Basic Comparator

The Comparable interface is a good choice to use for defining the default ordering, or in other words, if it's the main way of comparing objects. So why use a Comparator if we already have Comparable?

There are several reasons why: - Sometimes we can't modify the source code of the class whose objects we want to sort, thus making the use of Comparable impossible - Using Comparators allows us to avoid adding additional code to our domain classes - We can define multiple different comparison strategies, which isn't possible when using Comparable

public class Player implements Comparable<Player> {
    @Override
    public int compareTo(Player otherPlayer) {
        return Integer.compare(getRanking(), otherPlayer.getRanking());
    }
}

If the comparison returns	Then ...
>= 1	this.rank > other.rank
0	this.rank == other.rank
<= -1	this.rank < other.rank

public class PlayerRankingComparator implements Comparator<Player> {

    @Override
    public int compare(Player firstPlayer, Player secondPlayer) {
       return Integer.compare(firstPlayer.getRanking(), secondPlayer.getRanking());
    }
}

Lambda

Comparator<Player> comparator = (p1, p2) -> p1.getRanking() - p2.getRanking();