接口实现多继承

interface Flyer {
    fun fly()
    fun kind() = "flying Flyer"
}
interface Animal {
    val name: String
    fun eat()
    fun kind() = "flying animals"
}
class Bird(override val name: String) : Flyer, Animal {
    override fun fly() {
        println("I can fly")
    }
    override fun kind(): String {
        // 通过super调用某个父类
        return super<Flyer>.kind()
    }
    override fun eat() {
        println("I can eat")
    }
}
fun main() {
    Bird("AAA").fly()
    Bird("AAA").eat()
    Bird("AAA").kind()
}

使用内部类实现多继承

// 使用内部类实现多继承
open class Horse {
    fun runFast() {
        println("I can run fast")
    }
}

open class Donkey {
    fun doLongTimeThing() {
        println("I can do something long time")
    }
}

class Mule {
    fun runFast() {
        HoeseC().runFast()
    }

    fun doLongTimeThing() {
        DonkeyC().doLongTimeThing()
    }

    // 定义多个内部类来实现多继承，每个内部类的实例都有自己独立的状态，它们与外部对象的信息相互独立
    private inner class HoeseC : Horse()
    private inner class DonkeyC : Donkey()
}

fun main() {
    Mule().doLongTimeThing()
    Mule().runFast()
}

使用内部类实现多继承

interface CanFly {
    fun fly()
}

interface CanEat {
    fun eat()
}

open class Flyer : CanFly {
    override fun fly() {
        println("I can Fly")
    }
}

open class Animal : CanEat {
    override fun eat() {
        println("I can Eat")
    }
}

class Bird(flyer: Flyer, animal: Animal) : CanFly by flyer, CanEat by animal {

}

fun main() {
    val flyer = Flyer()
    val animal = Animal()

    val bird = Bird(flyer, animal)
    bird.eat()
    bird.fly()
}

反编译后得字节码，可以看到，委托实现的Bird 实际上还是实现了CanFly和CanEat接口，然后通过Field delegate_0:LFlyer和Field delegate_1:LAnimal成员变量去调用代理对象的方法。

Compiled from "Bird.kt"
public final class Bird implements CanFly,CanEat {
  public Bird(Flyer, Animal);
    Code:
       0: aload_1
       1: ldc           #13                 // String flyer
       3: invokestatic  #19                 // Method kotlin/jvm/internal/Intrinsics.checkParameterIsNotNull:(Ljava/lang/Object;Ljava/lang/String;)V
       6: aload_2
       7: ldc           #21                 // String animal
       9: invokestatic  #19                 // Method kotlin/jvm/internal/Intrinsics.checkParameterIsNotNull:(Ljava/lang/Object;Ljava/lang/String;)V
      12: aload_0
      13: invokespecial #24                 // Method java/lang/Object."<init>":()V
      16: aload_0
      17: aload_1
      18: putfield      #28                 // Field $$delegate_0:LFlyer;
      21: aload_0
      22: aload_2
      23: putfield      #32                 // Field $$delegate_1:LAnimal;
      26: return

  public void fly();
    Code:
       0: aload_0
       1: getfield      #28                 // Field $$delegate_0:LFlyer;
       4: invokevirtual #39                 // Method Flyer.fly:()V
       7: return

  public void eat();
    Code:
       0: aload_0
       1: getfield      #32                 // Field $$delegate_1:LAnimal;
       4: invokevirtual #44                 // Method Animal.eat:()V
       7: return
}