超类调用子类方法

Question

I have a subclass extending a superclass. I want the superclass to never call methods on the subclass. Is this possible?

It seems unintuitive that a method explicitly invoked on the superclass would call the subclass again.

Trivial example:

public class Demo {
    public static void main(String[] args) {
        A a = new A();
        B b = new B();
        System.out.println("A.a() = " + a.a()); // -1
        System.out.println("A.b() = " + a.b()); // -1
        System.out.println("B.a() = " + b.a()); // 99
        // B.b() -> A.b() -> B.a()
        System.out.println("B.b() = " + b.b()); // 99
    }
}

class A {
    public int a() { return -1; }
    public int b() { return a(); } // tried, doesn't work: A.a(), A.this.a()
}

class B extends A {
    public int a() { return 99; }
    public int b() { return super.b(); }
}

Note: Actual case is implementing a Deque; the descending iterator class inherits from the forward iterator class, just starting from the tail instead of the head. I'd like to swap the method names, which I'd like to do with:

public boolean hasNext() {
    return super.hasPrevious();
}

But that doesn't work because the forward iterator calls the wrong methods on the backwards iterator. My current workaround is storing the forward iterator as a field and call methods on that, but that seems clumsy / inelegant.

Answer 1

By default, subclass can override methods of its superclass. To prevent that from happening, you can add a final modifier to the superclass's methods to prevent its subclasses from overriding that method.

Answer 2

If the superclass method being called is private , then when you're in the superclass code, it will go for that superclass method and not the subclass method.

The following will work, even if it's ugly.

class A {
    public int a() { return privateA(); }
    public int b() { return privateA();}  
    // It's impossible for anything to override privateA, even if a subclass has a 
    // method with the same name, because it's private.
    private int privateA(){return -1}
}

class B extends A {
    public int a() { return 99; }
    public int b() { return super.b(); }
}