本期教程给大家分享Kotlin与Java上的主要异同，希望大家利用现有的Java知识能快速上手Kotlin开发。

打印日志

Java

System.out.print("hello world");System.out.println("hello world");

Kotlin

print("hello world")println("hello world")

定义变量与常量常

Java

String name = "hello world";final String name = "hello world";

Kotlin

var name = "hello world"val name = "hello world"

null声明

Java

String otherName;otherName = null;

Kotlin

var otherName : String?otherName = null

空判断

Java

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

Kotlin

text?.let {    val length = text.length}// or simplyval length = text?.length

字符串拼接

Java

String firstName = "Android";String lastName = "Architect";String message = "My name is: " + firstName + " " + lastName;

Kotlin

val firstName = "Android"val lastName = "Architect"val message = "My name is: $firstName $lastName"

换行

Java

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

Kotlin

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

三元表达式

Java

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

Kotlin

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

操作符

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;final int unsignedRightShift = a >>> 2;

Kotlin

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

类型判断和转换 (声明式)

Java

Car car = (Car) object;

Kotlin

var car = object as Car

类型判断和转换 (隐式)

Java

if (object instanceof Car) {   Car car = (Car) object;}

Kotlin

if (object is Car) {   var car = object // 自动识别}

多重条件

Java

if (score >= 0 && score <= 300) { }

Kotlin

if (score in 0..300) { }

更灵活的case语句

Java

int score = // some score;String grade;switch (score) {    case 10:    case 9:        grade = "Excellent";        break;    case 8:    case 7:    case 6:        grade = "Good";        break;    case 5:    case 4:        grade = "OK";        break;    case 3:    case 2:    case 1:        grade = "Fail";        break;    default:        grade = "Fail";}

Kotlin

var score = // some scorevar grade = when (score) {    9, 10 -> "Excellent"    in 6..8 -> "Good"    4, 5 -> "OK"    in 1..3 -> "Fail"    else -> "Fail"}

for循环

Java

for (int i = 1; i <= 10 ; i++) { }for (int i = 1; i < 10 ; i++) { }for (int i = 10; i >= 0 ; i--) { }for (int i = 1; i <= 10 ; i+=2) { }for (int i = 10; i >= 0 ; i-=2) { }for (String item : collection) { }for (Map.Entry<String, String> entry: map.entrySet()) { }

Kotlin

for (i in 1..10) { }for (i in 1 until 10) { }for (i in 10 downTo 0) { }for (i in 1..10 step 2) { }for (i in 10 downTo 0 step 2) { }for (item in collection) { }for ((key, value) in map) { }

更方便的集合操作

Java

final List<Integer> listOfNumber = Arrays.asList(1, 2, 3, 4);final Map<Integer, String> keyValue = new HashMap<Integer, String>();map.put(1, "Android");map.put(2, "Ali");map.put(3, "Mindorks");// Java 9final List<Integer> listOfNumber = List.of(1, 2, 3, 4);final Map<Integer, String> keyValue = Map.of(1, "Android",                                             2, "Ali",                                             3, "Mindorks");

Kotlin

val listOfNumber = listOf(1, 2, 3, 4)val keyValue = mapOf(1 to "Android",                     2 to "Ali",                     3 to "Mindorks")

遍历

Java

// Java 7 and belowfor (Car car : cars) {  System.out.println(car.speed);}// Java 8+cars.forEach(car -> System.out.println(car.speed));// Java 7 and belowfor (Car car : cars) {  if (car.speed > 100) {    System.out.println(car.speed);  }}// Java 8+cars.stream().filter(car -> car.speed > 100).forEach(car -> System.out.println(car.speed));

Kotlin

cars.forEach {    println(it.speed)}cars.filter { it.speed > 100 }      .forEach { println(it.speed)}

方法定义

Java

void doSomething() {   // logic here}void doSomething(int... numbers) {   // logic here}

Kotlin

fun doSomething() {   // logic here}fun doSomething(vararg numbers: Int) {   // logic here}

带返回值的方法

Java

int getScore() {   // logic here   return score;}

Kotlin

fun getScore(): Int {   // logic here   return score}// as a single-expression functionfun getScore(): Int = score

无结束符号

Java

int getScore(int value) {    // logic here    return 2 * value;}

Kotlin

fun getScore(value: Int): Int {   // logic here   return 2 * value}// as a single-expression functionfun getScore(value: Int): Int = 2 * value

constructor 构造器

Java

public class Utils {    private Utils() {      // This utility class is not publicly instantiable    }    public static int getScore(int value) {        return 2 * value;    }}

Kotlin

class Utils private constructor() {    companion object {        fun getScore(value: Int): Int {            return 2 * value        }    }}// another wayobject Utils {    fun getScore(value: Int): Int {        return 2 * value    }}

Get Set 构造器

Java

public class Developer {    private String name;    private int age;    public Developer(String name, int age) {        this.name = name;        this.age = age;    }    public String getName() {        return name;    }    public void setName(String name) {        this.name = name;    }    public int getAge() {        return age;    }    public void setAge(int age) {        this.age = age;    }}

Kotlin

data class Developer(val name: String, val age: Int)