列举一些 Java 和 Kotlin 语法上差别，让有 Java 基础的程序员可以快速上手 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"声明null
- 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
}
// 简化写法
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) { }case语句
- Java
int score = 0;
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 = 0
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) { }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 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() {
// 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构造器
- Java
public class Utils {
private Utils() {
// 私有构造，不能new
}
public static int getScore(int value) {
return 2 * value;
}
}- Kotlin
class Utils private constructor() {
companion object {
fun getScore(value: Int): Int {
return 2 * value
}
}
}
// 方法二
object Utils {
fun getScore(value: Int): Int {
return 2 * value
}
}Getter/Setter
- 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)