Annotation use-site targets
Hello World using an Object Declaration
You can alternatively use an Object Declaration that contains the main function for a Kotlin
program.
https://riptutorial.com/
3
package my.program
object App {
@JvmStatic fun main(args: Array<String>) {
println("Hello World")
}
}
The class name that you will run is the name of your object, in this case is my.program.App.
The advantage to this method over a top-level function is that the class name to run is more self-
evident, and any other functions you add are scoped into the class App. You then also have a
singleton instance of App to store state and do other work.
See also:
•
Static Methods including the @JvmStatic annotation
Hello World using a Companion Object
Similar to using an Object Declaration, you can define the main function of a Kotlin program using a
Companion Object of a class.
package my.program
class App {
companion object {
@JvmStatic fun main(args: Array<String>) {
println("Hello World")
}
}
}
The class name that you will run is the name of your class, in this case is my.program.App.
The advantage to this method over a top-level function is that the class name to run is more self-
evident, and any other functions you add are scoped into the class App. This is similar to the Object
Declaration example, other than you are in control of instantiating any classes to do further work.
A slight variation that instantiates the class to do the actual "hello":
class App {
companion object {
@JvmStatic fun main(args: Array<String>) {
App().run()
}
}
fun run() {
println("Hello World")
}
}
https://riptutorial.com/
4
See also:
•
Static Methods including the @JvmStatic annotation
Main methods using varargs
All of these main method styles can also be used with varargs:
package my.program
fun main(vararg args: String) {
println("Hello, world!")
}
Compile and Run Kotlin Code in Command Line
As java provide two different commands to compile and run Java code. Same as Kotlin also
provide you different commands.
javac to compile java files. java to run java files.
Same as kotlinc to compile kotlin files kotlin to run kotlin files.
Reading input from Command Line
The arguments passed from the console can be received in the Kotlin program and it can be used
as an input. You can pass N (1 2 3 and so on) numbers of arguments from the command prompt.
A simple example of a command-line argument in Kotlin.
fun main(args: Array<String>) {
println("Enter Two number")
var (a, b) = readLine()!!.split(' ') // !! this operator use for
NPE(NullPointerException).
println("Max number is : ${maxNum(a.toInt(), b.toInt())}")
}
fun maxNum(a: Int, b: Int): Int {
var max = if (a > b) {
println("The value of a is $a");
a
} else {
println("The value of b is $b")
b
}
return max;
}
https://riptutorial.com/
5
Here, Enter two number from the command line to find the maximum number. Output :
Enter Two number
71 89 // Enter two number from command line
The value of b is 89
Max number is: 89
For !! Operator Please check Null Safety.
Note: Above example compile and run on Intellij.
Read Getting started with Kotlin online: https://riptutorial.com/kotlin/topic/490/getting-started-with-
kotlin
https://riptutorial.com/
6
Chapter 2: Annotations
Examples
Declaring an annotation
Annotations are means of attaching metadata to code. To declare an annotation, put the
annotation modifier in front of a class:
annotation class Strippable
Annotations can have meta-anotations:
@Target(AnnotationTarget.CLASS, AnnotationTarget.FUNCTION,
AnnotationTarget.VALUE_PARAMETER, AnnotationTarget.EXPRESSION)
annotation class Strippable
Annotations, like other classes, can have constructors:
annotation class Strippable(val importanceValue: Int)
But unlike other classes, is limited to the following types:
•
•
•
•
•
•
types that correspond to Java primitive types (Int, Long etc.);
strings
classes ( Foo:: class)
enums
other annotations
arrays of the types listed above
Meta-annotations
When declaring an annotation, meta-info can be included using the following meta-annotations:
•
•
•
•
@Target: specifies the possible kinds of elements which can be annotated with the annotation
(classes, functions, properties, expressions etc.)
@Retention specifies whether the annotation is stored in the compiled class files and whether
it's visible through reflection at runtime (by default, both are true.)
@Repeatable allows using the same annotation on a single element multiple times.
@MustBeDocumented specifies that the annotation is part of the public API and should be
included in the class or method signature shown in the generated API documentation.
Example:
https://riptutorial.com/
7
@Target(AnnotationTarget.CLASS, AnnotationTarget.FUNCTION,
AnnotationTarget.VALUE_PARAMETER, AnnotationTarget.EXPRESSION)
@Retention(AnnotationRetention.SOURCE)
@MustBeDocumented
annotation class Fancy
Read Annotations online: https://riptutorial.com/kotlin/topic/4074/annotations
https://riptutorial.com/
8
Chapter 3: Arrays
Examples
Generic Arrays
Generic arrays in Kotlin are represented by Array<T>.
To create an empty array, use emptyArray<T>() factory function:
val empty = emptyArray<String>()
To create an array with given size and initial values, use the constructor:
var strings = Array<String>(size = 5, init = { index -> "Item #$index" })
print(Arrays.toString(a)) // prints "[Item #0, Item #1, Item #2, Item #3, Item #4]"
print(a.size) // prints 5
Arrays have get(index: Int): T and set(index: Int, value: T) functions:
strings.set(2, "ChangedItem")
print(strings.get(2)) // prints "ChangedItem"
// You can use subscription as well:
strings[2] = "ChangedItem"
print(strings[2]) // prints "ChangedItem"
Arrays of Primitives
These types do not inherit from Array<T> to avoid boxing, however, they have the same attributes
and methods.
Kotlin type
Factory function
JVM type
BooleanArray
booleanArrayOf(true, false)
boolean[]
ByteArray
byteArrayOf(1, 2, 3)
byte[]
CharArray
charArrayOf('a', 'b', 'c')
char[]
DoubleArray
doubleArrayOf(1.2, 5.0)
double[]
FloatArray
floatArrayOf(1.2, 5.0)
float[]
IntArray
intArrayOf(1, 2, 3)
int[]
LongArray
longArrayOf(1, 2, 3)
long[]
ShortArray
shortArrayOf(1, 2, 3)
short[]
https://riptutorial.com/
9
Extensions
average() is defined for Byte, Int, Long, Short, Double, Float and always returns Double:
val doubles = doubleArrayOf(1.5, 3.0)
print(doubles.average()) // prints 2.25
val ints = intArrayOf(1, 4)
println(ints.average()) // prints 2.5
component1(), component2(), ... component5() return an item of the array
getOrNull(index: Int) returns null if index is out of bounds, otherwise an item of the array
first(), last()
toHashSet() returns a HashSet<T> of all elements
sortedArray(), sortedArrayDescending() creates and returns a new array with sorted elements of
current
sort(), sortDescending sort the array in-place
min(), max()
Iterate Array
You can print the array elements using the loop same as the Java enhanced loop, but you need to
change keyword from : to in.
val asc = Array(5, { i -> (i * i).toString() })
for(s : String in asc){
println(s);
}
You can also change data type in for loop.
val asc = Array(5, { i -> (i * i).toString() })
for(s in asc){
println(s);
}
Create an array
val a = arrayOf(1, 2, 3) // creates an Array<Int> of size 3 containing [1, 2, 3].
Create an array using a closure
val a = Array(3) { i -> i * 2 } // creates an Array<Int> of size 3 containing [0, 2, 4]
https://riptutorial.com/
10
Create an uninitialized array
val a = arrayOfNulls<Int>(3) // creates an Array<Int?> of [null, null, null]
The returned array will always have a nullable type. Arrays of non-nullable items can't be created
uninitialized.
Read Arrays online: https://riptutorial.com/kotlin/topic/5722/arrays
https://riptutorial.com/
11
Chapter 4: Basic Lambdas
Syntax
•
•
•
•
•
•
•
•
•
•
Explicit parameters:
{ parameterName: ParameterType, otherParameterName: OtherParameterType ->
anExpression() }
Inferred parameters:
val addition: (Int, Int) -> Int = { a, b -> a + b }
Single parameter it shorthand
val square: (Int) -> Int = { it*it }
Signature:
() -> ResultType
(InputType) -> ResultType
(InputType1, InputType2) -> ResultType
Remarks
Input type parameters can be left out when they can be left out when they can be inferred from the
context. For example say you have a function on a class that takes a function:
data class User(val fistName: String, val lastName: String) {
fun username(userNameGenerator: (String, String) -> String) =
userNameGenerator(firstName, secondName)
}
You can use this function by passing a lambda, and since the parameters are already specified in
the function signature there's no need to re-declare them in the lambda expression:
val user = User("foo", "bar")
println(user.userName { firstName, secondName ->
"${firstName.toUppercase}"_"${secondName.toUppercase}"
}) // prints FOO_BAR
This also applies when you are assigning a lambda to a variable:
//valid:
val addition: (Int, Int) = { a, b -> a + b }
//valid:
val addition = { a: Int, b: Int -> a + b }
https://riptutorial.com/
12
//error (type inference failure):
val addition = { a, b -> a + b }
When the lambda takes one parameter, and the type can be inferred from the context, you can
refer to the parameter by it.
listOf(1,2,3).map { it * 2 } // [2,4,6]
Examples
Lambda as parameter to filter function
val allowedUsers = users.filter { it.age > MINIMUM_AGE }
Lambda passed as a variable
val isOfAllowedAge = { user: User -> user.age > MINIMUM_AGE }
val allowedUsers = users.filter(isOfAllowedAge)
Lambda for benchmarking a function call
General-purpose stopwatch for timing how long a function takes to run:
object Benchmark {
fun realtime(body: () -> Unit): Duration {
val start = Instant.now()
try {
body()
} finally {
val end = Instant.now()
return Duration.between(start, end)
}
}
}
Usage:
val time = Benchmark.realtime({
// some long-running code goes here ...
})
println("Executed the code in $time")
Read Basic Lambdas online: https://riptutorial.com/kotlin/topic/5878/basic-lambdas
https://riptutorial.com/
13
Chapter 5: Basics of Kotlin
Introduction
This topic covers the basics of Kotlin for beginners.
Remarks
1.
2.
3.
4.
5.
Kotlin file has an extension .kt.
All classes in Kotlin have a common superclass Any, that is a default super for a class with
no supertypes declared(similar to Object in Java).
Variables can be declared as val(immutable- assign once) or var(mutables- value can be
changed)
Semicolon is not needed at end of statement.
If a function does not return any useful value, its return type is Unit.It is also optional.
6.Referential equality is checked by the === operation. a === b evaluates to true if and only
if a and b point to the same object.
Examples
Basic examples
1.The Unit return type declaration is optional for functions. The following codes are equivalent.
fun printHello(name: String?): Unit {
if (name != null)
println("Hello ${name}")
}
fun printHello(name: String?) {
...
}
2.Single-Expression functions:When a function returns a single expression, the curly braces can
be omitted and the body is specified after = symbol
fun double(x: Int): Int = x * 2
Explicitly declaring the return type is optional when this can be inferred by the compiler
fun double(x: Int) = x * 2
3.String interpolation: Using string values is easy.
In java:
int num=10
https://riptutorial.com/
14
String s = "i =" + i;
In Kotlin
val num = 10
val s = "i = $num"
4.In Kotlin, the type system distinguishes between references that can hold null (nullable
references) and those that can not (non-null references). For example, a regular variable of type
String can not hold null:
var a: String = "abc"
a = null // compilation error
To allow nulls, we can declare a variable as nullable string, written String?:
var b: String? = "abc"
b = null // ok
5.In Kotlin,== actually checks for equality of values.By convention, an expression like a == b is
translated to
a?.equals(b) ?: (b === null)
Read Basics of Kotlin online: https://riptutorial.com/kotlin/topic/10648/basics-of-kotlin
https://riptutorial.com/
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Chapter 6: Class Delegation
Introduction
A Kotlin class may implement an interface by delegating its methods and properties to another
object that implements that interface. This provides a way to compose behavior using association
rather than inheritance.
Examples
Delegate a method to another class
interface Foo {
fun example()
}
class Bar {
fun example() {
println("Hello, world!")
}
}
class Baz(b : Bar) : Foo by b
Baz(Bar()).example()
The example prints Hello, world!
Read Class Delegation online: https://riptutorial.com/kotlin/topic/10575/class-delegation
https://riptutorial.com/
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Chapter 7: Class Inheritance
Introduction
Any object-oriented programming language has some form of class inheritance. Let me revise:
Imagine you had to program a bunch of fruit: Apples, Oranges and Pears. They all differ in size,
shape and color, that's why we have different classes.
But let's say their differences don't matter for a second and you just want a Fruit, no matter which
exactly? What return type would getFruit() have?
The answer is class Fruit. We create a new class and make all fruits inherit from it!
Syntax
•
•
•
•
open {Base Class}
class {Derived Class} : {Base Class}({Init Arguments})
override {Function Definition}
{DC-Object} is {Base Class} == true
Parameters
Parameter
Details
Base Class
Class that is inherited from
Derived Class
Class that inherits from Base Class
Init Arguments
Arguments passed to constructor of Base Class
Function
Definition
Function in Derived Class that has different code than the same in the
Base Class
DC-Object
”Derived Class-Object“ Object that has the type of the Derived Class
Examples
Basics: the 'open' keyword
In Kotlin, classes are final by default which means they cannot be inherited from.
To allow inheritance on a class, use the open keyword.
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open class Thing {
// I can now be extended!
}
Note: abstract classes, sealed classes and interfaces will be open by default.
