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 name = "Amit Shekhar";final String name = "Amit Shekhar";
Kotlin
var name = "Amit Shekhar"val name = "Amit Shekhar"
null声明
Java
final String name = null;String otherName; otherName = null;
Kotlin
val name : String? = nullvar otherName : String? otherName = null
空判断
Java
if(text != null){
int length = text.length();
}Kotlin
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"
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
Java
String text = x > 5 ? "x > 5" : "x <= 5";
Kotlin
val text = if (x > 5) "x > 5" else "x <= 5"
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 // smart casting}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 = // some score;String grade;switch (score) { case 10:
case 9:
grade = "Excellent"; break; case 8:
case 7:
case 6:
grade = "Good"; break; default:
grade = "Fail";
}Kotlin
var score = // some scorevar grade = when (score) { 9, 10 -> "Excellent" in 6..8 -> "Good"
else -> "Fail"}for循环
Java
for (int i = 1; i <= 10 ; i++) { }for (int i = 1; i <= 10 ; i+=2) { }for (String item : collection) { }for (Map.Entry<String, String> entry: map.entrySet()) { }Kotlin
for (i in 1..10) { }for (i in 1..10 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");
Kotlin
val listOfNumber = listOf(1, 2, 3, 4)val keyValue = mapOf(1 to "Amit", 2 to "Ali", 3 to "Mindorks")
遍历
Java
for (Car car : cars) {
System.out.println(car.speed);
}for (Car car : cars) {
if(cars.speed > 100) {
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() {
// logic here}Kotlin
fun doSomething() {
// logic here}带返回值的方法
Java
int getScore() {
// logic here return score;
}Kotlin
fun getScore(): Int {
// logic here return 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
}constructor 构造器
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
}
}
}// other way is also thereobject Utils {
fun getScore(value: Int): Int {
return 2 * value
}
}Get Set 构造器
Java
public class Developer {
private String name;
public Developer(String name) {
this.name = name;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
@Override public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
Developer developer = (Developer) o;
return name != null ? name.equals(developer.name) : developer.name == null;
}
@Override public int hashCode() {
return name != null ? name.hashCode() : 0;
}
@Override public String toString() {
return "Developer{" +"name='" + name +'}';
}
}Kotlin
data class Developer(val name: String)
工具类
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()
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