String name = "Amit Shekhar";
final String name = "Amit Shekhar";
Kotlin
var name = "Amit Shekhar"
val name = "Amit Shekhar"
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
}
// or simply
val length = text?.length
字符串拼接
Java
String firstName = "Amit";
String lastName = "Shekhar";
String message = "My name is: " + firstName + " " + lastName;
Kotlin
val firstName = "Amit"
val lastName = "Shekhar"
val message = "My name is: $firstName $lastName"
換行
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;
}
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循環(huán)
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) { }
void doSomething() {// logic here
}void doSomething(int... numbers) {// logic here
}
Kotlin
fun doSomething() {// logic here
}fun doSomething(vararg numbers: Int) {// logic here
}
帶返回值的方法
Java
int getScore() {// logic herereturn score;
}
Kotlin
fun getScore(): Int {// logic herereturn score
}// as a single-expression functionfun getScore(): Int = score
無結束符號
Java
int getScore(int value) {// logic herereturn 2 * value;
}
Kotlin
fun getScore(value: Int): Int {// logic herereturn 2 * value
}// as a single-expression functionfun getScore(value: Int): Int = 2 * value
constructor 構造器
Java
public class Utils {private Utils() { // This utility class is not publicly instantiable }public static int getScore(int value) {return 2 * value;}}
Kotlin
class Utils private constructor() {companion object {fun getScore(value: Int): Int {return 2 * value}}
}// another wayobject 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;}@Overridepublic 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;}@Overridepublic int hashCode() {int result = name != null ? name.hashCode() : 0;result = 31 * result + age;return result;}@Overridepublic 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() { // 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()
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);
}