Inheriting fields from a class
Defining the base class:
open class BaseClass {
val x = 10
}
Defining the derived class:
class DerivedClass: BaseClass() {
fun foo() {
println("x is equal to " + x)
}
}
Using the subclass:
fun main(args: Array<String>) {
val derivedClass = DerivedClass()
derivedClass.foo() // prints: 'x is equal to 10'
}
Inheriting methods from a class
Defining the base class:
open class Person {
fun jump() {
println("Jumping...")
}
}
Defining the derived class:
class Ninja: Person() {
fun sneak() {
println("Sneaking around...")
}
}
The Ninja has access to all of the methods in Person
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fun main(args: Array<String>) {
val ninja = Ninja()
ninja.jump() // prints: 'Jumping...'
ninja.sneak() // prints: 'Sneaking around...'
}
Overriding properties and methods
Overriding properties (both read-only and mutable):
abstract class Car {
abstract val name: String;
open var speed: Int = 0;
}
class BrokenCar(override val name: String) : Car() {
override var speed: Int
get() = 0
set(value) {
throw UnsupportedOperationException("The car is bloken")
}
}
fun main(args: Array<String>) {
val car: Car = BrokenCar("Lada")
car.speed = 10
}
Overriding methods:
interface Ship {
fun sail()
fun sink()
}
object Titanic : Ship {
var canSail = true
override fun sail() {
sink()
}
override fun sink() {
canSail = false
}
}
Read Class Inheritance online: https://riptutorial.com/kotlin/topic/5622/class-inheritance
https://riptutorial.com/
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Chapter 8: Collections
Introduction
Unlike many languages, Kotlin distinguishes between mutable and immutable collections (lists,
sets, maps, etc). Precise control over exactly when collections can be edited is useful for
eliminating bugs, and for designing good APIs.
Syntax
•
•
•
listOf, mapOf and setOf returns read-only objects that you cannot add or remove items.
If you want to add or remove items you have to use arrayListOf, hashMapOf, hashSetOf,
linkedMapOf (LinkedHashMap), linkedSetOf (LinkedHashSet), mutableListOf (The Kotlin
MultableList collection), mutableMapOf (The Kotlin MultableMap collection), mutableSetOf
(The Kotlin MultableSet collection), sortedMapOf or sortedSetOf
Each collection has methods like first(), last(), get() and lambda functions like filter, map, join,
reduce and many others.
Examples
Using list
// Create a new read-only List<String>
val list = listOf("Item 1", "Item 2", "Item 3")
println(list) // prints "[Item 1, Item 2, Item 3]"
Using map
// Create a new read-only Map<Integer, String>
val map = mapOf(Pair(1, "Item 1"), Pair(2, "Item 2"), Pair(3, "Item 3"))
println(map) // prints "{1=Item 1, 2=Item 2, 3=Item 3}"
Using set
// Create a new read-only Set<String>
val set = setOf(1, 3, 5)
println(set) // prints "[1, 3, 5]"
Read Collections online: https://riptutorial.com/kotlin/topic/8846/collections
https://riptutorial.com/
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Chapter 9: Conditional Statements
Remarks
In contrast to Java's switch, the when statement has no fall-through behavior. This means, that if a
branch is matched, the control flow returns after its execution and no break statement is required. If
you want to combine the bahaviors for multiple arguments, you can write multiple arguments
separated by commas:
when (x) {
"foo", "bar" -> println("either foo or bar")
else -> println("didn't match anything")
}
Examples
Standard if-statement
val str = "Hello!"
if (str.length == 0) {
print("The string is empty!")
} else if (str.length > 5) {
print("The string is short!")
} else {
print("The string is long!")
}
The else-branches are optional in normal if-statements.
If-statement as an expression
If-statements can be expressions:
val str = if (condition) "Condition met!" else "Condition not met!"
Note that the else-branch is not optional if the if-statement is used as an expression.
This can also been done with a multi-line variant with curly brackets and multiple else if
statements.
val str = if (condition1){
"Condition1 met!"
} else if (condition2) {
"Condition2 met!"
} else {
"Conditions not met!"
}
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TIP: Kotlin can infer the type of the variable for you but if you want to be sure of the
type just annotate it on the variable like: val str: String = this will enforce the type and
will make it easier to read.
When-statement instead of if-else-if chains
The when-statement is an alternative to an if-statement with multiple else-if-branches:
when {
str.length == 0 -> print("The string is empty!")
str.length > 5 -> print("The string is short!")
else -> print("The string is long!")
}
Same code written using an if-else-if chain:
if (str.length == 0) {
print("The string is empty!")
} else if (str.length > 5) {
print("The string is short!")
} else {
print("The string is long!")
}
Just like with the if-statement, the else-branch is optional, and you can add as many or as few
branches as you like. You can also have multiline-branches:
when {
condition -> {
doSomething()
doSomeMore()
}
else -> doSomethingElse()
}
When-statement argument matching
When given an argument, the when-statement matches the argument against the branches in
sequence. The matching is done using the == operator which performs null checks and compares
the operands using the equals function. The first matching one will be executed.
when (x) {
"English" -> print("How are you?")
"German" -> print("Wie geht es dir?")
else -> print("I don't know that language yet :(")
}
The when statement also knows some more advanced matching options:
val names = listOf("John", "Sarah", "Tim", "Maggie")
when (x) {
in names -> print("I know that name!")
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!in 1..10 -> print("Argument was not in the range from 1 to 10")
is String -> print(x.length) // Due to smart casting, you can use String-functions here
}
When-statement as expression
Like if, when can also be used as an expression:
val greeting = when (x) {
"English" -> "How are you?"
"German" -> "Wie geht es dir?"
else -> "I don't know that language yet :("
}
print(greeting)
To be used as an expression, the when-statement must be exhaustive, i.e. either have an else
branch or cover all possibilities with the branches in another way.
When-statement with enums
when can be used to match enum values:
enum class Day {
Sunday,
Monday,
Tuesday,
Wednesday,
Thursday,
Friday,
Saturday
}
fun doOnDay(day: Day) {
when(day) {
Day.Sunday -> // Do something
Day.Monday, Day.Tuesday -> // Do other thing
Day.Wednesday -> // ...
Day.Thursday -> // ...
Day.Friday -> // ...
Day.Saturday -> // ...
}
}
As you can see in second case line (Monday and Tuedsay) it is also possible to combine two or more
enum values.
If your cases are not exhaustive the compile will show an error. You can use else to handle default
cases:
fun doOnDay(day: Day) {
when(day) {
Day.Monday -> // Work
Day.Tuesday -> // Work hard
Day.Wednesday -> // ...
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Day.Thursday -> //
Day.Friday -> //
else -> // Party on weekend
}
}
Though the same can be done using if-then-else construct, when takes care of missing enum values
and makes it more natural.
Check here for more information about kotlin enum
Read Conditional Statements online: https://riptutorial.com/kotlin/topic/2685/conditional-statements
https://riptutorial.com/
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Chapter 10: Configuring Kotlin build
Examples
Gradle configuration
kotlin-gradle-plugin is used to compile Kotlin code with Gradle. Basically, its version should
correspond to the Kotlin version you want to use. E.g. if you want to use Kotlin 1.0.3, then you
need to aplly kotlin-gradle-plugin version 1.0.3 too.
It's a good idea to externalize this version in gradle.properties or in ExtraPropertiesExtension:
buildscript {
ext.kotlin_version = '1.0.3'
repositories {
mavenCentral()
}
dependencies {
classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
}
}
Then you need to apply this plugin to your project. The way you do this differs when targeting
different platforms:
Targeting JVM
apply plugin: 'kotlin'
Targeting Android
apply plugin: 'kotlin-android'
Targeting JS
apply plugin: 'kotlin2js'
These are the default paths:
•
•
•
•
kotlin sources: src/main/kotlin
java sources: src/main/java
kotlin tests: src/test/kotlin
java tests: src/test/java
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•
•
runtime resources: src/main/resources
test resources: src/test/resources
You may need to configure SourceSets if you're using custom project layout.
Finally, you'll need to add Kotlin standard library dependency to your project:
dependencies {
compile "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
}
If you want to use Kotlin Reflection you'll also need to add compile "org.jetbrains.kotlin:kotlin-
reflect:$kotlin_version"
Using Android Studio
Android Studio can configure Kotlin automatically in an Android project.
Install the plugin
To install the Kotlin plugin, go to File > Settings > Editor > Plugins > Install JetBrains Plugin... >
Kotlin > Install, then restart Android Studio when prompted.
Configure a project
Create an Android Studio project as normal, then press Ctrl + Shift + A. In the search box, type
"Configure Kotlin in Project" and press Enter.
Android Studio will alter your Gradle files to add all the necessary dependencies.
Converting Java
To convert your Java files to Kotlin files, press Ctrl + Shift + A and find "Convert Java File to Kotlin
File". This will change the current file's extension to .kt and convert the code to Kotlin.
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Migrating from Gradle using Groovy script to Kotlin script
Steps:
•
•
•
•
clone the gradle-script-kotlin project
copy/paste from the cloned project to your project:
○
○
○
○
build.gradle.kts
gradlew
gradlew.bat
settings.gradle
update the content of the build.gradle.kts based on your needs, you can use as inspiration
the scripts in the project just cloned or in one of its samples
now open Intellij and open your project, in the explorer window, it should be recognized as a
Gradle project, if not, expand it first.
•
after opening, let Intellij works, open build.gradle.kts and check if there are any error. If the
https://riptutorial.com/
27
highlighting is not working and/or is everything marked red, then close and reopen Intellij
•
open the Gradle window and refresh it
If you are on Windows, you may encounter this bug, download the full Gradle 3.3 distribution and
use that instead the one provided. Related.
OSX and Ubuntu work out of the box.
Small bonus, if you want to avoid all the hassle of publicing on Maven and similar, use Jitpack, the
lines to add are almost identical compared to Groovy. You can take inspiration from this project of
mine.
Read Configuring Kotlin build online: https://riptutorial.com/kotlin/topic/2501/configuring-kotlin-build
https://riptutorial.com/
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Chapter 11: coroutines
Introduction
Examples of Kotlin's experimental(yet) implementation of coroutines
Examples
Simple coroutine which delay's 1 second but not blocks
(from official doc)
fun main(args: Array<String>) {
launch(CommonPool) { // create new coroutine in common thread pool
delay(1000L) // non-blocking delay for 1 second (default time unit is ms)
println("World!") // print after delay
}
println("Hello,") // main function continues while coroutine is delayed
Thread.sleep(2000L) // block main thread for 2 seconds to keep JVM alive
}
result
Hello,
World!
Read coroutines online: https://riptutorial.com/kotlin/topic/10936/coroutines
https://riptutorial.com/
29
Chapter 12: Delegated properties
Introduction
Kotlin can delegate the implementation of a property to a handler object. Some standard handlers
are included, such as lazy initialization or observable properties. Custom handlers can also be
created.
Examples
Lazy initialization
val foo : Int by lazy { 1 + 1 }
println(foo)
The example prints 2.
Observable properties
var foo : Int by Delegates.observable("1") { property, oldValue, newValue ->
println("${property.name} was changed from $oldValue to $newValue")
}
foo = 2
The example prints foo was changed from 1 to 2
Map-backed properties
val map = mapOf("foo" to 1)
val foo : String by map
println(foo)
The example prints 1
Custom delegation
class MyDelegate {
operator fun getValue(owner: Any?, property: KProperty<*>): String {
return "Delegated value"
}
}
val foo : String by MyDelegate()
println(foo)
The example prints Delegated value
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Delegate Can be used as a layer to reduce boilerplate
Consider Kotlin's Null Type system and WeakReference<T>.
So let's say we have to save some sort of reference and we wanted to avoid memory leaks, here
is where WeakReference comes in.
take for example this:
class MyMemoryExpensiveClass {
companion object {
var reference: WeakReference<MyMemoryExpensiveClass>? = null
fun doWithReference(block: (MyMemoryExpensiveClass) -> Unit) {
reference?.let {
it.get()?.let(block)
}
}
}
init {
reference = WeakReference(this)
}
}
Now this is just with one WeakReference. To Reduce this boilerplate, we can use a custom
property delegate to help us like so:
class WeakReferenceDelegate<T>(initialValue: T? = null) : ReadWriteProperty<Any, T?> {
var reference = WeakReference(initialValue)
private set
override fun getValue(thisRef: Any, property: KProperty<*>): T? = reference.get()
override fun setValue(thisRef: Any, property: KProperty<*>, value: T?) {
reference = WeakReference(value)
}
}
So Now we can use variables that are wrapped with WeakReference just like normal nullable
variables !
class MyMemoryExpensiveClass {
companion object {
var reference: MyMemoryExpensiveClass? by
WeakReferenceDelegate<MyMemoryExpensiveClass>()
fun doWithReference(block: (MyMemoryExpensiveClass) -> Unit) {
reference?.let(block)
}
}
init {
reference = this
}
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}
Read Delegated properties online: https://riptutorial.com/kotlin/topic/10571/delegated-properties
https://riptutorial.com/
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Chapter 13: DSL Building
Introduction
Focus on the syntax details to design internal DSLs in Kotlin.
