Java VS Kotlin：基础语法比较-蒲公英云

在这里插入图片描述

转载自 https://github.com/MindorksOpenSource/from-java-to-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”

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 = “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 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

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) { }

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 = new HashMap();
map.put(1, “Amit”);
map.put(2, “Ali”);
map.put(3, “Mindorks”);

// Java 9
final List listOfNumber = List.of(1, 2, 3, 4);

final Map 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 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 function

fun 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 function

fun 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 way

object 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;
}
@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)

原型扩展

Java

public class Utils {

private Utils() {  
  // This utility class is not publicly instantiable 
}
public static int triple(int value) { 
    return 3 * value;
}

}

int result = Utils.triple(3);

Kotlin

fun Int.triple(): Int {
return this * 3
}

var result = 3.triple()

Java

public enum Direction {

    NORTH(1),
    SOUTH(2),
    WEST(3),
    EAST(4);
    int direction;
    Direction(int direction) { 
        this.direction = direction;
    }
    public int getDirection() { 
        return direction;
    }
}