从 Java 开始进入 Kotlin的世界： From Java to Kotlin: JKotlinor
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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
System.out.print("Java");
System.out.println("Java");
- Kotlin
print("Kotlin")
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。
@kotlin.internal.InlineOnly
public inline fun print(message: Any?) {
System.out.print(message)
}
常量与变量
- Java
String name = "KotlinVSJava";
final String name = "KotlinVSJava";
- Kotlin
var name = "KotlinVSJava"
val name = "KotlinVSJava"
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
}
// 或者
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
String firstName = "Jack";
String lastName = "Chen";
String message = "My name is: " + firstName + " " + lastName;
- Kotlin
val firstName = "Jack"
val lastName = "Chen"
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
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;
- 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
类型判断和转换（显式）
- 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 // Kotlin智能转换
}
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
if (score >= 0 && score <= 300) { }
- Kotlin
if (score in 0..300) { }
更灵活的case语句
- Java
public String getGrade(int score) {
String grade;
switch (score) {
case 10:
case 9:
grade = "A";
break;
case 8:
case 7:
case 6:
grade = "B";
break;
case 5:
case 4:
grade = "C";
break;
case 3:
case 2:
case 1:
grade = "D";
break;
default:
grade = "E";
}
return grade;
}
- Kotlin
fun getGrade(score: Int): String {
var grade = when (score) {
9, 10 -> "A"
in 6..8 -> "B"
4, 5 -> "C"
in 1..3 -> "D"
else -> "E"
}
return grade
}
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 1 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> map = new HashMap<Integer, String>();
map.put(1, "Jack");
map.put(2, "Ali");
map.put(3, "Mindorks");
- Kotlin
val listOfNumber = listOf(1, 2, 3, 4)
val map = mapOf(1 to "Jack", 2 to "Ali", 3 to "Mindorks")
遍历
- Java
// Java 7
for (Car car : cars) {
System.out.println(car.speed);
}
// Java 8+
cars.forEach(car -> System.out.println(car.speed));
// Java 7
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(函数)定义
- Java
void doSomething() {
// 实现
}
void doSomething(int... numbers) {
// 实现
}
- Kotlin
fun doSomething() {
// 实现
}
fun doSomething(vararg numbers: Int) {
// 实现
}
带返回值的方法（函数）
- Java
int getScore() {
// logic here
return score;
}
- Kotlin
fun getScore(): Int {
// logic here
return score
}
// 单表达式函数
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
public class Utils {
private Utils() {
// 外部无法来调用实例化
}
public static int getScore(int value) {
return 2 * value;
}
}
- Kotlin
class Utils private constructor() {
companion object {
fun getScore(value: Int): Int {
return 2 * value
}
}
}
// 或者直接声明一个object对象
object Utils {
fun getScore(value: Int): Int {
return 2 * value
}
}
JavaBean与Kotlin数据类
这段Kotlin中的数据类的代码：
data class Developer(val name: String, val age: Int)
对应下面这段Java实体类的代码：
- Java
public final class Developer {
@NotNull
private final String name;
private final int age;
@NotNull
public final String getName() {
return this.name;
}
public final int getAge() {
return this.age;
}
public Developer(@NotNull String name, int age) {
Intrinsics.checkParameterIsNotNull(name, "name");
super();
this.name = name;
this.age = age;
}
@NotNull
public final String component1() {
return this.name;
}
public final int component2() {
return this.age;
}
@NotNull
public final Developer copy(@NotNull String name, int age) {
Intrinsics.checkParameterIsNotNull(name, "name");
return new Developer(name, age);
}
// $FF: synthetic method
// $FF: bridge method
@NotNull
public static Developer copy$default(Developer var0, String var1, int var2, int var3, Object var4) {
if((var3 & 1) != 0) {
var1 = var0.name;
}
if((var3 & 2) != 0) {
var2 = var0.age;
}
return var0.copy(var1, var2);
}
public String toString() {
return "Developer(name=" + this.name + ", age=" + this.age + ")";
}
public int hashCode() {
return (this.name != null?this.name.hashCode():0) * 31 + this.age;
}
public boolean equals(Object var1) {
if(this != var1) {
if(var1 instanceof Developer) {
Developer var2 = (Developer)var1;
if(Intrinsics.areEqual(this.name, var2.name) && this.age == var2.age) {
return true;
}
}
return false;
} else {
return true;
}
}
}