Examples
Infix approach to build DSL
If you have:
infix fun <T> T?.shouldBe(expected: T?) = assertEquals(expected, this)
you can write the following DSL-like code in your tests:
@Test
fun test() {
100.plusOne() shouldBe 101
}
Overriding invoke method to build DSL
If you have:
class MyExample(val i: Int) {
operator fun <R> invoke(block: MyExample.() -> R) = block()
fun Int.bigger() = this > i
}
you can write the following DSL-like code in your production code:
fun main2(args: Array<String>) {
val ex = MyExample(233)
ex {
// bigger is defined in the context of `ex`
// you can only call this method inside this context
if (777.bigger()) kotlin.io.println("why")
}
}
Using operators with lambdas
If you have:
val r = Random(233)
infix inline operator fun Int.rem(block: () -> Unit) {
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if (r.nextInt(100) < this) block()
}
You can write the following DSL-like code:
20 % { println("The possibility you see this message is 20%") }
Using extensions with lambdas
If you have:
operator fun <R> String.invoke(block: () -> R) = {
try { block.invoke() }
catch (e: AssertException) { System.err.println("$this\n${e.message}") }
}
You can write the following DSL-like code:
"it should return 2" {
parse("1 + 1").buildAST().evaluate() shouldBe 2
}
If you feel confused with shouldBe above, see the example Infix approach to build DSL.
Read DSL Building online: https://riptutorial.com/kotlin/topic/10042/dsl-building
https://riptutorial.com/
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Chapter 14: Enum
Remarks
Just like in Java, enum classes in Kotlin have synthetic methods allowing to list the defined enum
constants and to get an enum constant by its name. The signatures of these methods are as
follows (assuming the name of the enum class is EnumClass):
EnumClass.valueOf(value: String): EnumClass
EnumClass.values(): Array<EnumClass>
The valueOf() method throws an IllegalArgumentException if the specified name does not match
any of the enum constants defined in the class.
Every enum constant has properties to obtain its name and position in the enum class declaration:
val name: String
val ordinal: Int
The enum constants also implement the Comparable interface, with the natural order being the
order in which they are defined in the enum class.
Examples
Initialization
Enum classes as any other classes can have a constructor and be initialized
enum class Color(val rgb: Int) {
RED(0xFF0000),
GREEN(0x00FF00),
BLUE(0x0000FF)
}
Functions and Properties in enums
Enum classes can also declare members (i.e. properties and functions). A semicolon (;) must be
placed between the last enum object and the first member declaration.
If a member is abstract, the enum objects must implement it.
enum class Color {
RED {
override val rgb: Int = 0xFF0000
},
GREEN {
override val rgb: Int = 0x00FF00
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},
BLUE {
override val rgb: Int = 0x0000FF
}
;
abstract val rgb: Int
fun colorString() = "#%06X".format(0xFFFFFF and rgb)
}
Simple enum
enum class Color {
RED, GREEN, BLUE
}
Each enum constant is an object. Enum constants are separated with commas.
Mutability
Enums can be mutable, this is another way to obtain a singleton behavior:
enum class Planet(var population: Int = 0) {
EARTH(7 * 100000000),
MARS();
override fun toString() = "$name[population=$population]"
}
println(Planet.MARS) // MARS[population=0]
Planet.MARS.population = 3
println(Planet.MARS) // MARS[population=3]
Read Enum online: https://riptutorial.com/kotlin/topic/2286/enum
https://riptutorial.com/
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Chapter 15: Exceptions
Examples
Catching exception with try-catch-finally
Catching exceptions in Kotlin looks very similar to Java
try {
doSomething()
}
catch(e: MyException) {
handle(e)
}
finally {
cleanup()
}
You can also catch multiple exceptions
try {
doSomething()
}
catch(e: FileSystemException) {
handle(e)
}
catch(e: NetworkException) {
handle(e)
}
catch(e: MemoryException) {
handle(e)
}
finally {
cleanup()
}
try is also an expression and may return value
val s: String? = try { getString() } catch (e: Exception) { null }
Kotlin doesn't have checked exceptions, so you don't have to catch any exceptions.
fun fileToString(file: File) : String {
//readAllBytes throws IOException, but we can omit catching it
fileContent = Files.readAllBytes(file)
return String(fileContent)
}
Read Exceptions online: https://riptutorial.com/kotlin/topic/7246/exceptions
https://riptutorial.com/
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Chapter 16: Extension Methods
Syntax
•
•
•
fun TypeName.extensionName(params, ...) { /* body */ } // Declaration
fun <T: Any> TypeNameWithGenerics<T>.extensionName(params, ...) { /* body */ } //
Declaration with Generics
myObj.extensionName(args, ...) // invocation
Remarks
Extensions are resolved statically. This means that the extension method to be used is
determined by the reference-type of the variable you are accessing; it doesn't matter what the
variable's type is at runtime, the same extension method will always be called. This is because
declaring an extension method doesn't actually add a member to the receiver type.
Examples
Top-Level Extensions
Top-level extension methods are not contained within a class.
fun IntArray.addTo(dest: IntArray) {
for (i in 0 .. size - 1) {
dest[i] += this[i]
}
}
Above an extension method is defined for the type IntArray. Note that the object for which the
extension method is defined (called the receiver) is accessed using the keyword this.
This extension can be called like so:
val myArray = intArrayOf(1, 2, 3)
intArrayOf(4, 5, 6).addTo(myArray)
Potential Pitfall: Extensions are Resolved Statically
The extension method to be called is determined at compile-time based on the reference-type of
the variable being accessed. It doesn't matter what the variable's type is at runtime, the same
extension method will always be called.
open class Super
class Sub : Super()
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fun Super.myExtension() = "Defined for Super"
fun Sub.myExtension() = "Defined for Sub"
fun callMyExtension(myVar: Super) {
println(myVar.myExtension())
}
callMyExtension(Sub())
The above example will print "Defined for Super", because the declared type of the variable myVar
is Super.
Sample extending long to render a human readable string
Given any value of type Int or Long to render a human readable string:
fun Long.humanReadable(): String {
if (this <= 0) return "0"
val units = arrayOf("B", "KB", "MB", "GB", "TB", "EB")
val digitGroups = (Math.log10(this.toDouble())/Math.log10(1024.0)).toInt();
return DecimalFormat("#,##0.#").format(this/Math.pow(1024.0, digitGroups.toDouble())) + "
" + units[digitGroups];
}
fun Int.humanReadable(): String {
return this.toLong().humanReadable()
}
Then easily used as:
println(1999549L.humanReadable())
println(someInt.humanReadable())
Sample extending Java 7+ Path class
A common use case for extension methods is to improve an existing API. Here are examples of
adding exist, notExists and deleteRecursively to the Java 7+ Path class:
fun Path.exists(): Boolean = Files.exists(this)
fun Path.notExists(): Boolean = !this.exists()
fun Path.deleteRecursively(): Boolean = this.toFile().deleteRecursively()
Which can now be invoked in this example:
val dir = Paths.get(dirName)
if (dir.exists()) dir.deleteRecursively()
Using extension functions to improve readability
In Kotlin you could write code like:
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val x: Path = Paths.get("dirName").apply {
if (Files.notExists(this)) throw IllegalStateException("The important file does not
exist")
}
But the use of apply is not that clear as to your intent. Sometimes it is clearer to create a similar
extension function to in effect rename the action and make it more self-evident. This should not be
allowed to get out of hand, but for very common actions such as verification:
infix inline fun <T> T.verifiedBy(verifyWith: (T) -> Unit): T {
verifyWith(this)
return this
}
infix inline fun <T: Any> T.verifiedWith(verifyWith: T.() -> Unit): T {
this.verifyWith()
return this
}
You could now write the code as:
val x: Path = Paths.get("dirName") verifiedWith {
if (Files.notExists(this)) throw IllegalStateException("The important file does not
exist")
}
Which now let's people know what to expect within the lambda parameter.
Note that the type parameter T for verifiedBy is same as T: Any? meaning that even nullable types
will be able to use that version of the extension. Although verifiedWith requires non-nullable.
Sample extending Java 8 Temporal classes to render an ISO formatted string
With this declaration:
fun Temporal.toIsoString(): String = DateTimeFormatter.ISO_INSTANT.format(this)
You can now simply:
val dateAsString = someInstant.toIsoString()
Extension functions to Companion Objects (appearance of Static functions)
If you want to extend a class as-if you are a static function, for example for class Something add
static looking function fromString, this can only work if the class has a companion object and that
the extension function has been declared upon the companion object:
class Something {
companion object {}
}
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class SomethingElse {
}
fun Something.Companion.fromString(s: String): Something = ...
fun SomethingElse.fromString(s: String): SomethingElse = ...
fun main(args: Array<String>) {
Something.fromString("") //valid as extension function declared upon the
//companion object
SomethingElse().fromString("") //valid, function invoked on instance not
//statically
SomethingElse.fromString("") //invalid
}
Lazy extension property workaround
Assume you want to create an extension property that is expensive to compute. Thus you would
like to cache the computation, by using the lazy property delegate and refer to current instance (
this), but you cannot do it, as explained in the Kotlin issues KT-9686 and KT-13053. However,
there is an official workaround provided here.
In the example, the extension property is color. It uses an explicit colorCache which can be used
with this as no lazy is necessary:
class KColor(val value: Int)
private val colorCache = mutableMapOf<KColor, Color>()
val KColor.color: Color
get() = colorCache.getOrPut(this) { Color(value, true) }
Extensions for easier reference View from code
You can use extensions for reference View, no more boilerplate after you created the views.
Original Idea is by Anko Library
Extensions
inline fun <reified T : View> View.find(id: Int): T = findViewById(id) as T
inline fun <reified T : View> Activity.find(id: Int): T = findViewById(id) as T
inline fun <reified T : View> Fragment.find(id: Int): T = view?.findViewById(id) as T
inline fun <reified T : View> RecyclerView.ViewHolder.find(id: Int): T =
itemView?.findViewById(id) as T
inline fun <reified T : View> View.findOptional(id: Int): T? = findViewById(id) as? T
inline fun <reified T : View> Activity.findOptional(id: Int): T? = findViewById(id) as? T
inline fun <reified T : View> Fragment.findOptional(id: Int): T? = view?.findViewById(id) as?
T
inline fun <reified T : View> RecyclerView.ViewHolder.findOptional(id: Int): T? =
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itemView?.findViewById(id) as? T
Usage
val yourButton by lazy { find<Button>(R.id.yourButtonId) }
val yourText by lazy { find<TextView>(R.id.yourTextId) }
val yourEdittextOptional by lazy { findOptional<EditText>(R.id.yourOptionEdittextId) }
Read Extension Methods online: https://riptutorial.com/kotlin/topic/613/extension-methods
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Chapter 17: Functions
Syntax
•
•
•
•
•
•
•
•
•
fun Name(Params) = ...
fun Name(Params) {...}
fun Name(Params): Type {...}
fun <Type Argument> Name(Params): Type {...}
inline fun Name(Params): Type {...}
{ ArgName: ArgType -> ... }
{ ArgName -> ... }
{ ArgNames -> ... }
{ (ArgName: ArgType): Type -> ... }
Parameters
Parameter
Details
Name
Name of the function
Params
Values given to the function with a name and type: Name:Type
Type
Return type of the function
Type Argument Type parameter used in generic programming (not necessarily return type)
ArgName
Name of value given to the function
ArgType
Type specifier for ArgName
ArgNames
List of ArgName separated by commas
Examples
Functions Taking Other Functions
As seen in "Lambda Functions", functions can take other functions as a parameter. The "function
type" which you'll need to declare functions which take other functions is as follows:
# Takes no parameters and returns anything
() -> Any?
# Takes a string and an integer and returns ReturnType
(arg1: String, arg2: Int) -> ReturnType
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For example, you could use the vaguest type, () -> Any?, to declare a function which executes a
lambda function twice:
fun twice(x: () -> Any?) {
x(); x();
}
fun main() {
twice {
println("Foo")
} # => Foo
# => Foo
}
Lambda Functions
Lambda functions are anonymous functions which are usually created during a function call to act
as a function parameter. They are declared by surrounding expressions with {braces} - if
arguments are needed, these are put before an arrow ->.
{ name: String ->
"Your name is $name" //This is returned
}
The last statement inside a lambda function is automatically the return value.
The type's are optional, if you put the lambda on a place where the compiler can infer the types.
Multiple arguments:
{ argumentOne:String, argumentTwo:String ->
"$argumentOne - $argumentTwo"
}
If the lambda function only needs one argument, then the argument list can be omitted and the
single argument be referred to using it instead.
{ "Your name is $it" }
If the only argument to a function is a lambda function, then parentheses can be completely
omitted from the function call.
# These are identical
listOf(1, 2, 3, 4).map { it + 2 }
listOf(1, 2, 3, 4).map({ it + 2 })
Function References
We can reference a function without actually calling it by prefixing the function's name with ::. This
can then be passed to a function which accepts some other function as a parameter.
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fun addTwo(x: Int) = x + 2
listOf(1, 2, 3, 4).map(::addTwo) # => [3, 4, 5, 6]
Functions without a receiver will be converted to (ParamTypeA, ParamTypeB, ...) -> ReturnType
where ParamTypeA, ParamTypeB ... are the type of the function parameters and `ReturnType1 is the
type of function return value.
fun foo(p0: Foo0, p1: Foo1, p2: Foo2): Bar {
//...
}
println(::foo::class.java.genericInterfaces[0])
// kotlin.jvm.functions.Function3<Foo0, Foo1, Foo2, Bar>
// Human readable type: (Foo0, Foo1, Foo2) -> Bar
Functions with a receiver (be it an extension function or a member function) has a different syntax.
You have to add the type name of the receiver before the double colon:
class Foo
fun Foo.foo(p0: Foo0, p1: Foo1, p2: Foo2): Bar {
//...
