public class Shape{}

public class Circle extends Shape{}

public class DoIt{
      public static void main(String[] args){

          Shape c = new Circle();
      }
} 

public class GeometricObject {
  public void move (double difX, double difY) {
    //move the object on the x/y grid.
  }
}
 
public class A extends GeometricObject{
  public void specialFunctionA() {
  //do something very important
  }
}
 
public class B extends GeometricObject{
  public void specialFunctionB() {
  //do something very important
  }
}
 
 
public class container() {
  public static void main(String[] args) {
    GeometricObject[] geos = {new A(), new A(), new B(), new B()};
     for (int i = 0; i < geos.length; i++)
       geos[i].move (5,5);
  //As you see, it does not matter, if it's A or B.
  //this is the same as if a graphical method requests a Component Object,
  // because to this method it does not matter if it's a Button or a Textfield or whatever.
  }
}