时刻对新事物保持好奇心
Google I/O 2017 宣布在 Android Studio 3.0 将默认支持 Kotlin 用于 Android 开发. 请注意 Google 只是宣布 Kotlin 作为 Android 开发的一级语言，并没有说要用其取代 Java ，这里很多无脑媒体进行了过度解读。作为一名合格的程序员，首先要理性看待技术迭代更新，同时要对新事物保持强烈的好奇心。
Kotlin 来自于捷克一家牛逼软件研发公司 JetBrains ，很多优秀的 IDE，如 IntelliJ IDEA、PyCharm、WebStorm 等都是它的杰作，包括 Google 官方的 Android IDE -- Android Studio ，也是 IntelliJ IDEA 的插件版。
Kotlin 源于 JetBrains 的圣彼得堡团队，名称取自圣彼得堡附近的一个小岛 ( Kotlin Island ) ，JetBrains 在 2010 年首次推出 Kotlin 编程语言，并在次年将之开源。
Kotlin 是一个基于 JVM 的编程语言，与 Java 有很深的渊源。它能与 Java 100% 互通，具备诸多 Java 尚不支持的新特性。Kotlin 可以编译成 Java 字节码，也可以编译成 JavaScript ，方便在没有 JVM 的设备上运行，可用于移动端以及 Web 开发，是一种全栈式开发语言。
下面是 Java to Kotlin 主要语法，以便于你快速认识 Kotlin 这门语言。
Java
String lan = "Hello Kotlin";
final String lan = "Hello Kotlin";Kotlin
var lan = "Hello Kotlin"
val lan = "Hello Kotlin"Java
System.out.print("Hello Kotlin");
System.out.println("Hello Kotlin");Kotlin
print("Hello Kotlin");
println("Hello Kotlin");Java
String name = "haohao";
int age = 25;
String msg = "I am " + name + ", " + age + " years old.";
String text = "Hello Kotlin\n" +
"Hello Android\n" +
"Hello TensorFlow";Kotlin
var name = "haohao";
var age = 25;
var msg = "I am $name , $age years old.";
var text = """
|Hello Kotlin
|Hello Android
|Hello TensorFlow
""".trimMargin()
// trimMargin() 去除连接符 `|`
Java
String name = null;Kotlin
var name: String? = null // `?` 声明变量可为 null Java
if (name != null) {
int len = name.length;
}Kotlin
name?.let{
val len = name.length
}Java
String y = x > 6 ? "x > 6" : "x <= 6";Kotlin
var y = if (x > 6)
"x > 6"
else "x <= 6"Java
if (obj instanceof Person) {
Person p = (Person) obj;
}Kotlin
if (obj is Person) {
var p = obj //智能转换
}Java
if (value >= 0 && value <= 100) {
}
for (int i = 1; i <= 100 ; i++) {}
for (int i = 1; i < 100 ; i++) {}
for (int i = 100; i >= 0 ; i--) {}
for (int i = 1; i <= 100 ; i+=2) {}
for (int i = 100; i >= 0 ; i-=2) {}Kotlin
if (value is 0..100) {
}
for (i in 1..100 ) {}
for (i in 1 until 100) {} // 半开区间：不包含 100
for (i in 100 downTo 0) {}
for (i in 1..100 step 2) {}
for (i in 100 donwTo 1 step 2) {}Java
List<Integer> list = Arrays.asList(1, 2, 3, 4);
for(Integer value : list) {
System.out.println(value);
}
Map<Integer, String> map = new HashMap<>();
map.put(1, "Java");
map.put(2, "Kotlin");
map.put(3, "iOS");
map.put(4, "React Native");
Iterator<Map.Entry<Integer, String>> iterator = map.entrySet().iterator();
while (iterator.hasNext()) {
Map.Entry<Integer, String> entry = iterator.next();
System.out.println(entry.getKey() + " , " + entry.getValue());
}
Kotlin
var list = listOf(1, 2, 3, 4);
list.forEach {
println(it)
}
var map = mapOf(1 to "Java", 2 to "Kotlin", 3 to "iOS", 4 to "React Native")
map.forEach { key, value ->
println("$key, $value")
}
// 或者
for ((key, value) in map) {
println("$key, $value")
}
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"
}Java
List<String> list = Arrays.asList("java", "c++", "Android", "Kotlin", "iOS");
for(int i = 0; i < list.size(); i++ ) {
list.set(i, "Hello " + list.get(i));
if (!list.get(i).contains("c")) {
System.out.println(list.get(i));
}
}
//输出：
// Hello java
// Hello Android
// Hello Kotlin
// Hello iOSKotlin
var list = arrayOf("java", "c++", "Android", "Kotlin", "iOS")
list.map {
"Hello $it"
}.filter {
!it.contains("c")
}.forEach {
println(it)
}
//输出：
// Hello java
// Hello Android
// Hello Kotlin
// Hello iOSJava