}
val ref = Foo::foo
println(ref::class.java.genericInterfaces[0])
// kotlin.jvm.functions.Function4<Foo, Foo0, Foo1, Foo2, Bar>
// Human readable type: (Foo, Foo0, Foo1, Foo2) -> Bar
// takes 4 parameters, with receiver as first and actual parameters following, in their order
// this function can't be called like an extension function, though
val ref = Foo::foo
Foo().ref(Foo0(), Foo1(), Foo2()) // compile error
class Bar {
fun bar()
}
print(Bar::bar) // works on member functions, too.
However, when a function's receiver is an object, the receiver is omitted from parameter list,
because these is and only is one instance of such type.
object Foo
fun Foo.foo(p0: Foo0, p1: Foo1, p2: Foo2): Bar {
//...
}
val ref = Foo::foo
println(ref::class.java.genericInterfaces[0])
// kotlin.jvm.functions.Function3<Foo0, Foo1, Foo2, Bar>
// Human readable type: (Foo0, Foo1, Foo2) -> Bar
// takes 3 parameters, receiver not needed
object Bar {
fun bar()
}
print(Bar::bar) // works on member functions, too.
Since kotlin 1.1, function reference can also be bounded to a variable, which is then called a
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bounded function reference.
1.1.0
fun makeList(last: String?): List<String> {
val list = mutableListOf("a", "b", "c")
last?.let(list::add)
return list
}
Note this example is given only to show how bounded function reference works. It's bad practice in all other senses.
There is a special case, though. An extension function declared as a member can't be referenced.
class Foo
class Bar {
fun Foo.foo() {}
val ref = Foo::foo // compile error
}
Basic Functions
Functions are declared using the fun keyword, followed by a function name and any parameters.
You can also specify the return type of a function, which defaults to Unit. The body of the function
is enclosed in braces {}. If the return type is other than Unit, the body must issue a return
statement for every terminating branch within the body.
fun sayMyName(name: String): String {
return "Your name is $name"
}
A shorthand version of the same:
fun sayMyName(name: String): String = "Your name is $name"
And the type can be omitted since it can be inferred:
fun sayMyName(name: String) = "Your name is $name"
Shorthand Functions
If a function contains just one expression, we can omit the brace brackets and use an equals
instead, like a variable assignment. The result of the expression is returned automatically.
fun sayMyName(name: String): String = "Your name is $name"
Inline Functions
Functions can be declared inline using the inline prefix, and in this case they act like macros in C
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- rather than being called, they are replaced by the function's body code at compile time. This can
lead to performance benefits in some circumstances, mainly where lambdas are used as function
parameters.
inline fun sayMyName(name: String) = "Your name is $name"
One difference from C macros is that inline functions can't access the scope from which they're
called:
inline fun sayMyName() = "Your name is $name"
fun main() {
val name = "Foo"
sayMyName() # => Unresolved reference: name
}
Operator functions
Kotlin allows us to provide implementations for a predefined set of operators with fixed symbolic
representation (like + or *) and fixed precedence. To implement an operator, we provide a member
function or an extension function with a fixed name, for the corresponding type. Functions that
overload operators need to be marked with the operator modifier:
data class IntListWrapper (val wrapped: List<Int>) {
operator fun get(position: Int): Int = wrapped[position]
}
val a = IntListWrapper(listOf(1, 2, 3))
a[1] // == 2
More operator functions can be found in here
Read Functions online: https://riptutorial.com/kotlin/topic/1280/functions
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Chapter 18: Generics
Introduction
A List can hold numbers, words or really anything. That's why we call the List generic.
Generics are basically used to define which types a class can hold and which type an object
currently holds.
Syntax
•
•
•
•
•
class ClassName<TypeName>
class ClassName<*>
ClassName<in UpperBound>
ClassName<out LowerBound>
class Name<TypeName:UpperBound>
Parameters
Parameter
Details
TypeName
Type Name of generic parameter
UpperBound Covariant Type
LowerBound Contravariant Type
ClassName
Name of the class
Remarks
Implied Upper Bound is Nullable
In Kotlin Generics, the upper bound of type parameter T would be Any?. Therefore for this class:
class Consumer<T>
The type parameter T is really T: Any?. To make a non-nullable upper bound, explicitly specific T:
Any. For example:
class Consumer<T: Any>
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Examples
Declaration-site variance
Declaration-site variance can be thought of as declaration of use-site variance once and for all the
use-sites.
class Consumer<in T> { fun consume(t: T) { ... } }
fun charSequencesConsumer() : Consumer<CharSequence>() = ...
val stringConsumer : Consumer<String> = charSequenceConsumer() // OK since in-projection
val anyConsumer : Consumer<Any> = charSequenceConsumer() // Error, Any cannot be passed
val outConsumer : Consumer<out CharSequence> = ... // Error, T is `in`-parameter
Widespread examples of declaration-site variance are List<out T>, which is immutable so that T
only appears as the return value type, and Comparator<in T>, which only receives T as argument.
Use-site variance
Use-site variance is similar to Java wildcards:
Out-projection:
val takeList : MutableList<out SomeType> = ... // Java: List<? extends SomeType>
val takenValue : SomeType = takeList[0] // OK, since upper bound is SomeType
takeList.add(takenValue) // Error, lower bound for generic is not specified
In-projection:
val putList : MutableList<in SomeType> = ... // Java: List<? super SomeType>
val valueToPut : SomeType = ...
putList.add(valueToPut) // OK, since lower bound is SomeType
putList[0] // This expression has type Any, since no upper bound is specified
Star-projection
val starList : MutableList<*> = ... // Java: List<?>
starList[0] // This expression has type Any, since no upper bound is specified
starList.add(someValue) // Error, lower bound for generic is not specified
See also:
•
Variant Generics interoperability when calling Kotlin from Java.
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Read Generics online: https://riptutorial.com/kotlin/topic/1147/generics
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Chapter 19: Idioms
Examples
Creating DTOs (POJOs/POCOs)
Data classes in kotlin are classes created to do nothing but hold data. Such classes are marked as
data:
data class User(var firstname: String, var lastname: String, var age: Int)
The code above creates a User class with the following automatically generated:
•
•
•
•
•
•
Getters and Setters for all properties (getters only for vals)
equals()
hashcode()
toString()
copy()
componentN() (where N is the corresponding property in order of declaration)
Just as with a function, default values can also be specified:
data class User(var firstname: String = "Joe", var lastname: String = "Bloggs", var age: Int =
20)
More details can be found here Data Classes.
Filtering a list
val list = listOf(1,2,3,4,5,6)
//filter out even numbers
val even = list.filter { it % 2 == 0 }
println(even) //returns [2,4]
Delegate to a class without providing it in the public constructor
Assume you want to delegate to a class but you do not want to provide the delegated-to class in
the constructor parameter. Instead, you want to construct it privately, making the constructor caller
unaware of it. At first this might seem impossible because class delegation allows to delegate only
to constructor parameters. However, there is a way to do it, as given in this answer:
class MyTable private constructor(table: Table<Int, Int, Int>) : Table<Int, Int, Int> by table
{
constructor() : this(TreeBasedTable.create()) // or a different type of table if desired
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}
With this, you can just call the constructor of MyTable like that: MyTable(). The Table<Int, Int, Int>
to which MyTable delegates will be created privately. Constructor caller knows nothing about it.
This example is based on this SO question.
Serializable and serialVersionUid in Kotlin
To create the serialVersionUID for a class in Kotlin you have a few options all involving adding a
member to the companion object of the class.
The most concise bytecode comes from a private const val which will become a private static
variable on the containing class, in this case MySpecialCase:
class MySpecialCase : Serializable {
companion object {
private const val serialVersionUID: Long = 123
}
}
You can also use these forms, each with a side effect of having getter/setter methods which
are not necessary for serialization...
class MySpecialCase : Serializable {
companion object {
private val serialVersionUID: Long = 123
}
}
This creates the static field but also creates a getter as well getSerialVersionUID on the companion
object which is unnecessary.
class MySpecialCase : Serializable {
companion object {
@JvmStatic private val serialVersionUID: Long = 123
}
}
This creates the static field but also creates a static getter as well getSerialVersionUID on the
containing class MySpecialCase which is unnecessary.
But all work as a method of adding the serialVersionUID to a Serializable class.
Fluent methods in Kotlin
Fluent methods in Kotlin can be the same as Java:
fun doSomething() {
someOtherAction()
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return this
}
But you can also make them more functional by creating an extension function such as:
fun <T: Any> T.fluently(func: ()->Unit): T {
func()
return this
}
Which then allows more obviously fluent functions:
fun doSomething() {
return fluently { someOtherAction() }
}
Use let or also to simplify working with nullable objects
let in Kotlin creates a local binding from the object it was called upon. Example:
val str = "foo"
str.let {
println(it) // it
}
This will print "foo" and will return Unit.
The difference between let and also is that you can return any value from a let block. also in the
other hand will always reutrn Unit.
Now why this is useful, you ask? Because if you call a method which can return null and you want
to run some code only when that return value is not null you can use let or also like this:
val str: String? = someFun()
str?.let {
println(it)
}
This piece of code will only run the let block when str is not null. Note the null safety operator (?
).
Use apply to initialize objects or to achieve method chaining
The documentation of apply says the following:
calls the specified function block with this value as its receiver and returns this value.
While the kdoc is not so helpful apply is indeed an useful function. In layman's terms apply
establishes a scope in which this is bound to the object you called apply on. This enables you to
spare some code when you need to call multiple methods on an object which you will then return
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later. Example:
File(dir).apply { mkdirs() }
This is the same as writing this:
fun makeDir(String path): File {
val result = new File(path)
result.mkdirs()
return result
}
Read Idioms online: https://riptutorial.com/kotlin/topic/2273/idioms
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Chapter 20: Interfaces
Remarks
See also: Kotlin reference documentation for Interfaces: Interfaces
Examples
Basic Interface
A Kotlin interface contains declarations of abstract methods, and default method implementations
although they cannot store state.
interface MyInterface {
fun bar()
}
This interface can now be implemented by a class as follows:
class Child : MyInterface {
override fun bar() {
print("bar() was called")
}
}
Interface with default implementations
An interface in Kotlin can have default implementations for functions:
interface MyInterface {
fun withImplementation() {
print("withImplementation() was called")
}
}
Classes implementing such interfaces will be able to use those functions without reimplementing
class MyClass: MyInterface {
// No need to reimplement here
}
val instance = MyClass()
instance.withImplementation()
Properties
Default implementations also work for property getters and setters:
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interface MyInterface2 {
val helloWorld
get() = "Hello World!"
}
Interface accessors implementations can't use backing fields
interface MyInterface3 {
// this property won't compile!
var helloWorld: Int
get() = field
set(value) { field = value }
}
Multiple implementations
When multiple interfaces implement the same function, or all of them define with one or more
implementing, the derived class needs to manually resolve proper call
interface A {
fun notImplemented()
fun implementedOnlyInA() { print("only A") }
fun implementedInBoth() { print("both, A") }
fun implementedInOne() { print("implemented in A") }
}
interface B {
fun implementedInBoth() { print("both, B") }
fun implementedInOne() // only defined
}
class MyClass: A, B {
override fun notImplemented() { print("Normal implementation") }
// implementedOnlyInA() can by normally used in instances
// class needs to define how to use interface functions
override fun implementedInBoth() {
super<B>.implementedInBoth()
super<A>.implementedInBoth()
}
// even if there's only one implementation, there multiple definitions
override fun implementedInOne() {
super<A>.implementedInOne()
print("implementedInOne class implementation")
}
}
Properties in Interfaces
You can declare properties in interfaces. Since an interface cannot have state you can only
declare a property as abstract or by providing default implementation for the accessors.
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interface MyInterface {
val property: Int // abstract
val propertyWithImplementation: String
get() = "foo"
fun foo() {
print(property)
}
}
class Child : MyInterface {
override val property: Int = 29
}
Conflicts when Implementing Multiple Interfaces with Default Implementations
When implementing more than one interface that have methods of the same name that include
default implementations, it is ambiguous to the compiler which implementation should be used. In
the case of a conflict, the developer must override the conflicting method and provide a custom
implementation. That implementation may chose to delegate to the default implementations or not.
interface FirstTrait {
fun foo() { print("first") }
fun bar()
}
interface SecondTrait {
fun foo() { print("second") }
fun bar() { print("bar") }
}
class ClassWithConflict : FirstTrait, SecondTrait {
override fun foo() {
super<FirstTrait>.foo() // delegate to the default implementation of FirstTrait
super<SecondTrait>.foo() // delegate to the default implementation of SecondTrait
}
// function bar() only has a default implementation in one interface and therefore is ok.
}
super keyword
interface MyInterface {
fun funcOne() {
//optional body
print("Function with default implementation")
}
}
If the method in the interface has its own default implementation, we can use super keyword to
access it.
super.funcOne()
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Read Interfaces online: https://riptutorial.com/kotlin/topic/900/interfaces
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Chapter 21: Java 8 Stream Equivalents
Introduction
Kotlin provides many extension methods on collections and iterables for applying functional-style
operations. A dedicated Sequence type allows for lazy composition of several such operations.
Remarks
About laziness
If you want to lazy process a chain, you can convert to a Sequence using asSequence() before the
chain. At the end of the chain of functions, you usually end up with a Sequence as well. Then you
can use toList(), toSet(), toMap() or some other function to materialize the Sequence at the end.
