Kotlin与Java的异同点有哪些

这篇文章主要介绍“Kotlin与Java的异同点有哪些”，在日常操作中，相信很多人在Kotlin与Java的异同点有哪些问题上存在疑惑，小编查阅了各式资料，整理出简单好用的操作方法，希望对大家解答”Kotlin与Java的异同点有哪些”的疑惑有所帮助！接下来，请跟着小编一起来学习吧！

打印日志

Java

System.out.print("hello world");
System.out.println("hello world");

Kotlin

print("hello world")
println("hello world")

定义变量与常量常

Java

String name = 
"hello world";
final String name = 
"hello world";

Kotlin

var name = 
"hello world"
val name = 
"hello world"

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
}
// or simply
val length = text?.length

字符串拼接

Java

String firstName = 
"Android";
String lastName = 
"Architect";
String message = 
"My name is: " + firstName + 
" " + 
lastName;

Kotlin

val firstName = 
"Android"
val lastName = 
"Architect"
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

Car car = (Car) 
object;

Kotlin

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 // 自动识别
}

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 = 
// some score;
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 = 
// some score
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) { }

更方便的集合操作

Java

final List<Integer> listOfNumber = Arrays.asList(1, 
2, 
3, 
4);

final Map<Integer, String> keyValue = 
new HashMap<Integer, String>();
map.put(1, 
"Android");
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, "Android",
                                             2, "Ali",
                                             3, "Mindorks");

Kotlin

val listOfNumber = listOf(1, 
2, 
3, 
4)
val keyValue = mapOf(1 to 
"Android",
                     
2 to 
"Ali",
                     
3 to 
"Mindorks")

遍历

Java

// Java 7 and below
for (Car car : cars) {
  System.out.println(car.speed);
}

// Java 8+
cars.forEach(car -> System.out.println(car.speed));

// Java 7 and below
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

constructor 构造器

Java

public class Utils {

    private Utils() {
      // This utility class is not publicly instantiable
    }

    public static int getScore(int value) {
        return 2 * value;
    }

}

Kotlin

class Utils private constructor() {

    companion object {

        fun getScore(value: 
Int): 
Int {
            return 2 * value
        }

    }
}

// another way

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;
    }
}

Kotlin

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

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