11.4 Comparators

We've just learned about the comparable interface, which imbeds into each Dog the ability to compare itself to another Dog. Now, we will introduce a new interface that looks very similar called Comparator.

Let's start off by defining some terminology.

• Natural order - used to refer to the ordering implied in the compareTo method of a particular class.

As an example, the natural ordering of Dogs, as we stated previously, is defined according to the value of size. What if we'd like to sort Dogs in a different way than their natural ordering, such as by alphabetical order of their name?

Java's way of doing this is by using Comparator's. Since a comparator is an object, the way we'll use Comparator is by writing a nested class inside Dog that implements the Comparator interface.

But first, what's inside this interface?

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

This shows that the Comparator interface requires that any implementing class implements the compare method. The rule for compare is just like compareTo:

• Return negative number if o1 < o2.

• Return 0 if o1 equals o2.

• Return positive number if o1 > o2.

Let's give Dog a NameComparator. To do this, we can simply defer to String's already defined compareTo method.

import java.util.Comparator;

public class Dog implements Comparable<Dog> {
    ...
    public int compareTo(Dog uddaDog) {
        return this.size - uddaDog.size;
    }

    private static class NameComparator implements Comparator<Dog> {
        public int compare(Dog a, Dog b) {
            return a.name.compareTo(b.name);
        }
    }

    public static Comparator<Dog> getNameComparator() {
        return new NameComparator();
    }
}

Note that we've declared NameComparator to be a static class. A minor difference, but we do so because we do not need to instantiate a Dog to get a NameComparator. Let's see how this Comparator works in action.

As you've seen, we can retrieve our NameComparator like so:

Comparator<Dog> nc = Dog.getNameComparator();

All in all, we have a Dog class that has a private NameComparator class and a method that returns a NameComparator we can use to compare dogs alphabetically by name.

Let's see how everything works in the inheritance hierarchy - we have a Comparator interface that's built-in to Java, which we can implement to define our own Comparators (NameComparator, SizeComparator, etc.) within Dog.

To summarize, interfaces in Java provide us with the ability to make callbacks. Sometimes, a function needs the help of another function that might not have been written yet (e.g. max needs compareTo). A callback function is the helping function (in the scenario, compareTo). In some languages, this is accomplished using explicit function passing; in Java, we wrap the needed function in an interface.

A Comparable says, "I want to compare myself to another object". It is imbedded within the object itself, and it defines the natural ordering of a type. A Comparator, on the other hand, is more like a third party machine that compares two objects to each other. Since there's only room for one compareTo method, if we want multiple ways to compare, we must turn to Comparator.