Kotlin与Java的异同，看完你就明白了

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

[code]System.out.print("hello world");
System.out.println("hello world");

Kotlin

[code]print("hello world")
println("hello world")

定义变量与常量常

Java

[code]String name = "hello world";
final String name = "hello world";

Kotlin

[code]var name = "hello world"
val name = "hello world"

null声明

Java

[code]String otherName;
otherName = null;

Kotlin

[code]var otherName : String?
otherName = null

空判断

Java

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

Kotlin

[code]text?.let {
val length = text.length
}
// or simply
val length = text?.length

字符串拼接

Java

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

Kotlin

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

换行

Java

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

Kotlin

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

三元表达式

Java

[code]String text = x > 5 ? "x > 5" : "x <= 5";

Kotlin

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

操作符

java

[code]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

[code]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

[code]Car car = (Car) object;

Kotlin

[code]var car = object as Car

类型判断和转换 (隐式)

Java

[code]if (object instanceof Car) {
Car car = (Car) object;
}

Kotlin

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

 

多重条件

Java

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

Kotlin

[code]if (score in 0..300) { }

更灵活的case语句

Java

[code]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

[code]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

[code]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

[code]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

[code]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 9
final List<Integer> listOfNumber = List.of(1, 2, 3, 4);

final Map<Integer, String> keyValue = Map.of(1, "Android",
2, "Ali",
3, "Mindorks");

Kotlin

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

遍历

Java

[code]// 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

[code]cars.forEach {
println(it.speed)
}

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

方法定义

Java

[code]void doSomething() {
// logic here
}

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

Kotlin

[code]fun doSomething() {
// logic here
}

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

带返回值的方法

Java

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

Kotlin

[code]fun getScore(): Int {
// logic here
return score
}

// as a single-expression function

fun getScore(): Int = score

无结束符号

Java

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

Kotlin

[code]fun getScore(value: Int): Int {
// logic here
return 2 * value
}

// as a single-expression function

fun getScore(value: Int): Int = 2 * value

constructor 构造器

Java

[code]public class Utils {

private Utils() {
// This utility class is not publicly instantiable
}

public static int getScore(int value) {
return 2 * value;
}

}

Kotlin

[code]class Utils private constructor() {

companion object {

fun getScore(value: Int): Int {
return 2 * value
}

}
}

// another way

object Utils {

fun getScore(value: Int): Int {
return 2 * value
}

}

Get Set 构造器

Java

[code]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

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