Java 内部类“覆盖"

Posted 2020-10-06 Always_July

Think in Java  P269

如果子类中的内部类和父类中内部类一样，这好像子类内部类“覆盖”了父类的内部类，但其实没有代表着什么。

public class BigEgg extends Egg {
    public class Yolk {
        public Yolk() {
            System.out.println("BigEgg.Yolk()");
        }
    }
    public static void main(String[] args) {
        new BigEgg();
    }
}
class Egg
{
    private Yolk y;
    protected class Yolk
    {
        public Yolk() {
            System.out.println("Egg.yolk( )");
        }
    }
    public Egg() {
        System.out.println("New Egg()");
        y = new Yolk();
    }
}

输出结果：

New Egg()
Egg.yolk( )

结果表明  内部类在继承中并没有表现其他的特性 ，两个是两个完全独立的实体。

 

class Egg2 {
    protected class Yolk {
        public Yolk() { print("Egg2.Yolk()"); }
        public void f() { print("Egg2.Yolk.f()");}
    }
    private Yolk y = new Yolk();
    public Egg2() {
        System.out.println("new Egg2");
    }
    public void insertYolk(Yolk yy) { y = yy; }
    public void g() {
        y.f(); }
    public void print(String msg)
    {
        System.out.println(msg);
    }
}
public class BigEgg2 extends Egg2 {
    public class Yolk extends Egg2.Yolk {
        public Yolk() { print("BigEgg2.Yolk()"); }
        public void f() { print("BigEgg2.Yolk.f()"); }
    }
    public BigEgg2() {
        insertYolk(new Yolk());
    }
    public static void main(String[] args) {
        Egg2 e2 = new BigEgg2();
        e2.g();
    }
    public void print(String msg)
    {
        System.out.println(msg);
    }
}

输出：

Egg2.Yolk()
new Egg2
Egg2.Yolk()
BigEgg2.Yolk()
BigEgg2.Yolk.f()

1.父类变量初始化  

private Yolk y = new Yolk();
此时调用的是父类的Yolk,所以输出的结果是Egg2.Yolk()

2.父类构造函数，输出 new Egg2

3.子类构造函数  

insertYolk(new Yolk());
此时使用的是子类的Yolk,实例化子类，需要先实例化父类，所以 输出的结果是
Egg2.Yolk()
BigEgg2.Yolk()

4.代码 e2.g();

此时父类中的Yolk实际上是子类的Yolk,所以输出的结果是BigEgg2.Yolk.f()