Java VS Kotlin：基础语法比较

向右看齐 2022-11-30 01:24 182阅读 0赞

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3ZpdGF2aXZh_size_16_color_FFFFFF_t_70_pic_center]

> 转载自 [https://github.com/MindorksOpenSource/from-java-to-kotlin][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<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);
    }

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

[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3ZpdGF2aXZh_size_16_color_FFFFFF_t_70_pic_center]: /images/20221124/6aea3b269b344b8fb8bbeef9c20b86ec.png
[https_github.com_MindorksOpenSource_from-java-to-kotlin]: https://github.com/MindorksOpenSource/from-java-to-kotlin