Java to Kotlin（java 与 Kotlin对比）

From Java to Kotlin

BASICS

Print
Java

System.out.print("Hello, World!");System.out.println("Hello, World!");

Kotlin

print("Hello, World!")println("Hello, World!")

Variables I
Java

final int x;final int y = 1;

Kotlin

val x: Intval y = 1

Variables II
Java

int w;int z = 2;z = 3;w = 1;

Kotlin

var w: Intvar z = 2z = 3w = 1 

Null I
Java

final String name = null;String lastName;lastName = null

Kotlin

val name: String? = nullvar lastName: String?lastName = nullvar firstName: StringfirstName = null // Compilation error!!

Null II
Java

if(text != null){  int length = text.length();}

Kotlin

val length = text?.lengthval length = text!!.length // NullPointerException if text == null

Strings I
Java

String name = "John";String lastName = "Smith";String text = "My name is: " + name + " " + lastName;String otherText = "My name is: " + name.substring(2);

Kotlin

val name = "John"val lastName = "Smith"val text = "My name is: $name $lastName"val otherText = "My name is: ${name.substring(2)}"

Strings II
Java

String text = "First Line\n" +              "Second Line\n" +              "Third Line";

Kotlin

val text = """        |First Line        |Second Line        |Third Line""".trimMargin()

Ternary Operator
Java

String text = x > 5 ? "x > 5" : "x <= 5";

Kotlin

val text = if (x > 5)              "x > 5"            else "x <= 5"

BASICS
Bits Operations
Java

final int andResult  = a & b;final int orResult   = a | b;final int xorResult  = a ^ b;final int rightShift = a >> 2;final int leftShift  = a << 2;

Kotlin

val andResult  = a and bval orResult   = a or bval xorResult  = a xor bval rightShift = a shr 2val leftShift  = a shl 2

Is As In
Java

if(x instanceof Integer){ }final String text = (String) other;if(x >= 0 && x <= 10 ){}

Kotlin

if (x is Int) { }val text = other as Stringif (x in 0..10) { }

Smart Cast
Java

if(a instanceof String){  final String result = ((String) a).substring(1);}

Kotlin

if (a is String) {  val result = a.substring(1)}

Switch / When
Java

final int x = // value;final String xResult;switch (x){  case 0:  case 11:    xResult = "0 or 11";    break;  case 1:  case 2:    //...  case 10:    xResult = "from 1 to 10";    break;  default:    if(x < 12 && x > 14) {      xResult = "not from 12 to 14";      break;    }    if(isOdd(x)) {      xResult = "is odd";      break;    }    xResult = "otherwise";}final int y = // value;final String yResult;if(isNegative(y)){  yResult = "is Negative";} else if(isZero(y)){  yResult = "is Zero";}else if(isOdd(y)){  yResult = "is Odd";}else {  yResult = "otherwise";}

Kotlin

val x = // valueval xResult = when (x) {  0, 11 -> "0 or 11"  in 1..10 -> "from 1 to 10"  !in 12..14 -> "not from 12 to 14"  else -> if (isOdd(x)) { "is odd" } else { "otherwise" }}val y = // valueval yResult = when {  isNegative(y) -> "is Negative"  isZero(y) -> "is Zero"  isOdd(y) -> "is odd"  else -> "otherwise"}

For
Java

for (int i = 1; i < 11 ; i++) { }for (int i = 1; i < 11 ; i+=2) { }for (String item : collection) { }for (Map.Entry<String, String> entry: map.entrySet()) { }

Kotlin

for (i in 1..10) { }for (i in 1..10 step 2) {}for (item in collection) {}for ((index, item) in collection.withIndex()) {}for ((key, value) in map) {}

Collections
Java

final List<Integer> numbers = Arrays.asList(1, 2, 3);final Map<Integer, String> map = new HashMap<Integer, String>();map.put(1, "One");map.put(2, "Two");map.put(3, "Three");// Java 9final List<Integer> numbers = List.of(1, 2, 3);final Map<Integer, String> map = Map.of(1, "One",                                        2, "Two",                                        3, "Three");

Kotlin

val numbers = listOf(1, 2, 3)val map = mapOf(1 to "One",                2 to "Two",                3 to "Three")

Collections
Java

for (int number : numbers) {  System.out.println(number);}for (int number : numbers) {  if(number > 5) {    System.out.println(number);  }}

Kotlin

numbers.forEach {    println(it)}numbers.filter  { it > 5 }       .forEach { println(it) }

Collections
Java

final Map<String, List<Integer>> groups = new HashMap<>();for (int number : numbers) {  if((number & 1) == 0){    if(!groups.containsKey("even")){      groups.put("even", new ArrayList<>());    }    groups.get("even").add(number);    continue;  }  if(!groups.containsKey("odd")){    groups.put("odd", new ArrayList<>());  }  groups.get("odd").add(number);}

Kotlin

val groups = numbers.groupBy {                if (it and 1 == 0) "even" else "odd"             }

Collections
Java

final List<Integer> evens = new ArrayList<>();final List<Integer> odds = new ArrayList<>();for (int number : numbers){  if ((number & 1) == 0) {    evens.add(number);  }else {    odds.add(number);  }}

Kotlin

val (evens, odds) = numbers.partition { it and 1 == 0 }CollectionsJavafinal List<User> users = getUsers();Collections.sort(users, new Comparator<User>(){  public int compare(User user, User otherUser){    return user.lastname.compareTo(otherUser.lastname);  }});// orusers.sort(Comparator.comparing(user -> user.lastname));Kotlinval users = getUsers()users.sortedBy { it.lastname }

