Can I use the == operator to compare Point objects in Java?

Question

While i am solving the questions from the book "cracking the coding interview"... i got a doubt.the question is:

Given two squares on a two dimensional plane, find a line that would cut these two squares in half.

Solution:Any line that goes through the center of a rectangle must cut it in half. Therefore, if you drew a line connecting the centers of the two squares, it would cut both in half.

   public class Square {
       public double left;
       public double top;
       public double bottom;
       public double right;
       public Square(double left, double top, double size) {
              this.left = left;
              this.top = top;
              this.bottom = top + size;
              this.right = left + size;
       }

       public Point middle() {
              return new Point((this.left + this.right) / 2,
                                           (this.top + this.bottom) / 2);
       }

       public Line cut(Square other) {
              Point middle_s = this.middle();
              Point middle_t = other.middle();
              if (middle_s == middle_t) {
                     return new Line(new Point(left, top),
                     new Point(right, bottom));
              } else {
                     return new Line(middle_s, middle_t);
              }
       }
}

But now the doubt is '==' operator in cut method to check whether they are points of same square. Is Point immutable?? kindly help me... Thanks in advance.

Answer 1

It doesn't matter if Point is immutable. What matters is that the == is doing a pointer/reference comparison. It is checking to see if middle_s and middle_t reference the same Object instance, not whether or not they are the same in terms of their internal fields.

What you probably want to do is implement/override equals() for the Point class such that it compares the x and y coordinates, and returns true if they match. Then you can compare your points by doing if (middle_s.equals(middle_t)).

When you override equals(), be sure you also override hashCode() accordingly, as well.