java - Getters and setters in abstract class? -

i'm working on game in java , have problem using setters , getters abstract class. let's class looks this:

public abstract class objects {     protected int velx, vely;      public objects(int velx, int vely){         this.velx = velx;         this.vely = vely;     }      //getters, setters , other methods } 

now, if want use these methods in class not extend abstract class objects, not work properly. game run throws nullpointerexception when tries execute part of code getters , setters. example:

public class hud {     private objects o;      public hud(......)      public void somemethod(){         if (o.getvelx > 0){             o.setvely(3);         }     } } 

this bad example, if ran throw nullpointerexception. have initialize class objects in different way?

i order use o, must initialize reference instance of concrete class extends objects abstract class. otherwise o remains null, , call o.something throws nullpointerexception. has nothing setters or getters.