FUNCTIONS

Basic Function
Java

public void hello() {  System.out.print("Hello, World!");}

Kotlin

fun hello() {    println("Hello, World!")}

Arguments
Java

public void hello(String name){  System.out.print("Hello, " + name + "!");}

Kotlin

fun hello(name: String) {    println("Hello, $name!")}

Default Values
Java

public void hello(String name) {  if (name == null) {    name = "World";  }  System.out.print("Hello, " + name + "!");}

Kotlin

fun hello(name: String = "World") {    println("Hello, $name!")}

Return
Java

public boolean hasItems() {  return true;}

Kotlin

fun hasItems() : Boolean {    return true}

Single-Expression
Java

public double cube(double x) {  return x * x * x;}

Kotlin

fun cube(x: Double) : Double = x * x * x

FUNCTIONS
Vararg
Java

public int sum(int... numbers) { }

Kotlin

fun sum(vararg x: Int) { }

Main
Java

public class MyClass {    public static void main(String[] args){    }}

Kotlin

fun main(args: Array<String>) {}

Named Arguments
Java

public static void main(String[]args){  openFile("file.txt", true);}public static File openFile(String filename, boolean readOnly) { }Kotlinfun main(args: Array<String>) {  openFile("file.txt", readOnly = true)}fun openFile(filename: String, readOnly: Boolean) : File { }

Optional Arguments
Java

public static void main(String[]args){  createFile("file.txt");  createFile("file.txt", true);  createFile("file.txt", true, false);  createExecutableFile("file.txt");}public static File createFile(String filename) { }public static File createFile(String filename, boolean appendDate) { }public static File createFile(String filename, boolean appendDate,                              boolean executable) { }public static File createExecutableFile(String filename) { }

Kotlin

fun main(args: Array<String>) {  createFile("file.txt")  createFile("file.txt", true)  createFile("file.txt", appendDate = true)  createFile("file.txt", true, false)  createFile("file.txt", appendDate = true, executable = true)  createFile("file.txt", executable = true)}fun createFile(filename: String, appendDate: Boolean = false,               executable: Boolean = false): File { }

Generic Methods
Java

public void init() {  List<String> moduleInferred = createList("net");}public <T> List<T> createList(T item) { }

Kotlin

fun init() {  val module = createList<String>("net")  val moduleInferred = createList("net")}fun <T> createList(item: T): List<T> { }

Data Classes - Destructuring
Java

public static void main(String[]args) {    Book book = createBook();    System.out.println(book);    System.out.println("Title: " + book.title);}public static Book createBook(){    return new Book("title_01", "author_01");}public class Book {    final private String title;    final private String author;    public Book(String title, String author) {        this.title = title;        this.author = author;    }    public String getTitle() {        return title;    }    public String getAuthor() {        return author;    }    @Override    public String toString() {        return "Title: " + title + " Author: " + author;    }}

Kotlin

fun main(args: Array<String>) {    val book = createBook();    // or    val (title, author) = createBook()    println(book)    println("Title: $title")}fun createBook() : Book{    return Book("title_01", "author_01")}data class Book(val title: String, val author: String)

CLASSES

Constructor Call
Java

final File file = new File("file.txt");

Kotlin

val file = File("file.txt")

Class
Java

public final class User {}

Kotlin

class User

Open Class
Java

public class User {}

Kotlin

open class User

Final Attributes

Java

final class User {     private final String name;     public User(String name) {         this.name = name;     }     public String getName() {         return name;     } }

Kotlin

class User(val name: String)

Primary Constructor
Java

final class User {     private String name;     public User(String name) {         this.name = name;     }     public String getName() {         return name;     }     public void setName(String name) {         this.name = name;     } }

Kotlin

class User(var name: String)

Optional Arguments in Constructors
Java

final class User {     private String name;     private String lastName;     public User(String name) {         this(name, "");     }     public User(String name, String lastName) {         this.name = name;         this.lastName = lastName;     }     // And Getters & Setters }

Kotlin

class User(var name: String, var lastName: String = "")

Properties
Java

public class Document {    private String id = "00x";     public String getId() {         return id;     }     public void setId(String id) {         if(id != null && !id.isEmpty()) {             this.id = id;         }     } }

Kotlin

class Document{    var id : String = "00x"        set(value) {            if(value.isNotEmpty())  field = value        }}

FUNCTIONS
Abstract Class
Java

public abstract class Document{   public abstract int calculateSize();}public class Photo extends Document{    @Override    public int calculateSize() {    }}

Kotlin

abstract class Document {    abstract fun calculateSize(): Int}class Photo : Document() {    override fun calculateSize(): Int {    }}

Singleton
Java

public class Document {   private static final Document INSTANCE = new Document();   public static Document getInstance(){       return INSTANCE;   } }

Kotlin

object Document {}

Extensions
Java

public class ByteArrayUtils {      public static String toHexString(byte[] data) {      }  }  final byte[] dummyData = new byte[10];  final String hexValue = ByteArrayUtils.toHexString(dummyData);

Kotlin

fun ByteArray.toHex() : String {}val dummyData = byteArrayOf()val hexValue = dummyData.toHex()

Inner Class
Java

public class Documment {    class InnerClass {    }}

Kotlin

class Document {    inner class InnerClass}

Nested Class
Java

public class Documment {    public static class InnerClass {    }}

Kotlin

class Document {    class InnerClass}

Interface
Java

public interface Printable {    void print();}public class Document implements Printable {    @Override    public void print() {    }}

Kotlin

interface Printable{    fun print()}class Document : Printable{    override fun print() {    }}