code: public void setConer(int index, Vector3D v) {
          if (index <= 0) {
              this.x1 = v.getX();
              this.y1 = v.getY();
          } else if (index == 1) {
              this.x2 = v.getX();
              this.y2 = v.getY();
          } else if (index == 2) {
              this.x3 = v.getX();
              this.y3 = v.getY();
          } else if (3 <= index) {
              this.x4 = v.getX();
              this.y4 = v.getY();
          }
          calcG();
      }
docs: Sets coordinates of a corner.
  
  @param index The index of a corner.
  @param v The coordinates of a corner.