Android|还不知道Kotlin与Java的异同（看完这篇彻底明白了） java|kotlin|android|kotlin

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在这篇文章中呢，我将为大家分享Kotlin与Java上的主要异同，希望能够帮助到各位小伙伴利用现有的Java知识快速上手Kotlin开发。
相关视频资料：
Kotlin协程从实战到原理
Koltin语言核心点总结和实践
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 simply val 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 b val orResult= a or b val xorResult= a xor b val rightShift = a shr 2 val leftShift= a shl 2 val 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 score var 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 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 listOfNumber = Arrays.asList(1, 2, 3, 4); final Map keyValue = https://www.it610.com/article/new HashMap(); map.put(1, "Android"); map.put(2, "Ali"); map.put(3, "Mindorks"); // Java 9 final List listOfNumber = List.of(1, 2, 3, 4); final Map keyValue = https://www.it610.com/article/Map.of(1,"Android", 2, "Ali", 3, "Mindorks");

Kotlin

val listOfNumber = listOf(1, 2, 3, 4) val keyValue = https://www.it610.com/article/mapOf(1 to"Android", 2 to "Ali", 3 to "Mindorks")

遍历

Java

// Java 7 and below for (Car car : cars) { System.out.println(car.speed); }// Java 8+ cars.forEach(car -> System.out.println(car.speed)); // Java 7 and below 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; } >Kotlinfun getScore(value: Int): Int { // logic here return 2 * value }// as a single-expression functionfun getScore(value: Int): Int = 2 * value constructor 构造器 >Javapublic 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; } }