JAVA多态的理解

package thinking.in.java;
public class PrivateOverride {
    private void f(){
        System.out.println("private f()");
    }
    private void ss()
    {
        System.out.println("this is a base class private method");
    }
    public void se()
    {
        System.out.println("this is a base class public method");
    }
    public final void sf()
    {
        System.out.println("this is a base class final method");
    }
    public static void main(String[] args) {
        // TODO Auto-generated method stub
            PrivateOverride po=new Derived();
            po.f();
            po.ss();
            po.se();
            po.sf();
            ((Derived) po).sp();
            
            Derived dd=    new Derived();
            dd.f();
            dd.sp();
            dd.se();
            dd.sf();
    }

}
class Derived extends PrivateOverride{
    public void f(){
        System.out.println("public f()");
    }
    public void sp(){
        System.out.println("Base Class don‘t have");
    }
}