// switch to and from lazy
val someList = items.asSequence().filter { ... }.take(10).map { ... }.toList()
// switch to lazy, but sorted() brings us out again at the end
val someList = items.asSequence().filter { ... }.take(10).map { ... }.sorted()
Why are there no Types?!?
You will notice the Kotlin examples do not specify the types. This is because Kotlin has full type
inference and is completely type safe at compile time. More so than Java because it also has
nullable types and can help prevent the dreaded NPE. So this in Kotlin:
val someList = people.filter { it.age <= 30 }.map { it.name }
is the same as:
val someList: List<String> = people.filter { it.age <= 30 }.map { it.name }
Because Kotlin knows what people is, and that people.age is Int therefore the filter expression only
allows comparison to an Int, and that people.name is a String therefore the map step produces a
List<String> (readonly List of String).
Now, if people were possibly null, as-in a List<People>? then:
val someList = people?.filter { it.age <= 30 }?.map { it.name }
Returns a List<String>? that would need to be null checked (or use one of the other Kotlin
operators for nullable values, see this Kotlin idiomatic way to deal with nullable values and also
Idiomatic way of handling nullable or empty list in Kotlin)
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Reusing Streams
In Kotlin, it depends on the type of collection whether it can be consumed more than once. A
Sequence generates a new iterator every time, and unless it asserts "use only once" it can reset to
the start each time it is acted upon. Therefore while the following fails in Java 8 stream, but works
in Kotlin:
// Java:
Stream<String> stream =
Stream.of("d2", "a2", "b1", "b3", "c").filter(s -> s.startsWith("b"));
stream.anyMatch(s -> true); // ok
stream.noneMatch(s -> true); // exception
// Kotlin:
val stream = listOf("d2", "a2", "b1", "b3", "c").asSequence().filter { it.startsWith('b' ) }
stream.forEach(::println) // b1, b2
println("Any B ${stream.any { it.startsWith('b') }}") // Any B true
println("Any C ${stream.any { it.startsWith('c') }}") // Any C false
stream.forEach(::println) // b1, b2
And in Java to get the same behavior:
// Java:
Supplier<Stream<String>> streamSupplier =
() -> Stream.of("d2", "a2", "b1", "b3", "c")
.filter(s -> s.startsWith("a"));
streamSupplier.get().anyMatch(s -> true); // ok
streamSupplier.get().noneMatch(s -> true); // ok
Therefore in Kotlin the provider of the data decides if it can reset back and provide a new iterator
or not. But if you want to intentionally constrain a Sequence to one time iteration, you can use
constrainOnce() function for Sequence as follows:
val stream = listOf("d2", "a2", "b1", "b3", "c").asSequence().filter { it.startsWith('b' ) }
.constrainOnce()
stream.forEach(::println) // b1, b2
stream.forEach(::println) // Error:java.lang.IllegalStateException: This sequence can be
consumed only once.
See also:
•
•
•
•
API Reference for extension functions for Iterable
API reference for extension functions for Array
API reference for extension functions for List
API reference for extension functions to Map
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Examples
Accumulate names in a List
// Java:
List<String> list = people.stream().map(Person::getName).collect(Collectors.toList());
// Kotlin:
val list = people.map { it.name } // toList() not needed
Convert elements to strings and concatenate them, separated by commas
// Java:
String joined = things.stream()
.map(Object::toString)
.collect(Collectors.joining(", "));
// Kotlin:
val joined = things.joinToString() // ", " is used as separator, by default
Compute sum of salaries of employee
// Java:
int total = employees.stream()
.collect(Collectors.summingInt(Employee::getSalary)));
// Kotlin:
val total = employees.sumBy { it.salary }
Group employees by department
// Java:
Map<Department, List<Employee>> byDept
= employees.stream()
.collect(Collectors.groupingBy(Employee::getDepartment));
// Kotlin:
val byDept = employees.groupBy { it.department }
Compute sum of salaries by department
// Java:
Map<Department, Integer> totalByDept
= employees.stream()
.collect(Collectors.groupingBy(Employee::getDepartment,
Collectors.summingInt(Employee::getSalary)));
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// Kotlin:
val totalByDept = employees.groupBy { it.dept }.mapValues { it.value.sumBy { it.salary }}
Partition students into passing and failing
// Java:
Map<Boolean, List<Student>> passingFailing =
students.stream()
.collect(Collectors.partitioningBy(s -> s.getGrade() >= PASS_THRESHOLD));
// Kotlin:
val passingFailing = students.partition { it.grade >= PASS_THRESHOLD }
Names of male members
// Java:
List<String> namesOfMaleMembersCollect = roster
.stream()
.filter(p -> p.getGender() == Person.Sex.MALE)
.map(p -> p.getName())
.collect(Collectors.toList());
// Kotlin:
val namesOfMaleMembers = roster.filter { it.gender == Person.Sex.MALE }.map { it.name }
Group names of members in roster by gender
// Java:
Map<Person.Sex, List<String>> namesByGender =
roster.stream().collect(
Collectors.groupingBy(
Person::getGender,
Collectors.mapping(
Person::getName,
Collectors.toList())));
// Kotlin:
val namesByGender = roster.groupBy { it.gender }.mapValues { it.value.map { it.name } }
Filter a list to another list
// Java:
List<String> filtered = items.stream()
.filter( item -> item.startsWith("o") )
.collect(Collectors.toList());
// Kotlin:
val filtered = items.filter { item.startsWith('o') }
Finding shortest string a list
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// Java:
String shortest = items.stream()
.min(Comparator.comparing(item -> item.length()))
.get();
// Kotlin:
val shortest = items.minBy { it.length }
Different Kinds of Streams #2 - lazily using first item if exists
// Java:
Stream.of("a1", "a2", "a3")
.findFirst()
.ifPresent(System.out::println);
// Kotlin:
sequenceOf("a1", "a2", "a3").firstOrNull()?.apply(::println)
Different Kinds of Streams #3 - iterate a range of Integers
// Java:
IntStream.range(1, 4).forEach(System.out::println);
// Kotlin: (inclusive range)
(1..3).forEach(::println)
Different Kinds of Streams #4 - iterate an array, map the values, calculate the
average
// Java:
Arrays.stream(new int[] {1, 2, 3})
.map(n -> 2 * n + 1)
.average()
.ifPresent(System.out::println); // 5.0
// Kotlin:
arrayOf(1,2,3).map { 2 * it + 1}.average().apply(::println)
Different Kinds of Streams #5 - lazily iterate a list of strings, map the values,
convert to Int, find max
// Java:
Stream.of("a1", "a2", "a3")
.map(s -> s.substring(1))
.mapToInt(Integer::parseInt)
.max()
.ifPresent(System.out::println); // 3
// Kotlin:
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sequenceOf("a1", "a2", "a3")
.map { it.substring(1) }
.map(String::toInt)
.max().apply(::println)
Different Kinds of Streams #6 - lazily iterate a stream of Ints, map the values,
print results
// Java:
IntStream.range(1, 4)
.mapToObj(i -> "a" + i)
.forEach(System.out::println);
// a1
// a2
// a3
// Kotlin: (inclusive range)
(1..3).map { "a$it" }.forEach(::println)
Different Kinds of Streams #7 - lazily iterate Doubles, map to Int, map to
String, print each
// Java:
Stream.of(1.0, 2.0, 3.0)
.mapToInt(Double::intValue)
.mapToObj(i -> "a" + i)
.forEach(System.out::println);
// a1
// a2
// a3
// Kotlin:
sequenceOf(1.0, 2.0, 3.0).map(Double::toInt).map { "a$it" }.forEach(::println)
Counting items in a list after filter is applied
// Java:
long count = items.stream().filter( item -> item.startsWith("t")).count();
// Kotlin:
val count = items.filter { it.startsWith('t') }.size
// but better to not filter, but count with a predicate
val count = items.count { it.startsWith('t') }
How streams work - filter, upper case, then sort a list
// Java:
List<String> myList = Arrays.asList("a1", "a2", "b1", "c2", "c1");
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myList.stream()
.filter(s -> s.startsWith("c"))
.map(String::toUpperCase)
.sorted()
.forEach(System.out::println);
// C1
// C2
// Kotlin:
val list = listOf("a1", "a2", "b1", "c2", "c1")
list.filter { it.startsWith('c') }.map (String::toUpperCase).sorted()
.forEach (::println)
Different Kinds of Streams #1 - eager using first item if it exists
// Java:
Arrays.asList("a1", "a2", "a3")
.stream()
.findFirst()
.ifPresent(System.out::println);
// Kotlin:
listOf("a1", "a2", "a3").firstOrNull()?.apply(::println)
or, create an extension function on String called ifPresent:
// Kotlin:
inline fun String?.ifPresent(thenDo: (String)->Unit) = this?.apply { thenDo(this) }
// now use the new extension function:
listOf("a1", "a2", "a3").firstOrNull().ifPresent(::println)
See also: apply() function
See also: Extension Functions
See also: ?. Safe Call operator, and in general nullability:
http://stackoverflow.com/questions/34498562/in-kotlin-what-is-the-idiomatic-way-to-deal-with-
nullable-values-referencing-o/34498563#34498563
Collect example #5 - find people of legal age, output formatted string
// Java:
String phrase = persons
.stream()
.filter(p -> p.age >= 18)
.map(p -> p.name)
.collect(Collectors.joining(" and ", "In Germany ", " are of legal age."));
System.out.println(phrase);
// In Germany Max and Peter and Pamela are of legal age.
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// Kotlin:
val phrase = persons
.filter { it.age >= 18 }
.map { it.name }
.joinToString(" and ", "In Germany ", " are of legal age.")
println(phrase)
// In Germany Max and Peter and Pamela are of legal age.
And as a side note, in Kotlin we can create simple data classes and instantiate the test data as
follows:
// Kotlin:
// data class has equals, hashcode, toString, and copy methods automagically
data class Person(val name: String, val age: Int)
val persons = listOf(Person("Tod", 5), Person("Max", 33),
Person("Frank", 13), Person("Peter", 80),
Person("Pamela", 18))
Collect example #6 - group people by age, print age and names together
// Java:
Map<Integer, String> map = persons
.stream()
.collect(Collectors.toMap(
p -> p.age,
p -> p.name,
(name1, name2) -> name1 + ";" + name2));
System.out.println(map);
// {18=Max, 23=Peter;Pamela, 12=David}
Ok, a more interest case here for Kotlin. First the wrong answers to explore variations of creating a
Map from a collection/sequence:
// Kotlin:
val map1 = persons.map { it.age to it.name }.toMap()
println(map1)
// output: {18=Max, 23=Pamela, 12=David}
// Result: duplicates overridden, no exception similar to Java 8
val map2 = persons.toMap({ it.age }, { it.name })
println(map2)
// output: {18=Max, 23=Pamela, 12=David}
// Result: same as above, more verbose, duplicates overridden
val map3 = persons.toMapBy { it.age }
println(map3)
// output: {18=Person(name=Max, age=18), 23=Person(name=Pamela, age=23), 12=Person(name=David,
age=12)}
// Result: duplicates overridden again
val map4 = persons.groupBy { it.age }
println(map4)
// output: {18=[Person(name=Max, age=18)], 23=[Person(name=Peter, age=23), Person(name=Pamela,
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age=23)], 12=[Person(name=David, age=12)]}
// Result: closer, but now have a Map<Int, List<Person>> instead of Map<Int, String>
val map5 = persons.groupBy { it.age }.mapValues { it.value.map { it.name } }
println(map5)
// output: {18=[Max], 23=[Peter, Pamela], 12=[David]}
// Result: closer, but now have a Map<Int, List<String>> instead of Map<Int, String>
And now for the correct answer:
// Kotlin:
val map6 = persons.groupBy { it.age }.mapValues { it.value.joinToString(";") { it.name } }
println(map6)
// output: {18=Max, 23=Peter;Pamela, 12=David}
// Result: YAY!!
We just needed to join the matching values to collapse the lists and provide a transformer to
joinToString to move from Person instance to the Person.name.
Collect example #7a - Map names, join together with delimiter
// Java (verbose):
Collector<Person, StringJoiner, String> personNameCollector =
Collector.of(
() -> new StringJoiner(" | "), // supplier
(j, p) -> j.add(p.name.toUpperCase()), // accumulator
(j1, j2) -> j1.merge(j2), // combiner
StringJoiner::toString); // finisher
String names = persons
.stream()
.collect(personNameCollector);
System.out.println(names); // MAX | PETER | PAMELA | DAVID
// Java (concise)
String names = persons.stream().map(p -> p.name.toUpperCase()).collect(Collectors.joining(" |
"));
// Kotlin:
val names = persons.map { it.name.toUpperCase() }.joinToString(" | ")
Collect example #7b - Collect with SummarizingInt
// Java:
IntSummaryStatistics ageSummary =
persons.stream()
.collect(Collectors.summarizingInt(p -> p.age));
System.out.println(ageSummary);
// IntSummaryStatistics{count=4, sum=76, min=12, average=19.000000, max=23}
// Kotlin:
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// something to hold the stats...
data class SummaryStatisticsInt(var count: Int = 0,
var sum: Int = 0,
var min: Int = Int.MAX_VALUE,
var max: Int = Int.MIN_VALUE,
var avg: Double = 0.0) {
fun accumulate(newInt: Int): SummaryStatisticsInt {
count++
sum += newInt
min = min.coerceAtMost(newInt)
max = max.coerceAtLeast(newInt)
avg = sum.toDouble() / count
return this
}
}
// Now manually doing a fold, since Stream.collect is really just a fold
val stats = persons.fold(SummaryStatisticsInt()) { stats, person ->
stats.accumulate(person.age) }
println(stats)
// output: SummaryStatisticsInt(count=4, sum=76, min=12, max=23, avg=19.0)
But it is better to create an extension function, 2 actually to match styles in Kotlin stdlib:
// Kotlin:
inline fun Collection<Int>.summarizingInt(): SummaryStatisticsInt
= this.fold(SummaryStatisticsInt()) { stats, num -> stats.accumulate(num) }
inline fun <T: Any> Collection<T>.summarizingInt(transform: (T)->Int): SummaryStatisticsInt =
this.fold(SummaryStatisticsInt()) { stats, item -> stats.accumulate(transform(item)) }
Now you have two ways to use the new summarizingInt functions:
val stats2 = persons.map { it.age }.summarizingInt()
// or
val stats3 = persons.summarizingInt { it.age }
And all of these produce the same results. We can also create this extension to work on Sequence
and for appropriate primitive types.
