Java Comparable and Comparator

In this tutorial, I will go over two similar but very different interfaces in Java: comparable and comparator. For official documentations on these, refer to here and here.

Comparable and Comparator are both Java interfaces to compare two or more objects. The difference is that comparable is the default comparator of the object, whereas comparator is a custom comparison implementation.

For example, assume you are entering personal info. For each personal info object, you have one's name, year, month, and date of birth. One way to sort a list of personal info objects is by their names, but another way is to sort by the date of birth. In Java, one will be able to sort by name or date of birth by providing corresponding comparator to the class. On the other hand, if one is to simply sort the objects without any comparator provided, then the list will be sorted by the class' comparable interface.

Let's take a look at an example.

To run it, simply type in
$ javac Personal.java && java Personal

Sorting by default
Bob 1999, 1, 2
Charles 1955, 7, 4
Mike 2011, 4, 1
Tom 2011, 4, 4
Tony 2000, 5, 5
Tony 1987, 12, 11

Sorting by date of birth
Charles 1955, 7, 4
Tony 1987, 12, 11
Bob 1999, 1, 2
Tony 2000, 5, 5
Mike 2011, 4, 1
Tom 2011, 4, 4

Sorting by name
Bob 1999, 1, 2
Charles 1955, 7, 4
Mike 2011, 4, 1
Tom 2011, 4, 4
Tony 1987, 12, 11
Tony 2000, 5, 5

In the first sorting, no comparator is provided, so the list is sorted by Personal's comparable interface, namely compareTo method. For the second and third sorting, DobComparator and NameComparator is provided, so the list is sorted by the date of birth and name, respectively. The comparator classes provide compare method which specifies the order in which the list is to be sorted.