Java 到 Kotlin 入门，这一篇就够啦，java 开发岗高频面试题全解析祖国的花朵
Kotlin
if (object is Car) {
var car = object // 聪明的转换
}
[](
)多重条件
Java
if (num >= 0 && num <= 300) { }
Kotlin
if (num 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;
case 5:
case 4:
grade = "OK";
break;
case 3:
case 2:
case 1:
grade = "Fail";
break;
default:
grade = "Fail";
}
Kotlin
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
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) { }
[](
)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
// 不可变
val listOfNumber = listOf(1, 2, 3, 4)
// 不可变
val keyValue = mapOf(1 to "Amit",
2 to "Ali",
3 to "Mindorks")
// 可变
val listOfNumber = mutableListOf(1, 2, 3, 4)
// 可变
val keyValue = mutableMapOf(1 to "Amit",
2 to "Ali",
3 to "Mindorks")
[](
)遍历
Java
// 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
cars.forEach {
println(it.speed)
}
cars.filter { it.speed > 100 }
.forEach { println(it.speed)}
[](
)方法定义
Java
void doSomething() {
// logic here
}
void doSomething(int... numbers) {
// logic here
}
Kotlin
fun doSomething() {
// logic here
}
fun doSomething(vararg numbers: Int) {
// logic here
}
[](
)有返回值的方法
Java
int getScore() {
// logic here
return score;
}
Kotlin
fun getScore(): Int {
// logic here
return score
}
// as a single-expression function
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
}
// as a single-expression function
fun getScore(value: Int): Int = 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
}
}
}
// another way
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 +
'}';
}
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