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# 打印日志

Java

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

Kotlin

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

# 定义变量与常量常

Java

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

Kotlin

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

# 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

# 字符串拼接

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

Kotlin

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

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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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Car car = (Car) object;

Kotlin

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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 // 自动识别
}

# 多重条件

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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, "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

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

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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

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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
    }

}

# 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;
    }
}

Kotlin

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

# 参考

  • 慕课网移动端架构师成长体系课