Kotlin与Java对比

在前面的内容里，我们已经看到了Java与Kotlin的互操作的基本方式。为了更好的认识Java与Kotlin这两门语言，我们在这里给出一些基本功能，同时使用Java与Kotlin来实现的代码实例。通过横向对比，从中我们可以看出它们的异同。

（此处可整理成表格形式）

打印日志

  • Java
  1. System.out.print("Java");
  2. System.out.println("Java");
  • Kotlin
  1. print("Kotlin")
  2. println("Kotlin")

其实，Kotlin中的println函数是一个内联函数，它其实就是通过封装java.lang.System类的System.out.println来实现的。

  1. @kotlin.internal.InlineOnly
  2. public inline fun print(message: Any?) {
  3.     System.out.print(message)
  4. }

常量与变量

  • Java
  1. String name = "KotlinVSJava";
  2. final String name = "KotlinVSJava";
  • Kotlin
  1. var name = "KotlinVSJava"
  2. val name = "KotlinVSJava"

null声明

  • Java
  1. String otherName;
  2. otherName = null;
  • Kotlin
  1. var otherName : String?
  2. otherName = null

空判断

  • Java
  1. if (text != null) {
  2.     int length = text.length();
  3. }
  • Kotlin
  1. text?.let {
  2.     val length = text.length
  3. }
  4. // 或者
  5. val length = text?.length

在Kotlin中，我们只使用一个问号安全调用符号就省去了Java中烦人的if - null 判断。

字符串拼接

  • Java
  1. String firstName = "Jack";
  2. String lastName = "Chen";
  3. String message = "My name is: " + firstName + " " + lastName;
  • Kotlin
  1. val firstName = "Jack"
  2. val lastName = "Chen"
  3. val message = "My name is: $firstName $lastName"

Kotlin中使用$和${}（花括号里面是表达式的时候）占位符来实现字符串的拼接，这个比在Java中每次使用加号来拼接要方便许多。

换行

  • Java
  1. String text = "First Line\n" +
  2.               "Second Line\n" +
  3.               "Third Line";
  • Kotlin
  1. val text = """
  2.         |First Line
  3.         |Second Line
  4.         |Third Line
  5.         """.trimMargin()

三元表达式

  • Java
  1. String text = x > 5 ? "x > 5" : "x <= 5";
  • Kotlin
  1. val text = if (x > 5)
  2.               "x > 5"
  3.            else "x <= 5"

操作符

  • java
  1. final int andResult  = a & b;
  2. final int orResult   = a | b;
  3. final int xorResult  = a ^ b;
  4. final int rightShift = a >> 2;
  5. final int leftShift  = a << 2;
  • Kotlin
  1. val andResult  = a and b
  2. val orResult   = a or b
  3. val xorResult  = a xor b
  4. val rightShift = a shr 2
  5. val leftShift  = a shl 2

类型判断和转换（显式）

  • Java
  1. if (object instanceof Car) {
  2. }
  3. Car car = (Car) object;
  • Kotlin
  1. if (object is Car) {
  2. }
  3. var car = object as Car

类型判断和转换 (隐式)

  • Java
  1. if (object instanceof Car) {
  2.    Car car = (Car) object;
  3. }
  • Kotlin
  1. if (object is Car) {
  2.    var car = object // Kotlin智能转换
  3. }

Kotlin的类型系统具备一定的类型推断能力，这样也省去了不少在Java中类型转换的样板式代码。

Range区间

  • Java
  1. if (score >= 0 && score <= 300) { }
  • Kotlin
  1. if (score in 0..300) { }

更灵活的case语句

  • Java
  1. public String getGrade(int score) {
  2.     String grade;
  3.     switch (score) {
  4.         case 10:
  5.         case 9:
  6.             grade = "A";
  7.             break;
  8.         case 8:
  9.         case 7:
  10.         case 6:
  11.             grade = "B";
  12.             break;
  13.         case 5:
  14.         case 4:
  15.             grade = "C";
  16.             break;
  17.         case 3:
  18.         case 2:
  19.         case 1:
  20.             grade = "D";
  21.             break;
  22.         default:
  23.             grade = "E";
  24.     }
  25.     return grade;
  26. }
  • Kotlin
  1. fun getGrade(score: Int): String {
  2.     var grade = when (score) {
  3.         9, 10 -> "A"
  4.         in 6..8 -> "B"
  5.         4, 5 -> "C"
  6.         in 1..3 -> "D"
  7.         else -> "E"
  8.     }
  9.     return grade
  10. }

for循环

  • Java
  1. for (int i = 1; i <= 10 ; i++) { }
  3. for (int i = 1; i < 10 ; i++) { }
  5. for (int i = 10; i >= 0 ; i--) { }
  7. for (int i = 1; i <= 10 ; i+=2) { }
  9. for (int i = 10; i >= 0 ; i-=2) { }
  11. for (String item : collection) { }
  13. for (Map.Entry<String, String> entry: map.entrySet()) { }
  • Kotlin
  1. for (i in 1..10) { }
  3. for (i in 1 until 10) { }
  5. for (i in 10 downTo 0) { }
  7. for (i in 1..10 step 2) { }
  9. for (i in 10 downTo 1 step 2) { }
  11. for (item in collection) { }
  13. for ((key, value) in map) { }

