Kotlin简单语法

打印日志

• Java

System.out.print("打印");System.out.println("换行打印");

• Kotlin

print("打印")println("换行打印")

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常量与变量

• Java

String name = "变量";final String name = "常量";

• Kotlin

var name = "变量"val name = "常量"

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null声明

• Java

String otherName;otherName = null;

• Kotlin

var otherName : String?otherName = null

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空判断

• Java

if (text != null) {    int length = text.length();}

• Kotlin

text?.let {    val length = text.length}// 或者val length = text?.length

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字符串拼接

• Java

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

• Kotlin

val firstName = "firstName "val lastName = "lastName "val message = "My name is: $firstName $lastName"

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换行

• Java

String text = "First Line\n" +              "Second Line\n" +              "Third Line";

• Kotlin

val text = """        |First Line        |Second Line        |Third Line        """.trimMargin()

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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"

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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 bval orResult   = a or bval xorResult  = a xor bval rightShift = a shr 2val leftShift  = a shl 2

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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

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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 // smart casting}

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多重条件

• Java

if (score >= 0 && score <= 300) { }

• Kotlin

if (score in 0..300) { }

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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 scorevar grade = when (score) {    9, 10 -> "Excellent"    in 6..8 -> "Good"    4, 5 -> "OK"    in 1..3 -> "Fail"    else -> "Fail"}

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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) { }

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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, "Amit");map.put(2, "Ali");map.put(3, "Mindorks");// Java 9final 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")

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遍历

• Java

// Java 7 and belowfor (Car car : cars) {  System.out.println(car.speed);}// Java 8+cars.forEach(car -> System.out.println(car.speed));// Java 7 and belowfor (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)}

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方法定义

• Java

void doSomething() {}void doSomething(int... numbers) {}

• Kotlin

fun doSomething() {}fun doSomething(vararg numbers: Int) {}

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带返回值的方法

• Java

int getScore() {   return score;}

• Kotlin

fun getScore(): Int {   return score}fun getScore(): Int = score

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无结束符号

• Java

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

• Kotlin

fun getScore(value: Int): Int {   return 2 * value}// 作为一个单一表达式函数fun getScore(value: Int): Int = 2 * value

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constructor 构造器

• Java

public class Utils {    private Utils() {     }    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    }}

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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)

• Java

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

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