Больше ада

A.java

package a;

public abstract class A {
    abstract void foo();

    public void callFooA() {
        System.out.print("callFooA: ");
        foo();
    }
}

B.java

package b;

import a.A;

public abstract class B extends A {
    void foo() {
        System.out.println("B.foo()");
    }

    public void callFooB() {
        System.out.print("callFooB: ");
        foo();
    }
}

C.java

package a;

import b.B;

public class C extends B {
    void foo() {
        System.out.println("C.foo()");
    }

    public void callFooC() {
        System.out.print("callFooC: ");
        foo();
    }
}

D.java

package b;

import a.C;

public class D extends C {
    void foo() {
        System.out.println("D.foo()");
    }

    void callFooD() {
        System.out.print("callFooD: ");
        foo();
    }

    public static void main(String[] args) {
        D d = new D();
        d.foo();
        d.callFooA();
        d.callFooB();
        d.callFooC();
        d.callFooD();
    }
}

/*
Produces:

D.foo()
callFooA: D.foo()
callFooB: D.foo()
callFooC: C.foo()
callFooD: D.foo()

*/