Kotlin学习（3）类，对象和接口

实现接口

interface Clickable{    //类似Java8默认方法    fun showOff() = println("I was clickable")    //要自己实现    fun click()}interface Focusable{    //可以直接使用    fun setFocus(b: Boolean) =             println("I ${if (b) "got" else "lose"} focus")    fun showOff() = println("I was focused")}class Button : Clickable, Focusable {    //两个接口都有这个方法，必须重写    override fun showOff() {        super<Clickable>.showOff()        super<Focusable>.showOff()    }    override fun click() {        println("i was a button")    }}

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open关键字

Kotlin默认类都是public final的

可继承的类和方法必须声明open

open class RichButton : Clickable{    //默认是不可重写    fun disable() {}    //可以被重写    open fun animate(){}    //覆盖接口的方法，默认也可以重写    //不想被重写，可以加上final    override fun click() {}}

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abstract关键字

abstract class Animated {    //必须覆盖    abstract fun animate()    open fun stopAnimating() {}    fun animateTwice() {}}

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访问修饰

internal open class TalkativeButton : Focusable {     private fun yell() = println("Hey!")    protected fun whisper() = println("Let's talk!")}fun TalkativeButton.giveSpeech() {//报错    yell()        //报错    whisper()     //报错}

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内部类的实现

/* Java *///因为这个ButtonState类保存一个指向外部的作用域//所以不能序列化ButtonState,因为Button不能被序列化public class Button implements View {    @Override    public State getCurrentState(){         return new ButtonState();    }    @Override    public void restoreState(State state) { /*...*/ }    public class ButtonState implements State,Serializable { /*...*/ }}class Button : View {    override fun getCurrentState(): State = ButtonState()    override fun restoreState(state: State) { /*...*/ }    class ButtonState : State { /*...*/ }}class Outer {    inner class Inner {        //指向外部作用域        fun getOuterReference(): Outer = this@Outer    }}

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sealed关键字

限定地创建子类，除了sealed类作用域里面的子类以外，不能创建别的子类

sealed class Expr {    class Num(val value: Int) : Expr()    class Sum(val left: Expr, val right: Expr) : Expr()}fun eval(e: Expr): Int =        when (e) {            is Expr.Num -> e.value            is Expr.Sum -> eval(e.right) + eval(e.left)        }

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默认构造器

class User(val nickname: String)//完整写法class User constructor(_nickname: String){     val nickname: String    init {        nickname = _nickname    }}

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私有构造器

class Secretive private constructor() {}class Secretive {    private constructor()}

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次级构造器

open class View {     constructor(ctx: Context) {        // some code    }    constructor(ctx: Context, attr: AttributeSet) {        // some code    }}

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子类调用父类构造器

open class View {    constructor(ctx: Context) {        // some code    }    constructor(ctx: Context, attr: AttributeSet) {        // some code    }}class MyButton : View {     constructor(ctx: Context) : super(ctx) {        // ...    }    constructor(ctx: Context, attr: AttributeSet) : super(ctx, attr) {        // ...    }}class MyButton : View {    constructor(ctx: Context): this(ctx, MY_STYLE) {        // ...    }    constructor(ctx: Context, attr: AttributeSet): super(ctx, attr) {        // ...    }}

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属性覆盖

interface User {    val nickname: String}class PrivateUser(override val nickname: String) : Userclass SubscribingUser(val email: String) : User {    override val nickname: String        get() = email.substringBefore('@')}class FacebookUser(val accountId: Int) : User {    override val nickname =        getFacebookName(accountId)}

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data class

data class会自动重写toString(), equals(), hashCode()方法。

==是equals()的语法糖

===和Java中的==是一样的，比较是不是一个对象

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代理类–by关键字

使用简化代理类，可以只重写需要的方法，不想重写的方法，会自动代理给代理对象

//复杂的代理类写法class DelegatingCollection<T> : Collection<T>{    private val innerList = arrayListOf<T>()    override val size: Int        get() = innerList.size    override fun contains(element: T): Boolean = innerList.contains(element)    override fun containsAll(elements: Collection<T>): Boolean = innerList.containsAll(elements)    override fun isEmpty(): Boolean = innerList.isEmpty()    override fun iterator(): Iterator<T> = innerList.iterator()}//简化代理类class DelegatingCollection<T> (        innerList: Collection<T> = ArrayList<T>()) : Collection<T> by innerList 

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object单例对象

想想javascript的对象就懂了

data class Person(val name: String)object Payroll{    val allEmployees = arrayListOf<Person>()    fun calculateSalary(){        for (person in allEmployees){            println(person.name)        }    }}

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伴生对象：代替工厂方法和静态方法

其实是有个隐含的Companion对象

//静态方法class A {    companion object{        fun bar(){            println("Companion object called")        }    }}fun main(vararg args: String){    A.bar()}//工厂方法class User2(val nickname: String){    companion object{        fun newSubscribingUser(email: String){            User2(email.substringBefore('@'))        }        fun newFacebookUser(accountId: Int){            User2(accountId.toString())        }    }}fun main(vararg args: String){    User2.newFacebookUser(1111)    User2.newSubscribingUser("[e-mail]")    User2.Companion.newSubscribingUser(...)}

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匿名对象

Java内的匿名内部类

window.addMouseListener(    object: MouseAdapter(){        override fun click(e: MouseEvent){            //....        }    })

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