Java TreeSet<Pair<Integer, Integer>> sorted by the value of the Pair element, how to insert a new pair that has the same value as an existing pair?

I currently have a TreeSet that contains elements of type Pair, and I’m sorting them by their value in descending order.

Here is the link to the original Pair class documentation: https://docs.oracle.com/javase/9/docs/api/javafx/util/Pair.html

This is the declaration:

TreeSet<Pair<Integer, Integer>> sortedSet = new TreeSet<Pair<Integer, Integer>>((a,b) -> b.getValue() - a.getValue());

My issue is that if I try to insert a few pairs into the set, for example: Pair(4, 51), Pair(8, 85), Pair(1, 16), Pair(2,51), the Pair(2,51) doesn’t get inserted.

Code for inserting the pairs:

sortedSet.add(new Pair<Integer, Integer>(4, 51));
sortedSet.add(new Pair<Integer, Integer>(8, 85));
sortedSet.add(new Pair<Integer, Integer>(1, 16));
sortedSet.add(new Pair<Integer, Integer>(2, 51));

I managed to figure out that the reason is because there already exists a pair element with the same value – P(4,51), however, they have different keys so they are different elements, and I expected the Pair(2,51) to be inserted before or after P(4,51).

Is there a way I can solve this issue?

>Solution :

The issue is caused by the definition of the Comparator: (a,b) -> b.getValue() - a.getValue()

Given this Comparator, two Pairs with the same value are "equal". If we want to store two Pairs with different values, we need to change the Comparator. I recommend to first sort by value (descending, hence the .reversed(), then by key (either ascending or descending, I choose ascending):

final TreeSet<Pair<Integer, Integer>> sortedSet =
    new TreeSet<>(Comparator
        .comparingInt(Pair<Integer, Integer>::getValue).reversed()
        .thenComparing(Pair::getKey));

Ideone demo