14. Comparable

Comparable means that two items in a class can be compared (less than, equal to, greater than). It is similar to the Ord typeclass in Haskell.

Comparable is an interface. You implement it. It is a template, so you put the class you are able to compare to in angle brackets afterwards. I expect this to be the same class as the one that is implementing the Comparable interface.

public class Whatever implements Comparable<Thing> { 
    public int compareTo (Thing other) {
        // ...
    }
}

The compareTo function gives an integer that indicates whether this is greater than other.

• negative means this < other. This could be read “this comes before other”.
• zero means this = other
• positive means this > other

Exercises

1. The Drow class holds a String, but when you compare two Drow instances you get the opposite answer from comparing two strings. Example:

    Drow a = new Drow("cat");
    Drow b = new Drow("dog");
    int cmp = a.compareTo(b);
    // cmp is positive, but would be negative for Strings
   

   Write the Drow class, including any instance variables, constructor, and compareTo method.

2. EBO. The class EBO is like Integer except all evens appear before any odd number in the ordering created by compareTo.

   Examples:

   a	b	a.compareTo(b)	a interpretation b
   2	4	-2	is before
   9	3	6	is after
   2	9	-7	is before
   2	-3	-1 (any neg)	is before
   3	8	+1 (any pos)	is after
   5	10	+1 (any pos)	is after

   The constructor is of the form new EBO(7).

3. The class Pt contains an (x,y) coordinate pair. The ordering of Pt is given by first comparing the x’s and then, if they are equal, comparing the y’s.

   Examples:

   a	b	a.compareTo(b)	a interpretation b
   (20,50)	(20,80)	-30	is before
   (10,5)	(10,2)	3	is after
   (10,30)	(31,19)	-21	is before

   Write a class header, instance variables, constructor, and compareTo method.