public class TuGiac {
  protected Diem d1, d2, d3, d4;
  public void setD1(Diem _d1) {d1=_d1;}
  public Diem getD1(){return d1;}
  public void printTuGiac(){...}
  …
}

public class HinhVuong extends TuGiac {
    public HinhVuong(){
        d1 = new Diem(0,0); d2 = new Diem(0,1);
        d3 = new Diem(1,0); d4 = new Diem(1,1);
    }
}
public class Test{
    public static void main(String args[]){
        HinhVuong hv = new HinhVuong();
        hv.printTuGiac();
    }
}