第10章 Kotlin与Java互操作 - 10.25 Kotlin与Java对比 - 《Kotlin极简教程》

Kotlin与Java对比

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. }