一個幫助Java開發者快速學會Kotlin的文章

今天有個小夥伴來問小灰，說自己JAVA用的賊溜，想再學習下Kotlin，問小灰上手難不難？

怎麼說呢？其實我一直覺得語言這方面，是觸類旁通、一通百通的，既然JAVA已經賊溜了，想必學其他的語言也一定事半功倍。

不過呢，如果有好的輔助，肯定就更加容易學會啦，今天小灰就和大家寫一篇對比展示Java和Kotlin兩種語言在語法上不同地方的文章，可以快速幫助會JAVA的小夥伴掌握Kotlin的編寫技巧！
廢話不多說，說完這句我就滾，用最實際的代碼來展示不同之處。

打印日誌

• Java

System.out.print("i小灰");
System.out.println("i小灰");

• Kotlin

print("i小灰")
println("i小灰")

---

常量與變量

• Java

String name = "i小灰";
final String name = "i小灰";

• Kotlin

var name = "i小灰"
val name = "i小灰"

---

null聲明

• Java

String otherName;
otherName = null;

• Kotlin

var otherName : String?
otherName = null

---

空判斷

• 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"

---

換行

• Java

String text = "一行\n" +
              "二行\n" +
              "三行";

• Kotlin

val text = """
        |一行
        |二行
        |三行
        """.trimMargin()

---

三元表達式

• 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

---

類型判斷和轉換 (聲明式)

• Java

if (object instanceof Car) {
}
Car car = (Car) object;

• Kotlin

if (object is Car) {
}
var car = object as Car

---

類型判斷和轉換 (隱式)

• Java

if (object instanceof Car) {
   Car car = (Car) object;
}

• Kotlin

if (object is Car) {
   var car = object // 智能的轉換
}

---

多重條件

• Java

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

• Kotlin

if (score in 0..300) { }

---

更靈活的case語句

• Java

int score = // 一些分數;
String grade;
switch (score) {
    case 10:
    case 9:
        grade = "優秀";
        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 = // 一些分數;
var grade = when (score) {
    9, 10 -> "優秀"
    in 6..8 -> "良"
    4, 5 -> ""
    in 1..3 -> "差"
    else -> "差"
}

---

for循環

• 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, "i");
map.put(2, "小");
map.put(3, "灰");

// Java 9
final List<Integer> listOfNumber = List.of(1, 2, 3, 4);

final Map<Integer, String> 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")

---

遍歷

• 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)}

---

方法定義

• Java

void doSomething() {
   // 邏輯...
}

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

• Kotlin

fun doSomething() {
   // 邏輯...
}

fun doSomething(vararg numbers: Int) {
   // 邏輯...
}

---

帶返回值的方法

• Java

int getScore() {
   // 邏輯...
   return score;
}

• Kotlin

fun getScore(): Int {
   // 邏輯...
   return score
}

// 作爲單表達式函數

fun getScore(): Int = score

---

無結束符號

• Java

int getScore(int value) {
   // 邏輯...
    return 2 * value;
}

• Kotlin

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

// 作爲單表達式函數

fun getScore(value: Int): Int = 2 * value

---

constructor 構造器

• Java

public class Utils {

    private Utils() { 
      // 此類不可公開實例化
    }
    
    public static int getScore(int value) {
        return 2 * value;
    }
    
}

• Kotlin

class Utils private constructor() {

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

// 另一種方式

object Utils {

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

}

---

Get Set 構造器

• 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)

---

原型擴展

• Java

public class Utils {

    private Utils() { 
      // 此類不可公開實例化
    }
    
    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);
}