帮助Java开发者快速学会Kotlin

ゝ一世哀愁。 2023-10-14 20:53 27阅读 0赞

其实我一直觉得语言这方面，是触类旁通、一通百通的，既然JAVA已经贼溜了，想必学其他的语言也一定事半功倍。

不过呢，如果有好的辅助，肯定就更加容易学会啦，今天小灰就和大家写一篇对比展示Java和Kotlin两种语言在语法上不同地方的文章，可以快速帮助会JAVA的小伙伴掌握Kotlin的编写技巧！  
废话不多说，说完这句我就滚，用最实际的代码来展示不同之处。

![Image 1][]

### 打印日志 ###

*  Java

System.out.print("i小灰");
    System.out.println("i小灰");

*  Kotlin

print("i小灰")
    println("i小灰")

--------------------

### 常量与变量 ###

*  Java

String name = "i小灰";
    final String name = "i小灰";

*  Kotlin

var name = "i小灰"
    val name = "i小灰"

--------------------

### 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
    }
    // 或
    val length = text?.length

--------------------

### 字符串拼接 ###

*  Java

String firstName = "小";
    String lastName = "灰";
    String message = "姓名: " + firstName + " " + lastName;

*  Kotlin

val firstName = "小"
    val lastName = "灰"
    val message = "姓名: $firstName $lastName"

--------------------

### 换行 ###

*  Java

String text = "一行\n" +
                  "二行\n" +
                  "三行";

*  Kotlin

val text = """
            |一行
            |二行
            |三行
            """.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 // 智能的转换
    }

--------------------

### 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 = // 一些分数;
    String grade;
    switch (score) {
        case 10:
        case 9:
            grade = "优秀";
            break;
        case 8:
        case 7:
        case 6:
            grade = "好";
            break;
        case 5:
        case 4:
            grade = "良";
            break;
        case 3:
        case 2:
        case 1:
            grade = "差";
            break;
        default:
            grade = "差";
    }

*  Kotlin

var score = // 一些分数;
    var grade = when (score) {
        9, 10 -> "优秀"
        in 6..8 -> "良"
        4, 5 -> ""
        in 1..3 -> "差"
        else -> "差"
    }

--------------------

### 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, "i");
    map.put(2, "小");
    map.put(3, "灰");
    
    // Java 9
    final List<Integer> listOfNumber = List.of(1, 2, 3, 4);
    
    final Map<Integer, String> keyValue = Map.of(1, "i",
                                                 2, "小",
                                                 3, "灰");

*  Kotlin

val listOfNumber = listOf(1, 2, 3, 4)
    val keyValue = mapOf(1 to "i",
                         2 to "小",
                         3 to "灰")

--------------------

### 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() {
       // 逻辑...
    }
    
    void doSomething(int... numbers) {
       // 逻辑...
    }

*  Kotlin

fun doSomething() {
       // 逻辑...
    }
    
    fun doSomething(vararg numbers: Int) {
       // 逻辑...
    }

--------------------

### 带返回值的方法 ###

*  Java

int getScore() {
       // 逻辑...
       return score;
    }

*  Kotlin

fun getScore(): Int {
       // 逻辑...
       return score
    }
    
    // 作为单表达式函数
    
    fun getScore(): Int = score

--------------------

### 无结束符号 ###

*  Java

int getScore(int value) {
       // 逻辑...
        return 2 * value;
    }

*  Kotlin

fun getScore(value: Int): Int {
       // 逻辑...
       return 2 * value
    }
    
    // 作为单表达式函数
    
    fun getScore(value: Int): Int = 2 * value

--------------------

### constructor 构造器 ###

*  Java

public class Utils {
    
        private Utils() { 
          // 此类不可公开实例化
        }
        
        public static int getScore(int value) {
            return 2 * value;
        }
        
    }

*  Kotlin

class Utils private constructor() {
    
        companion object {
        
            fun getScore(value: Int): Int {
                return 2 * value
            }
            
        }
    }
    
    // 另一种方式
    
    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() { 
          // 此类不可公开实例化
        }
        
        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);
    }

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