Read Java 8 Stream Equivalents online: https://riptutorial.com/kotlin/topic/707/java-8-stream-
equivalents
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Chapter 22: JUnit
Examples
Rules
To add a JUnit rule to a test fixture:
@Rule @JvmField val myRule = TemporaryFolder()
The @JvmField annotation is necessary to expose the backing field with the same visibility (public)
as the myRule property (see answer). JUnit rules require the annotated rule field to be public.
Read JUnit online: https://riptutorial.com/kotlin/topic/6973/junit
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Chapter 23: Kotlin Android Extensions
Introduction
Kotlin has a built-in view injection for Android, allowing to skip manual binding or need for
frameworks such as ButterKnife. Some of the advantages are a nicer syntax, better static typing
and thus being less error-prone.
Examples
Configuration
Start with a properly configured gradle project.
In your project-local (not top-level) build.gradle append extensions plugin declaration below your
Kotlin plugin, on top-level indentation level.
buildscript {
...
}
apply plugin: "com.android.application"
...
apply plugin: "kotlin-android"
apply plugin: "kotlin-android-extensions"
...
Using Views
Assuming we have an activity with an example layout called activity_main.xml:
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent">
<Button
android:id="@+id/my_button"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="My button"/>
</LinearLayout>
We can use Kotlin extensions to call the button without any additional binding like so:
import kotlinx.android.synthetic.main.activity_main.my_button
class MainActivity: Activity() {
override fun onCreate(savedInstanceBundle: Bundle?) {
super.onCreate(savedInstanceBundle)
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setContentView(R.layout.activity_main)
// my_button is already casted to a proper type of "Button"
// instead of being a "View"
my_button.setText("Kotlin rocks!")
}
}
You can also import all ids appearing in layout with a * notation
// my_button can be used the same way as before
import kotlinx.android.synthetic.main.activity_main.*
Synthetic views can't be used outside of Activities/Fragments/Views with that layout inflated:
import kotlinx.android.synthetic.main.activity_main.my_button
class NotAView {
init {
// This sample won't compile!
my_button.setText("Kotlin rocks!")
}
}
Product flavors
Android extensions also work with multiple Android Product Flavors. For example if we have
flavors in build.gradle like so:
android {
productFlavors {
paid {
...
}
free {
...
}
}
}
And for example, only the free flavor has a buy button:
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent">
<Button
android:id="@+id/buy_button"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Buy full version"/>
</LinearLayout>
We can bind to the flavor specifically:
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import kotlinx.android.synthetic.free.main_activity.buy_button
Painfull listener for getting notice, when the view is completely drawn now is
so simple and awesome with Kotlin's extension
mView.afterMeasured {
// inside this block the view is completely drawn
// you can get view's height/width, it.height / it.width
}
Under the hood
inline fun View.afterMeasured(crossinline f: View.() -> Unit) {
viewTreeObserver.addOnGlobalLayoutListener(object : ViewTreeObserver.OnGlobalLayoutListener {
override fun onGlobalLayout() {
if (measuredHeight > 0 && measuredWidth > 0) {
viewTreeObserver.removeOnGlobalLayoutListener(this)
f()
}
}
})
}
Read Kotlin Android Extensions online: https://riptutorial.com/kotlin/topic/9474/kotlin-android-
extensions
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Chapter 24: Kotlin Caveats
Examples
Calling a toString() on a nullable type
A thing to look out for when using the toString method in Kotlin is the handling of null in
combination with the String?.
For example you want to get text from an EditText in Android.
You would have a piece of code like:
// Incorrect:
val text = view.textField?.text.toString() ?: ""
You would expect that if the field did not exists the value would be empty string but in this case it is
"null".
// Correct:
val text = view.textField?.text?.toString() ?: ""
Read Kotlin Caveats online: https://riptutorial.com/kotlin/topic/6608/kotlin-caveats
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Chapter 25: Kotlin for Java Developers
Introduction
Most people coming to Kotlin do have a programming background in Java.
This topic collects examples comparing Java to Kotlin, highlighting the most important differences
and those gems Kotlin offers over Java.
Examples
Declaring Variables
In Kotlin, variable declarations look a bit different than Java's:
val i : Int = 42
•
•
•
They start with either val or var, making the declaration final ("value") or variable.
The type is noted after the name, separated by a :
Thanks to Kotlin's type inference the explicit type declaration can be obmitted if there is an
assignment with a type the compiler is able to unambigously detect
Java
Kotlin
int i = 42;
var i = 42 (or var i : Int = 42)
final int i = 42;
val i = 42
Quick Facts
•
•
•
•
•
•
•
•
•
•
Kotlin does not need ; to end statements
Kotlin is null-safe
Kotlin is 100% Java interoperable
Kotlin has no primitives (but optimizes their object counterparts for the JVM, if possible)
Kotlin classes have properties, not fields
Kotlin offers data classes with auto-generated equals/hashCode methods and field accessors
Kotlin only has runtime Exceptions, no checked Exceptions
Kotlin has no new keyword. Creating objects is done just by calling the constructor like any
other method.
Kotlin supports (limited) operator overloading. For example, accessing a value of a map
can be written like: val a = someMap["key"]
Kotlin can not only be compiled to byte code for the JVM, but also into Java Script, enabling
you to write both backend and frontend code in Kotlin
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•
•
•
•
Kotlin is fully compatible with Java 6, which is especially interesting in regards for support
of (not so) old Android devices
Kotlin is an officially supported language for Android development
Kotlin's collections have built-in disctinction between mutable and immutable collections.
Kotlin supports Coroutines (experimental)
Equality & Identity
Kotlin uses == for equality (that is, calls equals internally) and === for referential identity.
Java
Kotlin
a.equals(b);
a == b
a == b;
a === b
a != b;
a !== b
See: https://kotlinlang.org/docs/reference/equality.html
IF, TRY and others are expressions, not statements
In Kotlin, if, try and others are expressions (so they do return a value) rather than (void)
statements.
So, for example, Kotlin does not have Java's ternary Elvis Operator, but you can write something
like this:
val i = if (someBoolean) 33 else 42
Even more unfamiliar, but equally expressive, is the try expression:
val i = try {
Integer.parseInt(someString)
}
catch (ex : Exception)
{
42
}
Read Kotlin for Java Developers online: https://riptutorial.com/kotlin/topic/10099/kotlin-for-java-
developers
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Chapter 26: logging in kotlin
Remarks
Related question: Idiomatic way of logging in Kotlin
Examples
kotlin.logging
class FooWithLogging {
companion object: KLogging()
fun bar() {
logger.info { "hello $name" }
}
fun logException(e: Exception) {
logger.error(e) { "Error occured" }
}
}
Using kotlin.logging framework
Read logging in kotlin online: https://riptutorial.com/kotlin/topic/3258/logging-in-kotlin
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Chapter 27: Loops in Kotlin
Remarks
In Kotlin, loops are compiled down to optimized loops wherever possible. For example, if you
iterate over a number range, the bytecode will be compiled down to a corresponding loop based
on plain int values to avoid the overhead of object creation.
Examples
Repeat an action x times
repeat(10) { i ->
println("This line will be printed 10 times")
println("We are on the ${i + 1}. loop iteration")
}
Looping over iterables
You can loop over any iterable by using the standard for-loop:
val list = listOf("Hello", "World", "!")
for(str in list) {
print(str)
}
Lots of things in Kotlin are iterable, like number ranges:
for(i in 0..9) {
print(i)
}
If you need an index while iterating:
for((index, element) in iterable.withIndex()) {
print("$element at index $index")
}
There is also a functional approach to iterating included in the standard library, without apparent
language constructs, using the forEach function:
iterable.forEach {
print(it.toString())
}
it in this example implicitly holds the current element, see Lambda Functions
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While Loops
While and do-while loops work like they do in other languages:
while(condition) {
doSomething()
}
do {
doSomething()
} while (condition)
In the do-while loop, the condition block has access to values and variables declared in the loop
body.
Break and continue
Break and continue keywords work like they do in other languages.
while(true) {
if(condition1) {
continue // Will immediately start the next iteration, without executing the rest of
the loop body
}
if(condition2) {
break // Will exit the loop completely
}
}
If you have nested loops, you can label the loop statements and qualify the break and continue
statements to specify which loop you want to continue or break:
outer@ for(i in 0..10) {
inner@ for(j in 0..10) {
break // Will break the inner loop
break@inner // Will break the inner loop
break@outer // Will break the outer loop
}
}
This approach won't work for the functional forEach construct, though.
Iterating over a Map in kotlin
//iterates over a map, getting the key and value at once
var map = hashMapOf(1 to "foo", 2 to "bar", 3 to "baz")
for ((key, value) in map) {
println("Map[$key] = $value")
}
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Recursion
Looping via recursion is also possible in Kotlin as in most programming languages.
fun factorial(n: Long): Long = if (n == 0) 1 else n * factorial(n - 1)
println(factorial(10)) // 3628800
In the example above, the factorial function will be called repeatedly by itself until the given
condition is met.
Functional constructs for iteration
The Kotlin Standard Library also provides numerous useful functions to iteratively work upon
collections.
For example, the map function can be used to transform a list of items.
val numbers = listOf(1, 2, 3, 4, 5, 6, 7, 8, 9, 0)
val numberStrings = numbers.map { "Number $it" }
One of the many advantages of this style is it allows to chain operations in a similar fashion. Only
a minor modification would be required if say, the list above were needed to be filtered for even
numbers. The filter function can be used.
val numbers = listOf(1, 2, 3, 4, 5, 6, 7, 8, 9, 0)
val numberStrings = numbers.filter { it % 2 == 0 }.map { "Number $it" }
Read Loops in Kotlin online: https://riptutorial.com/kotlin/topic/2727/loops-in-kotlin
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Chapter 28: Null Safety
Examples
Nullable and Non-Nullable types
Normal types, like String, are not nullable. To make them able to hold null values, you have to
explicitly denote that by putting a ? behind them: String?
var string : String = "Hello World!"
var nullableString: String? = null
string = nullableString // Compiler error: Can't assign nullable to non-nullable type.
nullableString = string // This will work however!
Safe call operator
To access functions and properties of nullable types, you have to use special operators.
The first one, ?., gives you the property or function you're trying to access, or it gives you null if the
object is null:
val string: String? = "Hello World!"
print(string.length) // Compile error: Can't directly access property of nullable type.
print(string?.length) // Will print the string's length, or "null" if the string is null.
Idiom: calling multiple methods on the same, null-checked
object
An elegant way to call multiple methods of a null-checked object is using Kotlin's apply like this:
obj?.apply {
foo()
bar()
}
This will call foo and bar on obj (which is this in the apply block) only if obj is non-null, skipping the
entire block otherwise.
To bring a nullable variable into scope as a non-nullable reference without making it the implicit
receiver of function and property calls, you can use let instead of apply:
nullable?.let { notnull ->
notnull.foo()
notnull.bar()
}
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notnull could be named anything, or even left out and used through the implicit lambda parameter
it.
Smart casts
If the compiler can infer that an object can't be null at a certain point, you don't have to use the
special operators anymore:
var string: String? = "Hello!"
print(string.length) // Compile error
if(string != null) {
// The compiler now knows that string can't be null
print(string.length) // It works now!
}
Note: The compiler won't allow you to smart cast mutable variables that could
potentially be modified between the null-check and the intended usage.
If a variable is accessible from outside the scope of the current block (because they are
members of a non-local object, for example), you need to create a new, local reference
which you can then smart cast and use.
Eliminate nulls from an Iterable and array
Sometimes we need to change type from Collection<T?> to Collections<T>. In that case,
filterNotNull is our solution.
val a: List<Int?> = listOf(1, 2, 3, null)
val b: List<Int> = a.filterNotNull()
Null Coalescing / Elvis Operator
Sometimes it is desirable to evaluate a nullable expression in an if-else fashion. The elvis
operator, ?:, can be used in Kotlin for such a situation.
For instance:
val value: String = data?.first() ?: "Nothing here."
The expression above returns "Nothing here" if data?.first() or data itself yield a null value else
the result of data?.first().
It is also possible to throw exceptions using the same syntax to abort code execution.
val value: String = data?.second()
?: throw IllegalArgumentException("Value can't be null!")
Reminder: NullPointerExceptions can be thrown using the assertion operator (e.g.
data!!.second()!!)
