Java动态绑定失败

Question

I don't really understand why the following code doesn't work. I expect Bf to be printed out.

public class Main {
    public static void main(String[] args) {
        B b = new B();
        b.g();
    }
}

class A {
    void g() {
        f();    /* or this.f() */
    }
}

class B extends A {
    void f() {
        System.out.println("B.f");
    }
}

The compiler complains:

Main.java:13: error: cannot find symbol
        f();    /* or this.f() */
        ^
  symbol:   method f()
  location: class A
1 error

From my understanding, the dynamic type of the this in Ag is always of B . As a result, Java's dynamic method lookup should be able to find Bf for this.f and then execute it.

Answer 1

Java's dynamic dispatch can only see methods defined on the current class or its superclasses. Since A does not define the method it cannot see that B defines it and you get an error.

However if the method is any access other than private, B can override it and hence calls to the method on an instance of B will be dynamically dispatched to B's implementation even when accessing the method through an "A" instance type.

Answer 2

This will not work as class A doesn't have any idea about method f(). If you do not want to define f() in A, you could make f() abstract instead for which You can make class A also abstract :

abstract class A {
    void g() {
        f();    /* or this.f() */
    }

    abstract void f();
}

class B extends A {
    void f() {
        System.out.println("B.f");
    }
}