public class ClassC extends ClassB {

    //additional functionality

}

public class Class1 extends ClassC {...}

public abstract class ParentOfB {

    // Additional functionality
}

public class B extends ParentOfB { // The only change in B

}

public class B1 {
        public void b1() {
            System.out.println("B1");
        }
    }


public class C1 extends B1 {
    public void c1() {
        System.out.println("C1");
    }
}

--------------------------  Adding New B1 Functionality to C1 ---------------------

public interface B_EXTRA {
    void b_extra();
}

public class B_EXTRA_IMPL implements B_EXTRA {

    public B1 m_b1;

    B_EXTRA_IMPL(B1 base) {
        m_b1 = base;
    }

    @Override
    public void b_extra() {
        System.out.println("b_extra");

    }
}

public class C1X extends C1 implements B_EXTRA {
    B_EXTRA_IMPL b_extra;

    C1X() {
        super();
        b_extra = new B_EXTRA_IMPL((B1)this);
    }

    public void b1()
    {
        System.out.println("B1");
    }

    @Override
    public void b_extra() {
        b_extra.b_extra();
    }
}

public class D1 extends C1X {
}

public class Main {
    public static void main(String[] args) {
        D1 d = new D1();
        d.b1();
        d.c1();
        d.b_extra();
    }
}