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Assertion
!! suffixes ignore nullability and returns a non-null version of that type. KotlinNullPointerException
will be thrown if the object is a null.
val message: String? = null
println(message!!) //KotlinNullPointerException thrown, app crashes
Elvis Operator (?:)
In Kotlin, we can declare variable which can hold null reference. Suppose we have a nullable
reference a, we can say "if a is not null, use it, otherwise use some non-null value x"
var a: String? = "Nullable String Value"
Now, a can be null. So when we need to access value of a, then we need to perform safety check,
whether it contains value or not. We can perform this safety check by conventional if...else
statement.
val b: Int = if (a != null) a.length else -1
But here comes advance operator Elvis(Operator Elvis : ?:). Above if...else can be expressed
with the Elvis operator as below:
val b = a?.length ?: -1
If the expression to the left of ?: (here : a?.length) is not null, the elvis operator returns it, otherwise
it returns the expression to the right (here: -1). Right-hand side expression is evaluated only if the
left-hand side is null.
Read Null Safety online: https://riptutorial.com/kotlin/topic/2080/null-safety
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Chapter 29: Ranges
Introduction
Range expressions are formed with rangeTo functions that have the operator form .. which is
complemented by in and !in. Range is defined for any comparable type, but for integral primitive
types it has an optimized implementation
Examples
Integral Type Ranges
Integral type ranges ( IntRange , LongRange , CharRange ) have an extra feature: they can be
iterated over. The compiler takes care of converting this analogously to Java's indexed for-loop,
without extra overhead
for (i in 1..4) print(i) // prints "1234"
for (i in 4..1) print(i) // prints nothing
downTo() function
if you want to iterate over numbers in reverse order? It's simple. You can use the downTo()
function defined in the standard library
for (i in 4 downTo 1) print(i) // prints "4321"
step() function
Is it possible to iterate over numbers with arbitrary step, not equal to 1? Sure, the step() function
will help you
for (i in 1..4 step 2) print(i) // prints "13"
for (i in 4 downTo 1 step 2) print(i) // prints "42"
until function
To create a range which does not include its end element, you can use the until function:
for (i in 1 until 10) { // i in [1, 10), 10 is excluded
println(i)
}
Read Ranges online: https://riptutorial.com/kotlin/topic/10121/ranges
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Chapter 30: RecyclerView in Kotlin
Introduction
I just want to share my little bit knowledge and code of RecyclerView using Kotlin.
Examples
Main class and Adapter
I am assuming that you have aware about the some syntax of Kotlin and how to use, just add
RecyclerView in activity_main.xml file and set with adapter class.
class MainActivity : AppCompatActivity(){
lateinit var mRecyclerView : RecyclerView
val mAdapter : RecyclerAdapter = RecyclerAdapter()
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
val toolbar = findViewById(R.id.toolbar) as Toolbar
setSupportActionBar(toolbar)
mRecyclerView = findViewById(R.id.recycler_view) as RecyclerView
mRecyclerView.setHasFixedSize(true)
mRecyclerView.layoutManager = LinearLayoutManager(this)
mAdapter.RecyclerAdapter(getList(), this)
mRecyclerView.adapter = mAdapter
}
private fun getList(): ArrayList<String> {
var list : ArrayList<String> = ArrayList()
for (i in 1..10) { // equivalent of 1 <= i && i <= 10
println(i)
list.add("$i")
}
return list
}
}
this one is your recycler view adapter class and create main_item.xml file what you want
class RecyclerAdapter : RecyclerView.Adapter<RecyclerAdapter.ViewHolder>() {
var mItems: ArrayList<String> = ArrayList()
lateinit var mClick : OnClick
fun RecyclerAdapter(item : ArrayList<String>, mClick : OnClick){
this.mItems = item
this.mClick = mClick;
}
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override fun onBindViewHolder(holder: ViewHolder, position: Int) {
val item = mItems[position]
holder.bind(item, mClick, position)
}
override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ViewHolder {
val layoutInflater = LayoutInflater.from(parent.context)
return ViewHolder(layoutInflater.inflate(R.layout.main_item, parent, false))
}
override fun getItemCount(): Int {
return mItems.size
}
class ViewHolder(view: View) : RecyclerView.ViewHolder(view) {
val card = view.findViewById(R.id.card) as TextView
fun bind(str: String, mClick: OnClick, position: Int){
card.text = str
card.setOnClickListener { view ->
mClick.onClickListner(position)
}
}
}
}
Read RecyclerView in Kotlin online: https://riptutorial.com/kotlin/topic/10143/recyclerview-in-kotlin
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Chapter 31: Reflection
Introduction
Reflection is a language's ability to inspect code at runtime instead of compile time.
Remarks
Reflection is a mechanism to introspect language constructs (classes and functions) at the
runtime.
When targeting JVM platform, runtime reflection features are distributed in separate JAR: kotlin-
reflect.jar. This is done to reduce runtime size, cut unused features and make it possible to
target another (like JS) platforms.
Examples
Referencing a class
To obtain a reference to a KClass object representing some class use double colons:
val c1 = String::class
val c2 = MyClass::class
Referencing a function
Functions are first-class citizens in Kotlin. You can obtain a reference on it using double colons
and then pass it to another function:
fun isPositive(x: Int) = x > 0
val numbers = listOf(-2, -1, 0, 1, 2)
println(numbers.filter(::isPositive)) // [1, 2]
Inter-operating with Java reflection
To obtain a Java's Class object from Kotlin's KClass use the .java extension property:
val stringKClass: KClass<String> = String::class
val c1: Class<String> = stringKClass.java
val c2: Class<MyClass> = MyClass::class.java
The latter example will be optimized by the compiler to not allocate an intermediate KClass
instance.
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Getting values of all properties of a class
Given Example class extending BaseExample class with some properties:
open class BaseExample(val baseField: String)
class Example(val field1: String, val field2: Int, baseField: String):
BaseExample(baseField) {
val field3: String
get() = "Property without backing field"
val field4 by lazy { "Delegated value" }
private val privateField: String = "Private value"
}
One can get hold of all properties of a class:
val example = Example(field1 = "abc", field2 = 1, baseField = "someText")
example::class.memberProperties.forEach { member ->
println("${member.name} -> ${member.get(example)}")
}
Running this code will cause an exception to be thrown. Property private val privateField is
declared private and calling member.get(example) on it will not succeed. One way to handle this it to
filter out private properties. To do that we have to check the visibility modifier of a property's Java
getter. In case of private val the getter does not exist so we can assume private access.
The helper function and it's usage might look like this:
fun isFieldAccessible(property: KProperty1<*, *>): Boolean {
return property.javaGetter?.modifiers?.let { !Modifier.isPrivate(it) } ?: false
}
val example = Example(field1 = "abc", field2 = 1, baseField = "someText")
example::class.memberProperties.filter { isFieldAccessible(it) }.forEach { member ->
println("${member.name} -> ${member.get(example)}")
}
Another approach is to make private properties accessible using reflection:
example::class.memberProperties.forEach { member ->
member.isAccessible = true
println("${member.name} -> ${member.get(example)}")
}
Setting values of all properties of a class
As an example we want to set all string properties of a sample class
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class TestClass {
val readOnlyProperty: String
get() = "Read only!"
var readWriteString = "asd"
var readWriteInt = 23
var readWriteBackedStringProperty: String = ""
get() = field + '5'
set(value) { field = value + '5' }
var readWriteBackedIntProperty: Int = 0
get() = field + 1
set(value) { field = value - 1 }
var delegatedProperty: Int by TestDelegate()
private var privateProperty = "This should be private"
private class TestDelegate {
private var backingField = 3
operator fun getValue(thisRef: Any?, prop: KProperty<*>): Int {
return backingField
}
operator fun setValue(thisRef: Any?, prop: KProperty<*>, value: Int) {
backingField += value
}
}
}
Getting mutable properties builds on getting all properties, filtering mutable properties by type. We
also need to check visibility, as reading private properties results in run time exception.
val instance = TestClass()
TestClass::class.memberProperties
.filter{ prop.visibility == KVisibility.PUBLIC }
.filterIsInstance<KMutableProperty<*>>()
.forEach { prop ->
System.out.println("${prop.name} -> ${prop.get(instance)")
}
To set all String properties to "Our Value" we can additionally filter by the return type. Since Kotlin
is based on Java VM, Type Erasure is in effect, and thus Properties returning generic types such
as List<String> will be the same as List<Any>. Sadly reflection is not a golden bullet and there is no
sensible way to avoid this, so you need to watch out in your use-cases.
val instance = TestClass()
TestClass::class.memberProperties
.filter{ prop.visibility == KVisibility.PUBLIC }
// We only want strings
.filter{ it.returnType.isSubtypeOf(String::class.starProjectedType) }
.filterIsInstance<KMutableProperty<*>>()
.forEach { prop ->
// Instead of printing the property we set it to some value
prop.setter.call(instance, "Our Value")
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}
Read Reflection online: https://riptutorial.com/kotlin/topic/2402/reflection
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Chapter 32: Regex
Examples
Idioms for Regex Matching in When Expression
Using immutable locals:
Uses less horizontal space but more vertical space than the "anonymous temporaries" template.
Preferable over the "anonymous temporaries" template if the when expression is in a loop--in that
case, regex definitions should be placed outside the loop.
import kotlin.text.regex
var string = /* some string */
val regex1 = Regex( /* pattern */ )
val regex2 = Regex( /* pattern */ )
/* etc */
when {
regex1.matches(string) -> /* do stuff */
regex2.matches(string) -> /* do stuff */
/* etc */
}
Using anonymous temporaries:
Uses less vertical space but more horizontal space than the "immutable locals" template. Should
not be used if then when expression is in a loop.
import kotlin.text.regex
var string = /* some string */
when {
Regex( /* pattern */ ).matches(string) -> /* do stuff */
Regex( /* pattern */ ).matches(string) -> /* do stuff */
/* etc */
}
Using the visitor pattern:
Has the benefit of closely emulating the "argument-ful" when syntax. This is beneficial because it
more clearly indicates the argument of the when expression, and also precludes certain
programmer mistakes that could arise from having to repeat the when argument in every whenEntry.
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Either the "immutable locals" or the "anonymous temporaries" template may be used with this
implementation the visitor pattern.
import kotlin.text.regex
var string = /* some string */
when (RegexWhenArgument(string)) {
Regex( /* pattern */ ) -> /* do stuff */
Regex( /* pattern */ ) -> /* do stuff */
/* etc */
}
And the minimal definition of the wrapper class for the when expression argument:
class RegexWhenArgument (val whenArgument: CharSequence) {
operator fun equals(whenEntry: Regex) = whenEntry.matches(whenArgument)
override operator fun equals(whenEntry: Any?) = (whenArgument == whenEntry)
}
Introduction to regular expressions in Kotlin
This post shows how to use most of the functions in the Regex class, work with null safely related to
the Regex functions, and how raw strings makes it easier to write and read regex patterns.
The RegEx class
To work with regular expressions in Kotlin, you need to use the Regex(pattern: String) class and
invoke functions like find(..) or replace(..) on that regex object.
An example on how to use the Regex class that returns true if the input string contains c or d:
val regex = Regex(pattern = "c|d")
val matched = regex.containsMatchIn(input = "abc") // matched: true
The essential thing to understand with all the Regex functions is that the result is based on
matching the regex pattern and the input string. Some of the functions requires a full match, while
the rest requires only a partial match. The containsMatchIn(..) function used in the example
requires a partial match and is explained later in this post.
Null safety with regular expressions
Both find(..) and matchEntire(..) will return a MatchResult? object. The ? character after
MatchResult is necessary for Kotlin to handle null safely.
An example that demonstrates how Kotlin handles null safely from a Regex function, when the
find(..) function returns null:
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val matchResult =
Regex("c|d").find("efg") // matchResult: null
val a = matchResult?.value // a: null
val b = matchResult?.value.orEmpty() // b: ""
a?.toUpperCase() // Still needs question mark. => null
b.toUpperCase() // Accesses the function directly. => ""
With the orEmpty() function, b can't be null and the ? character is unnecessary when you call
functions on b.
If you don't care about this safe handling of null values, Kotlin allows you to work with null values
like in Java with the !! characters:
a!!.toUpperCase() // => KotlinNullPointerException
Raw strings in regex patterns
Kotlin provides an improvement over Java with a raw string that makes it possible to write pure
regex patterns without double backslashes, that are necessary with a Java string. A raw string is
represented with a triple quote:
"""\d{3}-\d{3}-\d{4}""" // raw Kotlin string
"\\d{3}-\\d{3}-\\d{4}" // standard Java string
find(input: CharSequence, startIndex: Int):
MatchResult?
The input string will be matched against the pattern in the Regex object. It returns a Matchresult?
object with the first matched text after the startIndex, or null if the pattern didn't match the input
string. The result string is retrieved from the MatchResult? object's value property. The startIndex
parameter is optional with the default value 0.
To extract the first valid phone number from a string with contact details:
val phoneNumber :String? = Regex(pattern = """\d{3}-\d{3}-\d{4}""")
.find(input = "phone: 123-456-7890, e..")?.value // phoneNumber: 123-456-7890
With no valid phone number in the input string, the variable phoneNumber will be null.
findAll(input: CharSequence, startIndex: Int):
Sequence
Returns all the matches from the input string that matches the regex pattern.