public final void func() {}
public final int func(int value) {
return 1 * value;
}Kotlin
fun func() {}
fun func(value: Int): Int {
return 1 * value
}
// 或者
fun func(value: Int): Int = 1 * valueJava
public final class Person {
private String name = null;
private int age = 25;
private Person() {
}
public Person(String name, int age) {
this.name = name;
this.age = age;
}
}Kotlin
class Person private constructor() {
private var name: String? = null
private var age: Int = 25
constructor (name: String, age: Int): this() {
this.name = name
this.age = age
}
}Java
public final class Manager {
private Manager() {}
public static Manager getInstance () {
return new Manager();
}
}Kotlin
class Manager private constructor() {
companion object {
fun getInstance(): Manager = Manager()
}
}
Java
public class Person {
public String name;
public age;
public Person(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;
Person person = (Person) o;
if (age != person.age) return false;
return name != null ? name.equals(person.name) : person.name == null;
}
@Override
public int hashCode() {
int result = name != null ? name.hashCode() : 0;
result = 31 * result + age;
return result;
}
@Override
public String toString() {
return "Person{" +
"name='" + name + '\'' +
", age=" + age +
'}';
}
}Kotlin
data class Person(var name: String, var age: Int)
Java
Person p = new Person("haohao", 25);
String name = p.getName();
String age = p.getAge();Kotlin
var p = Person("name", 25)
var (name, age) = pJava
interface Readable {
String getContent();
}public final class Book implements Readable {
@override
public String getContent() {
return "Hello";
}
} Kotlin
// Kotlin 接口中可以设置抽象和非抽象方法,可以有属性但必须声明为抽象或提供访问器实现。
interface Readable {
fun getContent(): String
fun getVersion(): Int = 0
}class Book(): Readable {
override fun getContent(): String = "Hello"
override fun getVersion(): Int {
return super.getVersion()
}
}Java
public class Person {
private String name = null;
public Person(String name) {
this.name = name;
}
public void sayHello() {
System.out.println("Hello");
}
public final void sayGood() {
System.out.println("Good");
}
}public final class Student extends Person {
private String school = null;
public Student(String name, String school) {
super(name);
this.school = school;
}
@Override
public void sayHello() {
super.sayHello();
System.out.println("Hello Student");
}
}Kotlin
open class Person(private var name: String? = null) {
open fun sayHello() = println("Hello")
fun sayGood() = println("Good")
}class Student(private var school: String? = null, name: String): Person(name) {
override fun sayHello() {
super.sayHello()
println("Hello Student")
}
} Java
public final class Outer {
private int bar = 100;
public static class Nester {
public final String foo() {
return "Hello Kotlin!";
}
}
public final class Inner {
public final int foo() {
return bar; // 可以访问外部类成员
}
}
}
System.out.println(new Outer.Nester().foo());
// System.out.println(new Outer().Inner().foo()); 不能在其他类中实例化非静态内部类Kotlin
class Outer {
private var bar: Int = 100
// 嵌套类
class Nester {
// 不能访问外部类成员
fun foo() = "Hello Kotlin!"
}
// 内部类
inner class Inner {
// 可以访问外部类成员
fun foo() = bar
}
}
println(Outer.Nested().foo())
println(Outer().Inner().foo())
Java
view.setOnClickListener(new OnClickListener() {
@override
onClick(View view){
// to do something.
