Kotlin系列之Kotlin与Java语法区别
打印日志 Java System.out.print("Amit Shekhar"); System.out.println("Amit Shekhar"); Kotlin print("Amit Shekhar") println("Amit Shekhar") 常量与变量 Java String mName = "Java string name"; final String mName = "Java string name"; Kotlin var mName = "kotlin string name" val mName = "kotlin string name" null声明 Java String mName; mName = null; Kotlin var mName : String? mName = null 空判断 Java if (str != null) { int length = str.length(); } Kotlin str?.let { val length = str.length } // 更简单的写法 val length = str?.length // 为null赋予默认值 val length = str?.length?:0 字符串拼接 Java String firstName = "Jiang"; String lastName = "super"; String message = "My name is: " + firstName + " " + lastName; Kotlin val firstName = "Jiang" val lastName = "super" 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(三目运算符) 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 if (object instanceof Car) { } Car car = (Car) object; Kotlin if (object is Car) { } 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 // 智能转换 } 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 = 8// 定义分数; 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 = 8// 定义分数 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 1 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, "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 val listOfNumber = listOf(1, 2, 3, 4) val keyValue = mapOf(1 to "Amit", 2 to "Ali", 3 to "Mindorks") 遍历 Java // Java 7 以及更低 for (Car car : cars) { System.out.println(car.speed); } // Java 8+ cars.forEach(car -> System.out.println(car.speed)); // Java 7 以及更低 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() { // todo here } void doSomething(int... numbers) { // todo 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 } // 单表达式函数 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 } // 单表达式函数 fun getScore(value: Int): Int = 2 * value constructor 构造器 Java public class Utils { private Utils() { // 私有构造方法 } public static int getScore(int value) { return 2 * value; } } Kotlin class Utils private constructor() { companion object { fun getScore(value: Int): Int { return 2 * value } } } // 第二个例子 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; } @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) 原型扩展(拓展函数) Java public class Utils { private Utils() { // 私有构造方法 } 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 constructor(direction: Int) { NORTH(1), SOUTH(2), WEST(3), EAST(4); var direction: Int = 0 private set init { this.direction = direction } }
