Extends Error

Hi guys,
 I have a class("myplatform") which extends another("platform"). From a
main class I instanciate "myagent" but when I try to use any of the
methods inside Platform (the superclass of the class I just
instanciated) , I get a nullpointer error.
I checked the code many times and if I call the method directly from
the superclass it works correctly, (but I shouldn't do that really).
Somebody know why?
Here is simplified code of "platform" :

-----------------------------------------------------------------------------------------------------------------------------------------
package agentsLibrary;
public class Platform {
    private String listaserver;
    private static PathEl[] vettoreservers=null;

    public Platform(String listaserver){
        this.listaserver=listaserver;
        //vettoreservers is then retrieved from an xml, I checked and is
correct!
    }

    synchronized public static PathEl[] getRemainingServers(){
        return vettoreservers;
    }

//...and other get methods like that...
}
-------------------------------------------------------------------------------------------------------------------------------------------

and code of MyPlatform (the subclass)

package test;
import agentsLibrary.Platform;

public class MyPlatform extends Platform{

    private HashMap prodmap=null;
    private ProductType pt=null;
    private String nomefile;

    public MyPlatform(String listaserver) {
        super(listaserver);
        this.prodmap=new HashMap();
        nomefile="prodotti.xml";

    }

    public synchronized void loadInfo() {

        //loads some info

    }

    synchronized public HashMap getProductsMap(){
        return prodmap;
    }

}

----------------------------------------------------------------

A main class creates an instance of myplatform , then tries to call
"synchronized public static PathEl[] getRemainingServers()" (PathEl[]
dpath=platf.getRemainingServers();) from the superclass "Platform",
but get nullpointer!!
(note that if I check the content in platform and myplatform is correct
and not null!!!)

Thanks a lot javaboys!