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To print out all numbers separated with space, from a text with letters and digits:
val matchedResults = Regex(pattern = """\d+""").findAll(input = "ab12cd34ef")
val result = StringBuilder()
for (matchedText in matchedResults) {
result.append(matchedText.value + " ")
}
println(result) // => 12 34
The matchedResults variable is a sequence with MatchResult objects. With an input string without
digits, the findAll(..) function will return an empty sequence.
matchEntire(input: CharSequence):
MatchResult?
If all the characters in the input string matches the regex pattern, a string equal to the input will be
returned. Else, null will be returned.
Returns the input string if the whole input string is a number:
val a = Regex("""\d+""").matchEntire("100")?.value // a: 100
val b = Regex("""\d+""").matchEntire("100 dollars")?.value // b: null
matches(input: CharSequence): Boolean
Returns true if the whole input string matches the regex pattern. False otherwise.
Tests if two strings contains only digits:
val regex = Regex(pattern = """\d+""")
regex.matches(input = "50") // => true
regex.matches(input = "50 dollars") // => false
containsMatchIn(input: CharSequence):
Boolean
Returns true if part of the input string matches the regex pattern. False otherwise.
Test if two strings contains at least one digit:
Regex("""\d+""").containsMatchIn("50 dollars") // => true
Regex("""\d+""").containsMatchIn("Fifty dollars") // => false
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split(input: CharSequence, limit: Int): List
Returns a new list without all the regex matches.
To return lists without digits:
val a = Regex("""\d+""").split("ab12cd34ef") // a: [ab, cd, ef]
val b = Regex("""\d+""").split("This is a test") // b: [This is a test]
There is one element in the list for each split. The first input string has three numbers. That results
in a list with three elements.
replace(input: CharSequence, replacement:
String): String
Replaces all matches of the regex pattern in the input string with the replacement string.
To replace all digits in a string with an x:
val result = Regex("""\d+""").replace("ab12cd34ef", "x") // result: abxcdxef
Read Regex online: https://riptutorial.com/kotlin/topic/8364/regex
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Chapter 33: Singleton objects
Introduction
An object is a special kind of class, which can be declared using object keyword. Objects are
similar to Singletons (a design pattern) in java. It also functions as the static part of java.
Beginners who are switching from java to kotlin can vastly use this feature, in place of static, or
singletons.
Examples
Use as repalcement of static methods/fields of java
object CommonUtils {
var anyname: String ="Hello"
fun dispMsg(message: String) {
println(message)
}
}
From any other class, just invoke the variable and functions in this way:
CommonUtils.anyname
CommonUtils.dispMsg("like static call")
Use as a singleton
Kotlin objects are actually just singletons. Its primary advantage is that you don't have to use
SomeSingleton.INSTANCE to get the instance of the singleton.
In java your singleton looks like this:
public enum SharedRegistry {
INSTANCE;
public void register(String key, Object thing) {}
}
public static void main(String[] args) {
SharedRegistry.INSTANCE.register("a", "apple");
SharedRegistry.INSTANCE.register("b", "boy");
SharedRegistry.INSTANCE.register("c", "cat");
SharedRegistry.INSTANCE.register("d", "dog");
}
In kotlin, the equivalent code is
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object SharedRegistry {
fun register(key: String, thing: Object) {}
}
fun main(Array<String> args) {
SharedRegistry.register("a", "apple")
SharedRegistry.register("b", "boy")
SharedRegistry.register("c", "cat")
SharedRegistry.register("d", "dog")
}
It's obvoiusly less verbose to use.
Read Singleton objects online: https://riptutorial.com/kotlin/topic/10152/singleton-objects
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Chapter 34: Strings
Examples
Elements of String
Elements of String are characters that can be accessed by the indexing operation string[index].
val str = "Hello, World!"
println(str[1]) // Prints e
String elements can be iterated with a for-loop.
for (c in str) {
println(c)
}
String Literals
Kotlin has two types of string literals:
•
•
Escaped string
Raw string
Escaped string handles special characters by escaping them. Escaping is done with a backslash.
The following escape sequences are supported: \t, \b, \n, \r, \', \", \\ and \$. To encode any
other character, use the Unicode escape sequence syntax: \uFF00.
val s = "Hello, world!\n"
Raw string delimited by a triple quote """, contains no escaping and can contain newlines and
any other characters
val text = """
for (c in "foo")
print(c)
"""
Leading whitespace can be removed with trimMargin() function.
val text = """
|Tell me and I forget.
|Teach me and I remember.
|Involve me and I learn.
|(Benjamin Franklin)
""".trimMargin()
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Default margin prefix is pipe character |, this can be set as a parameter to trimMargin; e.g.
trimMargin(">").
String Templates
Both escaped strings and raw strings can contain template expressions. Template expression is a
piece of code which is evaluated and its result is concatenated into string. It starts with a dollar
sign $ and consists of either a variable name:
val i = 10
val s = "i = $i" // evaluates to "i = 10"
Or an arbitrary expression in curly braces:
val s = "abc"
val str = "$s.length is ${s.length}" // evaluates to "abc.length is 3"
To include a literal dollar sign in a string, escape it using a backslash:
val str = "\$foo" // evaluates to "$foo"
The exception is raw strings, which do not support escaping. In raw strings you can use the
following syntax to represent a dollar sign.
val price = """
${'$'}9.99
"""
String Equality
In Kotlin strings are compared with == operator which chect for their structural equality.
val str1 = "Hello, World!"
val str2 = "Hello," + " World!"
println(str1 == str2) // Prints true
Referential equality is checked with === operator.
val str1 = """
|Hello, World!
""".trimMargin()
val str2 = """
#Hello, World!
""".trimMargin("#")
val str3 = str1
println(str1 == str2) // Prints true
println(str1 === str2) // Prints false
println(str1 === str3) // Prints true
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Read Strings online: https://riptutorial.com/kotlin/topic/8285/strings
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Chapter 35: Type aliases
Introduction
With type aliases, we can give a alias to other type. It's ideal for giving a name to function types
like (String) -> Boolean or generic type like Pair<Person, Person>.
Type aliases support generics. An alias can replace a type with generics and an alias can be
generics.
Syntax
•
typealias alias-name = existing-type
Remarks
Type aliases is a feature of the compiler. Nothing is added in the generated code for the JVM. All
aliases will be replaced by the real type.
Examples
Function type
typealias StringValidator = (String) -> Boolean
typealias Reductor<T, U, V> = (T, U) -> V
Generic type
typealias Parents = Pair<Person, Person>
typealias Accounts = List<Account>
Read Type aliases online: https://riptutorial.com/kotlin/topic/9453/type-aliases
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Chapter 36: Type-Safe Builders
Remarks
A type-safe builder is a concept, rather than a language feature, so it is not strictly formalized.
A typical structure of a type-safe builder
A single builder function usually consists of 3 steps:
1.
2.
3.
Create an object.
Execute lambda to initialize the object.
Add the object to structure or return it.
Type-safe builders in Kotlin libraries
The concept of type-safe builders is widely used in some Kotlin libraries and frameworks, eg.:
•
•
•
•
Anko
Wasabi
Ktor
Spec
Examples
Type-safe tree structure builder
Builders can be defined as a set of extension functions taking lambda expressions with receivers
as arguments. In this example, a menu of a JFrame is being built:
import javax.swing.*
fun JFrame.menuBar(init: JMenuBar.() -> Unit) {
val menuBar = JMenuBar()
menuBar.init()
setJMenuBar(menuBar)
}
fun JMenuBar.menu(caption: String, init: JMenu.() -> Unit) {
val menu = JMenu(caption)
menu.init()
add(menu)
}
fun JMenu.menuItem(caption: String, init: JMenuItem.() -> Unit) {
val menuItem = JMenuItem(caption)
menuItem.init()
add(menuItem)
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}
These functions can then be used to build a tree structure of objects in an easy way:
class MyFrame : JFrame() {
init {
menuBar {
menu("Menu1") {
menuItem("Item1") {
// Initialize MenuItem with some Action
}
menuItem("Item2") {}
}
menu("Menu2") {
menuItem("Item3") {}
menuItem("Item4") {}
}
}
}
}
Read Type-Safe Builders online: https://riptutorial.com/kotlin/topic/6010/type-safe-builders
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Chapter 37: Vararg Parameters in Functions
Syntax
•
•
Vararg Keyword: vararg is used in a method declaration to indicate that a variable number
of parameters will be accepted.
Spread Operator: An asterisk (*) before an array that is used in function calls to "unfold" the
contents into individual parameters.
Examples
Basics: Using the vararg keyword
Define the function using the vararg keyword.
fun printNumbers(vararg numbers: Int) {
for (number in numbers) {
println(number)
}
}
Now you can pass as many parameters (of the correct type) into the function as you want.
printNumbers(0, 1) // Prints "0" "1"
printNumbers(10, 20, 30, 500) // Prints "10" "20" "30" "500"
Notes: Vararg parameters must be the last parameter in the parameter list.
Spread Operator: Passing arrays into vararg functions
Arrays can be passed into vararg functions using the Spread Operator, *.
Assuming the following function exists...
fun printNumbers(vararg numbers: Int) {
for (number in numbers) {
println(number)
}
}
You can pass an array into the function like so...
val numbers = intArrayOf(1, 2, 3)
printNumbers(*numbers)
// This is the same as passing in (1, 2, 3)
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The spread operator can also be used in the middle of the parameters...
val numbers = intArrayOf(1, 2, 3)
printNumbers(10, 20, *numbers, 30, 40)
// This is the same as passing in (10, 20, 1, 2, 3, 30, 40)
Read Vararg Parameters in Functions online: https://riptutorial.com/kotlin/topic/5835/vararg-
parameters-in-functions
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Chapter 38: Visibility Modifiers
Introduction
In Kotlin, there are 4 types of visibility modifiers are available.
Public: This can be accessed from anywhere.
Private: This can only be accessed from the module code.
Protected: This can only be accessed from the class defining it and any derived classes.
Internal: This can only be accessed from the scope of the class defining it.
Syntax
•
<visibility modifier> val/var <variable name> = <value>
Examples
Code Sample
Public: public val name = "Avijit"
Private: private val name = "Avijit"
Protected: protected val name = "Avijit"
Internal: internal val name = "Avijit"
Read Visibility Modifiers online: https://riptutorial.com/kotlin/topic/10019/visibility-modifiers
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Credits
S.
No
1
2
3
4
5
6
7
8
9
Chapters
Contributors
Getting started with
Kotlin
babedev, Community, cyberscientist, ganesshkumar, Ihor
Kucherenko, Jayson Minard, mnoronha, neworld, Parker Hoyes,
Ruckus T-Boom, Sach, Sean Reilly, Sheigutn, Simón Oroño,
UnKnown, Urko Pineda
Annotations
Brad Larson, Caelum, Héctor, Mood, piotrek1543, Sapan Zaveri
Arrays
egor.zhdan, Sam, UnKnown
Basic Lambdas
memoizr, Rich Kuzsma
Basics of Kotlin
Shinoo Goyal
Class Delegation
Sam
Class Inheritance
byxor, KeksArmee, piotrek1543, Slav
Collections
Ascension
Conditional
Statements
Abdullah, Alex Facciorusso, jpmcosta, Kirill Rakhman, Robin,
Spidfire
10
Configuring Kotlin
build
Aaron Christiansen, elect, madhead
11
coroutines
Jemo Mgebrishvili
12 Delegated properties Sam, Seaskyways
13 DSL Building
Dmitriy L, ice1000
14 Enum
David Soroko, Kirill Rakhman, SerCe
15 Exceptions
Brad Larson, jereksel, Sapan Zaveri
16 Extension Methods
Dávid Tímár, Jayson Minard, Kevin Robatel, Konrad Jamrozik,
olivierlemasle, Parker Hoyes, razzledazzle
17
Functions
Aaron Christiansen, baha, Caelum, glee8e, Jayson Minard,
KeksArmee, madhead, Spidfire
18 Generics
hotkey, Jayson Minard, KeksArmee
19
Idioms
Aaron Christiansen, Adam Arold, Brad Larson, Héctor, Jayson
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106
Minard, Konrad Jamrozik, madhead, mayojava, razzledazzle,
Sapan Zaveri, Serge Nikitin, yole
20
Interfaces
Divya, Jan Vladimir Mostert, Jayson Minard, Ritave, Robin
21
Java 8 Stream
Equivalents
Brad, Gerson, Jayson Minard, Piero Divasto, Sam
22
JUnit
jenglert
23
Kotlin Android
Extensions
Jemo Mgebrishvili, Ritave
24 Kotlin Caveats
Grigory Konushev, Spidfire
25
Kotlin for Java
Developers
Thorsten Schleinzer
26
logging in kotlin
Konrad Jamrozik, olivierlemasle, oshai
27
Loops in Kotlin
Ben Leggiero, JaseAnderson, mayojava, razzledazzle, Robin
28 Null Safety
KeksArmee, Kirill Rakhman, piotrek1543, razzledazzle, Robin,
SerCe, Spidfire, technerd, Thorsten Schleinzer
29 Ranges
Nihal Saxena
30
RecyclerView in
Kotlin
Mohit Suthar
31 Reflection
atok, Kirill Rakhman, madhead, Ritave, Sup
32 Regex
Espen, Travis
33 Singleton objects
Divya, glee8e
34 Strings
Januson, Sam
35
Type aliases
Kevin Robatel
36
Type-Safe Builders
Slav
37
Vararg Parameters
in Functions
byxor, piotrek1543, Sam
38 Visibility Modifiers
Avijit Karmakar
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