Comparables and Comparators

What is it? #

A Comparable is a generic type that allows standardized comparisons between objects.

In other words, anything that has a compareTo() method can be a Comparable!

Many Java libraries already use Comparable without you knowing! Some of the more well-known ones are Collection and String.

CompareTo can’t return anything you want! #

There are some very specific properties CompareTo needs to have! Usually, we take them for granted but might forget about them when making our own.

• If x and y are the same object, y.compareTo(x) must return 0.
• x.compareTo(y) must return the negative of y.compareTo(x). (if one throws an error, the other must too!)
• If x and y are the same object, x.compareTo(z) must equal y.compareTo(z) for all z.

Defining a Comparable subclass #

public class MyComparable implements Comparable<MyComparable> {
    public int foo;
    ...

    /** Instance method that has nothing to do with comparable */
    public void doSomething() {
        ...
    }

    /** Comparable method used to compare objects of this type */
    public int compareTo(Object o) {
        MyComparable mc = (MyComparable) o;
        return ...
    }
}

Comparators #

Comparators are used instead of higher order functions in order to provide a callback function to methods. One example of where it is used commonly is Collections.sort. You can pass in a comparator here to change how items are sorted- for example, you could sort Person objects by their height variable.

The interface is as follows:

public interface Comparable<T> {
 int compare(T o1, T o2);
}

How is it different from Comparables??? #

Comparable is used to compare itself to other objects; a Comparator compares two other objects but not itself.