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from-java-to-kotlin

发表于 更新于 分类于

根据 from-java-to-kotlin 的学习笔记

null声明

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String otherName;
otherName = null;
  • Kotlin
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var otherName : String?
otherName = null

空判断

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if (text != null) {
    int length = text.length();
}
  • Kotlin
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text?.let {
    val length = text.length
}
// or simply
val length = text?.length

数组初始化

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byte[] data = new byte[200];
int[] int_data = new int[100];
String[] data_str = new String[100];
  • Kotlin
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val data = ByteArray(200)
val int_data = IntArray(100)
val data_str = arrayOfNulls<String>(100)

字符串拼接

  • Java
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String firstName = "Amit";
String lastName = "Shekhar";
String message = "My name is: " + firstName + " " + lastName;
  • Kotlin
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val firstName = "Amit"
val lastName = "Shekhar"
val message = "My name is: $firstName $lastName"

换行

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String text = "First Line\n" +
              "Second Line\n" +
              "Third Line";
  • Kotlin
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val text = """
        |First Line
        |Second Line
        |Third Line
        """.trimMargin()

三元表达式

  • Java
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String text = x > 5 ? "x > 5" : "x <= 5";
  • Kotlin
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val text = if (x > 5)
              "x > 5"
           else "x <= 5"

操作符

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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
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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)

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if (object instanceof Car) {
}
Car car = (Car) object;
  • Kotlin
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if (object is Car) {
}
var car = object as Car

类型判断和转换 (隐式)

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if (object instanceof Car) {
   Car car = (Car) object;
}
  • Kotlin
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if (object is Car) {
   var car = object // 聪明的转换
}

多重条件

  • Java
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if (score >= 0 && score <= 300) { }
  • Kotlin
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if (score in 0..300) { }

更灵活的case语句

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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
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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

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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
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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

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

遍历

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// 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
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cars.forEach {
    println(it.speed)
}

cars.filter { it.speed > 100 }
      .forEach { println(it.speed)}

方法定义

  • Java
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void doSomething() {
   // logic here
}

void doSomething(int... numbers) {
   // logic here
}
  • Kotlin
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fun doSomething() {
   // logic here
}

fun doSomething(vararg numbers: Int) {
   // logic here
}

带返回值的方法

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int getScore() {
   // logic here
   return score;
}
  • Kotlin
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fun getScore(): Int {
   // logic here
   return score
}

// as a single-expression function

fun getScore(): Int = score

无结束符号

  • Java
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int getScore(int value) {
    // logic here
    return 2 * value;
}
  • Kotlin
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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

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public class Utils {

    private Utils() { 
      // This utility class is not publicly instantiable 
    }
    
    public static int getScore(int value) {
        return 2 * value;
    }
    
}
  • Kotlin
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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
    }

}

继承

  • Java
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public class User extends Persion{

   
}
  • Kotlin
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class User : Persion() {

}

实现接口

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public class User implements Persion{

    @Override
    public void onName() {
        
    }
   
}

new Persion(){
    @Override
    public void onName() {
        
    }
}

  • Kotlin
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class User : Persion {
	override fun onName() {}
}

object : Persion {
  override fun onName() {}
}

Get Set 构造器

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

原型扩展

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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
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fun Int.triple(): Int {
  return this * 3
}

var result = 3.triple()
  • Java
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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
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    enum class Direction(val direction: Int) {
        NORTH(1),
        SOUTH(2),
        WEST(3),
        EAST(4);
    }

let内联函数

  • Kotlin
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    val result = "testLet".let {
        println(it.length)
        1000
    }
    println(result) //1000

场景一: 最常用的场景就是使用let函数处理需要针对一个可null的对象统一做判空处理。
场景二: 然后就是需要去明确一个变量所处特定的作用域范围内可以使用

  • Kotlin

不用let

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mVideoPlayer?.setVideoView(activity.course_video_view)
mVideoPlayer?.setControllerView(activity.course_video_controller_view)
mVideoPlayer?.setCurtainView(activity.course_video_curtain_view)

用let

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mVideoPlayer?.let {
   it.setVideoView(activity.course_video_view)
   it.setControllerView(activity.course_video_controller_view)
   it.setCurtainView(activity.course_video_curtain_view)
}

also内联函数

also函数和let差不多，also返回传入对象，let返回最后一行的值。

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val result = "testAlso".also {
    println(it.length)
    1000
}
println(result) //testAlso

with内联函数

with函数是接收了两个参数，分别为T类型的对象和一个lambda函数块，所以with函数最原始样子:

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val result = with(user, {
    println("my name is $name, I am $age years old, my phone number is $phoneNum")
    1000
})

最后一行的 ‘1000’ 返回值赋值给 ‘result’

但是由于with函数最后一个参数是一个函数，可以把函数提到圆括号的外部，所以最终with函数的调用形式:

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val result = with(user) {
    println("my name is $name, I am $age years old, my phone number is $phoneNum")
    1000
}

使用场景：
适用于调用同一个类的多个方法时，可以省去类名重复，直接调用类的方法即可，经常用于Android中RecyclerView中onBinderViewHolder中，数据model的属性映射到UI上

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@Override
public void onBindViewHolder(ViewHolder holder, int position) {

   		MyItem item = getItem(position);
		if (item == null) {
			return;
		}
		holder.tvTitle.setText(item.title);
		holder.tvDate.setText(item.date);
		holder.tvInfo.setText(item.info);
		holder.tvCount.setText(item.count);
		...
}
  • Kotlin
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    override fun onBindViewHolder(holder: ViewHolder, position: Int){
       val item = getItem(position)?: return
       
       with(item){
           holder.tvTitle.text = title
    	   holder.tvDate.text = date
    	   holder.tvInfo.text = info
    	   holder.tvCount.text = count
           ...   
       }
    }

run内联函数

run函数实际上可以说是let和with两个函数的结合体，run函数只接收一个lambda函数为参数，以闭包形式返回，返回值为最后一行的值或者指定的return的表达式。

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val user = User("Kotlin", 1, "1111111")

val result = user.run {
    println("my name is $name, I am $age years old, my phone number is $phoneNum")
    1000
}
println("result: $result")

适用于let,with函数任何场景。因为run函数是let,with两个函数结合体，准确来说它弥补了let函数在函数体内必须使用it参数替代对象，在run函数中可以像with函数一样可以省略，直接访问实例的公有属性和方法，另一方面它弥补了with函数传入对象判空问题，在run函数中可以像let函数一样做判空处理。

  • Kotlin
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    override fun onBindViewHolder(holder: ViewHolder, position: Int){
       
       getItem(position)?.run{
           holder.tvTitle.text = title
    	   holder.tvDate.text = date
    	   holder.tvInfo.text = info
    	   holder.tvCount.text = count
           ...   
       }
    }

apply内联函数

和run差不多，apply返回的是对象本身。
使用场景：实例化一个对象，对属性进行赋值