How do I compare objects based on their location on a line

Question

i am trying to write a compare to method that will do this.

// If a and b lie on the same horizontal line, 
//    then a < b when a is to the left of b
// Otherwise, a < b when a is below b   

i dnt really know how to do this, usually i just compare if a>b return +ve int and -ve int if less than or 0 for equal.

My solution from your suggestions.........

i used ideas form Jim Blackler, Ralph and Peter Lawrey and came up with this. it works sorry i was a little confused and did not think of the Cartesian cordinates thanks Aasmund Eldhuset, this is my final compare method.. and it works

class Lexicographical implements Comparator{ //This needs to be rewritten so...
// If a and b lie on the same horizontal line, // then a < b when a is to the left of b // Otherwise, a < b when a is below b
public int compare(Point a, Point b) { if (ay == by) // y axis are the same(same line) { if(ax < bx)// to the left of b(on the x axis) return -1; else return 1;// to the right of b } else if(ay < by)// y axis are not the same(below not same line) { return -1;

    }
    else 
        return 0;
}

}

Answer 1

Do you mean like

class Point implements Comparable<Point> {
  double x,y;

  public int compareTo(Point b) {
      // If a and b lie on the same horizontal line, 
      if (y == b.y)
      //    then a < b when a is to the left of b
          return x < b.x ? -1 : x > b.x ? +1 : 0;
      // Otherwise, a < b when a is below b
      return y < b.y ? -1 : y > b.y ? +1 : 0;
  }
}

Answer 2

You'll need a class attribute for whatever types a and b are that you can compareTo. It's easy once you starting thinking more concretely.

What are the types for a and b?

Answer 3

if (a.y == b.y) {
  return a.x < b.x;
} else {
  return a.y < b.y;
}

Answer 4

From the text, it sounds like a and b are Cartesian coordinates (each with an x and a y value). Do you have a class that represents coordinates? If not, you should make one, and the compare method should take two instances of that class, and you can use their x and y values to determine whether they are on the same horizontal line, or which one is to the left of the other.

Answer 5

One way to implement a compare Method in java is the Comparable Interface

It has only one method int compareTo(T o) . This method compares this object ( the object on which the method is invoked ) with the specified object ( b ). Returns a negative integer, zero, or a positive integer as this object is less than, equal to, or greater than the specified object.

Assume the object is of class Pseudo ( because it is only pseudo code ), then the compare method should looks like this.

class Pseudo implements Comparable<Pseudo> {

   @Override
   int compareTo(Pseudo b) {

     if (this and b lie on the same horizontal line) {
        if (this is to the left of b) {
           return -1;
        } else {
          return 1;
        }
      } else {
         if (this is to the below of b) {
           return -1;
         } else {
            return 1;
         }
      }
   }
}