final方法修饰继承问题

class WithFinal{
    private final void f(){
        System.out.println("WithFinal.f()");
    }
    private  void g(){
        System.out.println("WithFinal.g()");
    }
}

class OverridingPrivate extends WithFinal{
    private final void f(){
        System.out.println("OverridingPrivate.f()");
    }
    private  void g(){
        System.out.println("OverridingPrivate.g()");
    }
}
class OverridingPrivate2 extends OverridingPrivate{
    public final void f(){
        System.out.println("OverridingPrivate2.f()");
    }
    public  void g(){
        System.out.println("OverridingPrivate2.g()");
    }
}
public class FinalOverriding {
    public static void main(String[] args) {
        OverridingPrivate2 op2 = new OverridingPrivate2();
        op2.f();
        op2.g();
        OverridingPrivate op = op2;
        op.f();//编译报错
        op.g();//编译报错

        WithFinal wf = op2;
        wf.f();//编译报错
　　　　 wf.g();//编译报错

    }
}
为什么？？？