}
});Kotlin
interface OnClickListener {
fun onClick()
}
class View(){
var listener: OnClickListener? = null
fun setOnClickListener(listener: OnClickListener) {
this.listener = listener
}
}
view.setOnClickListener(object : OnClickListener{
override fun onClick() {
TODO("not implemented")
}
})
Java
// Java 类不能直接进行扩展Kotlin
// 一般扩展
class MyClass {
fun foo() = println("member")
}
fun MyClass.bar() = println("extension")
MyClass().bar()
//输出： extension
// 扩展函数与成员函数相同
class MyClass {
fun foo() = println("member")
}
fun MyClass.foo() = println("extension") // 扩展函数与成员函数相同时, 成员函数优先
fun MyClass.foo(para: Int) = println("extension")
MyClass().foo()
MyClass().foo(0)
//输出：
// member
// extension
Java
public class Person {
private String name = null;
private int age = 0;
public Person(String name, int age) {
this.name = name;
this.age = age;
}
public Person() {
}
public String getName() {
return name;
}
public int getAge() {
return age;
}
public void setName(String name) {
this.name = name;
}
public void setAge(int age) {
this.age = age;
}
@Override
public String toString() {
return "name : " + name + ", age : " + age;
}
}Kotlin
var p = Person()
p.name = "haohao"
p.age = 25
println(p.toString())
// name : haohao, age : 25
Kotlin
// Example.kt
package demo
class MyClass
fun func() {
}
Java
new demo.MyClass();
demo.ExampleKt.func();Java
public final class Wrapper<T> {
private T item;
public Wrapper(T item) {
this.item = item;
}
public T getItem() {
return item;
}
}
Wrapper<String> wrapper = new Wrapper<>("Hello Kotlin");
System.out.println(wrapper.getItem());Kotlin
class Wrapper<T>(val item: T)
var wrapper = Wrapper("Hello Kotlin")
println(wrapper.item)
Java (Java 8)
new Thread(() -> {
System.out.println("Hello Kotlin");
}).start();
// 同下
new Thread(new Runnable() {
@Override
public void run() {
System.out.println("Hello Kotlin");
}
}).start();Kotlin
Thread(Runnable {
println("Hello Kotlin")
}).start()
// Kotlin Lambda 表达式语法
val sum = {x: Int, y: Int -> x + y }
val sum1: (Int, Int) -> Int = {x, y -> x + y }
val sum2 = fun(x: Int, y: Int): Int {
return x + y
}
println(sum(2,8))
println(sum1(2,8))
println(sum2(2,8))
//输出：
// 10
// 10
// 10
// Hello KotlinJava
// Java 不能直接实现高阶函数Kotlin
// 高阶函数，即一个函数可作为高阶函数的参数
fun superFunc(desc: String?, method: () -> String) {
print(desc)
println(method())
}
fun argFun() = "我是高阶函数的参数"
//使用高阶函数
superFunc("这是一个高阶函数 ", ::argFun)
superFunc("这是一个高阶函数 ", { argFun() })
superFunc("这是一个高阶函数 ") { argFun() }
//输出：
// 这是一个高阶函数 我是高阶函数的参数
// 这是一个高阶函数 我是高阶函数的参数
// 这是一个高阶函数 我是高阶函数的参数
Java (Java 8)
new Thread(() -> {
data = DataSource.obtain(); //耗时操作
runOnUiThread(() -> {
view.load(data); //更新 UI
});
}).start();Kotlin (Kotlin Anko)
async {
data = DataSource.obtain(); //耗时操作
uiThread {
view.load(data); //更新 UI
}
}Java
// Java 不能单独实现泛型函数Kotlin
fun <T> singletonList(item: T): List<T> {
return arrayListOf(item)
}
val list = singletonList<String>("kotlin")Java
// Java 不支持嵌套函数Kotlin
fun main(args: Array<String>) {
fun sayHello() {
println("Hello Kotlin")
}
sayHello();
}
// 输出：
// Hello KotlinKotlin
inline fun <reified T ： Activity> Activity.gotoActivity() {
val intent = Intent(this, T::class.java)
this.startActivity(intent)
}
gotoActivity<NewActivity>()
Kotlin
val lazyValue: String by lazy {
println("init") //第一次使用时才被初始化
"Hello Kotlin"
}
fun main(args: Array<String>) {
println(lazyValue)
println(lazyValue)
}
//输出：
// init
// Hello Kotlin
// Hello KotlinKotlin
class Person{
public var name: String by Delegates.observable("init .. "){
property,oldValue,newValue -> println("property : $property, oldValue : $oldValue, newValue : $newValue")
}
}
fun main(args: Array<String>) {
val person = Person()
println(person.name)
person.name = "haohao"
person.name = "nannan"
}
//输出：
// init ..
// property : var Person.name: kotlin.String, oldValue : init .. , newValue : haohao
// property : var Person.name: kotlin.String, oldValue : haohao, newValue : nannanKotlin
val plus = {x: Int, y: Int -> println("$x plus $y is ${x+y}")}
val hello = {println("Hello Kotlin")}
fun main(args: Array<String>) {
{x: Int, y: Int ->
println("$x plus $y is ${x+y}")
}(2, 8) // 自执行的闭包
plus(2, 8)
hello()
}
//输出：
// 2 plus 8 is 10
// 2 plus 8 is 10
// Hello Kotlin
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