Java Comparators in a More Declarative Way

Java 8 has added many intuitive utility methods in the Comparator interface, making it easier to work with comparators in a functional way.

For example, if I want to sort the list of strings in a descending manner with length and they have the same length, I will sort in reverse natural ordering, and, I would, then, do something like this in JDK 7 or prior:

public void sortOldWay(final List < String > words) {
    words.sort(new Comparator < String > () {
        @Override
        public int compare(String s1, String s2) {
            if (s2.length() == s1.length()) {
                return s1.compareTo(s2);
            } else {
                return s2.length() - s1.length();
            }
        }
    });
}

You can see this approach is more imperative, focusing on how to sort the list.

But, with Java 8, rather than specifying how to do that, this can be accomplished in a more declarative way with static methodscomparing(), thenComparing() and   reverseOrder() as: 

public void sortNewWay(final List<String> words) {
        words.sort(Comparator.comparing(String::length).thenComparing(Comparator.reverseOrder()).reversed());
}

Comparator.comparing() takes a Function argument. The type T is the type of object being compared and type U is the type of object that will actually be compared. Here, T is thestring and, since we are comparing lengths of words, the type U   is Integer . So, it possible to write the call to sort even more intuitively as:

public void sortNewWayIntuitive(final List<String> words) {
        final Function<String, Integer> byLength = s -> s.length();
        words.sort(Comparator.comparing(byLength)
                .thenComparing(Comparator.reverseOrder()).reversed());
}

Let us take this to the next level where we want Comparator to work consistently with equals  for objects.

I want to sort the list of Student, where Student has a name and age, by name , then I need to consider age as well or vice versa, to make Comparators  consistent with equals as below:

public enum SORT_METHOD {
    BYNAME,
    BYAGE
}
public void sort(final List < Student > students, final SORT_METHOD method) {
    Collections.sort(students, (s1, s2) - > {
        if (SORT_METHOD.BYNAME == method) {
            if (s1.getName().compareTo(s2.getName()) == 0) {
                if (s1.getAge() == s2.getAge()) return 0;
                else if (s1.getAge() < s2.getAge()) return -1;
                else return 1;
            }
            return s1.getName().compareTo(s2.getName());
        } else {
            if (s1.getAge() == s2.getAge()) return s1.getName().compareTo(s2.getName());
            else if (s1.getAge() < s2.getAge()) return -1;
            else return 1;
        }
    });
}

This can be done in a more declarative way with comparing() and thenComparing() as:

public enum SORT_METHOD {
    BYNAME,
    BYAGE
}
Function < Student, String > byName = e - > e.getName();
Function < Student, Float > byAge = e - > e.getAge();

public void sort(final List < Student > students, final SORT_METHOD method) {
    if (SORT_METHOD.BYNAME == method) {
        Collections.sort(students, Comparator.comparing(byName).thenComparing(byAge));
    } else {
        Collections.sort(students, Comparator.comparing(byAge).thenComparing(byName));
    }
}

This is a more intuitive and declarative method in comparison to the old technique.

Conclusion

There are other utility methods like thenComparingDouble() , thenComparingLong() ,  reverseOrder(),  naturalOrder(),  reversed(), nullsFirst() , etc., making it the easiest comparison easiest. Let me know your thoughts!

For the examples I presented above, all the source code is available on GitHub.