Custom Comparator - Java

Question

I am having trouble understanding the Sorting using a Custom Comparator in Java.

Queue<Integer> q = new PriorityQueue<>(arr.length, (a,b) -> ab);

gives me ascending order sort, while

Queue<Integer> q = new PriorityQueue<>(arr.length, (a,b) -> ba);

gives me a descending order sort.

What I can not understand is how is this working? Is b the incoming element and a already in the array or vice versa? Also, how is the ascending or descending order obtained?

If ab > 0 , this means that a>b , so shouldn't b be infront of a in ascending order?

Answer 1

Comparison-based sorts distinguish values by determining whether or not they are < , = , or > .

For example, 3 < 4 , 4 = 4 , 4 > 3 .

Java Comparators use the convention that

• cmp(a, b) < 0 means a < b
• cmp(a, b) = 0 means a = b
• cmp(a, b) > 0 means a > b

Note that this means if cmp(x, y) = x - y , then you get the normal ordering for the integers. You can check yourself that cmp(x, y) = -(x- y) = y - x gives you the reverse ordering.

When sorting (or doing something else that moves generic elements around by their order, like a PriorityQueue), the algorithm will (repeatedly) consult the comparator to determine whether or not values it has been given are < , = , or > .