code: public Vector3D getConer(int index) {
          Vector3D v = new Vector3D(0, 0, 0);
  
          if (index <= 0) {
              v.set(this.x1, this.y1);
          } else if (index == 1) {
              v.set(this.x2, this.y2);
          } else if (index == 2) {
              v.set(this.x3, this.y3);
          } else if (3 <= index) {
              v.set(this.x4, this.y4);
          }
          return v;
      }
docs: Gets coordinates of a corner.
  
  @param index The index of a corner.