打印日志
System.out.print("Amit Shekhar");
System.out.println("Amit Shekhar");
print("Amit Shekhar")
println("Amit Shekhar")
常量与变量
String mName = "Java string name";
final String mName = "Java string name";
var mName = "kotlin string name"
val mName = "kotlin string name"
null声明
String mName;
mName = null;
var mName : String?
mName = null
空判断
if (str != null) {
int length = str.length();
}
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
String firstName = "Jiang";
String lastName = "super";
String message = "My name is: " + firstName + " " + lastName;
val firstName = "Jiang"
val lastName = "super"
val message = "My name is: $firstName $lastName"
换行
String text = "First Line\n" +
"Second Line\n" +
"Third Line";
val text = """
|First Line
|Second Line
|Third Line
""".trimMargin()
三元表达式(三目运算符)
String text = x > 5 ? "x > 5" : "x <= 5";
val text = if (x > 5)
"x > 5"
else "x <= 5"
操作符
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;
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)
if (object instanceof Car) {
}
Car car = (Car) object;
if (object is Car) {
}
var car = object as Car
类型判断和转换 (隐式)
if (object instanceof Car) {
Car car = (Car) object;
}
if (object is Car) {
var car = object // 智能转换
}
多重条件
if (score >= 0 && score <= 300) { }
if (score in 0..300) { }
更灵活的case语句
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";
}
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
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()) { }
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
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");
val listOfNumber = listOf(1, 2, 3, 4)
val keyValue = mapOf(1 to "Amit",
2 to "Ali",
3 to "Mindorks")
遍历
// 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));
cars.forEach {
println(it.speed)
}
cars.filter { it.speed > 100 }
.forEach { println(it.speed)}
方法定义
void doSomething() {
// todo here
}
void doSomething(int... numbers) {
// todo here
}
fun doSomething() {
// logic here
}
fun doSomething(vararg numbers: Int) {
// logic here
}
带返回值的方法
int getScore() {
// logic here
return score;
}
fun getScore(): Int {
// logic here
return score
}
// 单表达式函数
fun getScore(): Int = score
无结束符号
int getScore(int value) {
// logic here
return 2 * value;
}
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
public class Utils {
private Utils() {
// 私有构造方法
}
public static int getScore(int value) {
return 2 * value;
}
}
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
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 +
'}';
}
}
data class Developer(val name: String, val age: Int)
原型扩展(拓展函数)
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);
fun Int.triple(): Int {
return this * 3
}
var result = 3.triple()
public enum Direction {
NORTH(1),
SOUTH(2),
WEST(3),
EAST(4);
int direction;
Direction(int direction) {
this.direction = direction;
}
public int getDirection() {
return direction;
}
}
enum class Direction constructor(direction: Int) {
NORTH(1),
SOUTH(2),
WEST(3),
EAST(4);
var direction: Int = 0
private set
init {
this.direction = direction
}
}
