【kotlin】Java和kotlin语法对比学习

986945193@qq.com (David) — Thu, 02 Jun 2022 10:03:48 +0800

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<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, "Amit");
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, "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;
 }
 }

Kotlin

enum class Direction(val direction: Int) {
 NORTH(1),
 SOUTH(2),
 WEST(3),
 EAST(4);
}

你需要知道一些比较重要的事

参考地址：

https://github.com/MindorksOpenSource/from-java-to-kotlin

Kotlin on 计算机知识分享网

【kotlin】Java和kotlin语法对比学习

打印日志

常量与变量

null声明

空判断

字符串拼接

换行

三元表达式

操作符

类型判断和转换 (声明式)

类型判断和转换 (隐式)

多重条件

更灵活的case语句

for循环

更方便的集合操作

遍历

方法定义

带返回值的方法

无结束符号

constructor 构造器

Get Set 构造器

原型扩展

你需要知道一些比较重要的事

参考地址：