列举一些 Java 和 Kotlin 语法上差别，让有 Java 基础的程序员可以快速上手 Kotlin。

打印日志Java

System.out.print("Amit Shekhar");System.out.println("Amit Shekhar");

Kotlin

print("Amit Shekhar")println("Amit Shekhar")

常量与变量Java

String name = "Amit Shekhar";final String name = "Amit Shekhar";

Kotlin

var name = "Amit Shekhar"val name = "Amit Shekhar"

声明nullJava

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}// 简化写法val length = text?.length

字符串拼接Java

String firstName = "Amit";String lastName = "Shekhar";String message = "My name is: " + firstName + " " + lastName;

Kotlin

val firstName = "Amit"val lastName = "Shekhar"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

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

Kotlin

if (object is Car) {}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 // 聪明的转换}

多重条件Java

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

Kotlin

if (score in 0..300) { }

case语句Java

int score = 0;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 = 0var 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) { }

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, "Amit");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, "Amit",                                             2, "Ali",                                             3, "Mindorks");

Kotlin

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

遍历Java

// Java 7 及以下for (Car car : cars) {  System.out.println(car.speed);}// Java 8+cars.forEach(car -> System.out.println(car.speed));// Java 7 及以下for (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

构造器Java

public class Utils {    private Utils() {       // 私有构造，不能new     }        public static int getScore(int value) {        return 2 * value;    }    }

Kotlin

class Utils private constructor() {    companion object {        fun getScore(value: Int): Int {            return 2 * value        }    }}// 方法二object Utils {    fun getScore(value: Int): Int {        return 2 * value    }}

Getter/SetterJava

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;    }    @Override    public boolean equals(Object o) {        if (this == o) return true;        if (o == null || getClass() != o.getClass()) return false;        Developer developer = (Developer) o;        if (age != developer.age) return false;        return name != null ? name.equals(developer.name) : developer.name == null;    }    @Override    public int hashCode() {        int result = name != null ? name.hashCode() : 0;        result = 31 * result + age;        return result;    }    @Override    public String toString() {        return "Developer{" +                "name='" + name + '\'' +                ", age=" + age +                '}';    }}

Kotlin

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