今天有个小伙伴来问小灰，说自己JAVA用的贼溜，想再学习下Kotlin，问小灰上手难不难？
怎么说呢？其实我一直觉得语言这方面，是触类旁通、一通百通的，既然JAVA已经贼溜了，想必学其他的语言也一定事半功倍。
不过呢，如果有好的辅助，肯定就更加容易学会啦，今天小灰就和大家写一篇对比展示Java和Kotlin两种语言在语法上不同地方的文章，可以快速帮助会JAVA的小伙伴掌握Kotlin的编写技巧！
废话不多说，说完这句我就滚，用最实际的代码来展示不同之处。
打印日志
- Java
System.out.print("i小灰");
System.out.println("i小灰");
- Kotlin
print("i小灰")
println("i小灰")
常量与变量
- Java
String name = "i小灰";
final String name = "i小灰";
- Kotlin
var name = "i小灰"
val name = "i小灰"
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
val length = text?.length
字符串拼接
- Java
String firstName = "小";
String lastName = "灰";
String message = "姓名: " + firstName + " " + lastName;
- Kotlin
val firstName = "小"
val lastName = "灰"
val message = "姓名: $firstName $lastName"
换行
- Java
String text = "一行\n" +
"二行\n" +
"三行";
- Kotlin
val text = """
|一行
|二行
|三行
""".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 // 智能的转换
}
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 = // 一些分数;
String grade;
switch (score) {
case 10:
case 9:
grade = "优秀";
break;
case 8:
case 7:
case 6:
grade = "好";
break;
case 5:
case 4:
grade = "良";
break;
case 3:
case 2:
case 1:
grade = "差";
break;
default:
grade = "差";
}
- Kotlin
var score = // 一些分数;
var grade = when (score) {
9, 10 -> "优秀"
in 6..8 -> "良"
4, 5 -> ""
in 1..3 -> "差"
else -> "差"
}
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 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 listOfNumber = Arrays.asList(1, 2, 3, 4);
final Map keyValue = new HashMap();
map.put(1, "i");
map.put(2, "小");
map.put(3, "灰");
// Java 9
final List listOfNumber = List.of(1, 2, 3, 4);
final Map keyValue = Map.of(1, "i",
2, "小",
3, "灰");
- Kotlin
val listOfNumber = listOf(1, 2, 3, 4)
val keyValue = mapOf(1 to "i",
2 to "小",
3 to "灰")
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() {
// 逻辑...
}
void doSomething(int... numbers) {
// 逻辑...
}
- Kotlin
fun doSomething() {
// 逻辑...
}
fun doSomething(vararg numbers: Int) {
// 逻辑...
}
带返回值的方法
- Java
int getScore() {
// 逻辑...
return score;
}
- Kotlin
fun getScore(): Int {
// 逻辑...
return score
}
// 作为单表达式函数
fun getScore(): Int = score
无结束符号
- Java
int getScore(int value) {
// 逻辑...
return 2 * value;
}
- Kotlin
fun getScore(value: Int): Int {
// 逻辑...
return 2 * value
}
// 作为单表达式函数
fun getScore(value: Int): Int = 2 * value
constructor 构造器
- Java
public class Utils {
private Utils() {
// 此类不可公开实例化
}
public static int getScore(int value) {
return 2 * value;
}
}
- Kotlin
class Utils private constructor() {
companion object {
fun getScore(value: Int): Int {
return 2 * value
}
}
}
// 另一种方式
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() {
// 此类不可公开实例化
}
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);
}