更方便的集合操作

  • Java
  1. final List<Integer> listOfNumber = Arrays.asList(1, 2, 3, 4);
  2. final Map<Integer, String> map = new HashMap<Integer, String>();
  3. map.put(1, "Jack");
  4. map.put(2, "Ali");
  5. map.put(3, "Mindorks");
  • Kotlin
  1. val listOfNumber = listOf(1, 2, 3, 4)
  2. val map = mapOf(1 to "Jack", 2 to "Ali", 3 to "Mindorks")

遍历

  • Java
  1. // Java 7 
  2. for (Car car : cars) {
  3.   System.out.println(car.speed);
  4. }
  6. // Java 8+
  7. cars.forEach(car -> System.out.println(car.speed));
  9. // Java 7 
  10. for (Car car : cars) {
  11.   if (car.speed > 100) {
  12.     System.out.println(car.speed);
  13.   }
  14. }
  16. // Java 8+
  17. cars.stream().filter(car -> car.speed > 100).forEach(car -> System.out.println(car.speed));
  • Kotlin
  1. cars.forEach {
  2.     println(it.speed)
  3. }
  5. cars.filter { it.speed > 100 }
  6.       .forEach { println(it.speed)}

方法(函数)定义

  • Java
  1. void doSomething() {
  2.    // 实现
  3. }
  5. void doSomething(int... numbers) {
  6.    // 实现
  7. }
  • Kotlin
  1. fun doSomething() {
  2.    // 实现
  3. }
  5. fun doSomething(vararg numbers: Int) {
  6.    // 实现
  7. }

带返回值的方法（函数）

  • Java
  1. int getScore() {
  2.    // logic here
  3.    return score;
  4. }
  • Kotlin
  1. fun getScore(): Int {
  2.    // logic here
  3.    return score
  4. }
  6. // 单表达式函数
  7. fun getScore(): Int = score

另外，Kotlin中的函数是可以直接传入函数参数，同时可以返回一个函数类型的。

constructor 构造器

  • Java
  1. public class Utils {
  3.     private Utils() { 
  4.       // 外部无法来调用实例化
  5.     }
  7.     public static int getScore(int value) {
  8.         return 2 * value;
  9.     }
  11. }
  • Kotlin
  1. class Utils private constructor() {
  3.     companion object {
  5.         fun getScore(value: Int): Int {
  6.             return 2 * value
  7.         }
  9.     }
  10. }
  12. // 或者直接声明一个object对象
  13. object Utils {
  15.     fun getScore(value: Int): Int {
  16.         return 2 * value
  17.     }
  19. }

JavaBean与Kotlin数据类

这段Kotlin中的数据类的代码：

  1. data class Developer(val name: String, val age: Int)

对应下面这段Java实体类的代码：

  • Java
  1. public final class Developer {
  2.    @NotNull
  3.    private final String name;
  4.    private final int age;
  6.    @NotNull
  7.    public final String getName() {
  8.       return this.name;
  9.    }
  11.    public final int getAge() {
  12.       return this.age;
  13.    }
  15.    public Developer(@NotNull String name, int age) {
  16.       Intrinsics.checkParameterIsNotNull(name, "name");
  17.       super();
  18.       this.name = name;
  19.       this.age = age;
  20.    }
  22.    @NotNull
  23.    public final String component1() {
  24.       return this.name;
  25.    }
  27.    public final int component2() {
  28.       return this.age;
  29.    }
  31.    @NotNull
  32.    public final Developer copy(@NotNull String name, int age) {
  33.       Intrinsics.checkParameterIsNotNull(name, "name");
  34.       return new Developer(name, age);
  35.    }
  37.    // $FF: synthetic method
  38.    // $FF: bridge method
  39.    @NotNull
  40.    public static Developer copy$default(Developer var0, String var1, int var2, int var3, Object var4) {
  41.       if((var3 & 1) != 0) {
  42.          var1 = var0.name;
  43.       }
  45.       if((var3 & 2) != 0) {
  46.          var2 = var0.age;
  47.       }
  49.       return var0.copy(var1, var2);
  50.    }
  52.    public String toString() {
  53.       return "Developer(name=" + this.name + ", age=" + this.age + ")";
  54.    }
  56.    public int hashCode() {
  57.       return (this.name != null?this.name.hashCode():0) * 31 + this.age;
  58.    }
  60.    public boolean equals(Object var1) {
  61.       if(this != var1) {
  62.          if(var1 instanceof Developer) {
  63.             Developer var2 = (Developer)var1;
  64.             if(Intrinsics.areEqual(this.name, var2.name) && this.age == var2.age) {
  65.                return true;
  66.             }
  67.          }
  69.          return false;
  70.       } else {
  71.          return true;
  72.       }
  73.    }
  74. }