In Java, you can sort objects that implement the Collection interface, using either the comparable interface or a comparator class. What's the difference?

A Comparable interfaced class must contain a method called compareTo to compare two objects (one being the object on which it is called and the other being passed as a paramater) which returns an integer, negative for a < b, positive for a > b and 0 otherwise. It must also contain an equals method, returning a boolean, on the same parameters.

A Comparator is a class in its own right, which implements the Comparator interface; that means it must contain a method called compare (two objects as parameters) which returns a negative, zero or positive integer depensing on whether the first object is less than, equal to, or greater than the second.

If you want to sort a collection using its comparable interface, you simply call the static Collections.sort method on it ... so if we had an ArrayList called Actable which implemented Comparable, we could write:
Collections.sort(Actable);

To sort a collection using a Comparator class, you need to pass an extra parameter into the Collections.sortmethod - that parameter being an instance of a Comparator object. Thus:
Collections.sort(Actable, new Bynum());

So in effect, using the comparable interface gives you a default sort order of your own making, whereas the comparator is more flexible as it allows you define a whole series of different types of sorting that can be applied to a whole lot of different types of objects.
(written 2007-02-02, updated 2007-02-05)

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