Java Lambdas：与 PriorityQueue 的比较器

Question

Consider a priority queue

PriorityQueue<Integer> heap = new PriorityQueue<Integer>();

I can define a comparator in two ways here, one by using lambda and the other using the Comparator() class

Assuming a simple integer comparison of 2 variables for both these ways, I would like to know which variable is the value we want to compare and which variable contains the existing value from the queue

For exammple:

    PriorityQueue<Integer> heap = new PriorityQueue<Integer>(new Comparator<Integer>() {
       @Override
        public int compare(Integer a, Integer b) {
            System.out.println(a+" "+b);
            return b-a;
        }
    });

When I call heap.add(2) followed by a heap.add(3) , the second add() call triggers the compare(a,b) function and prints a=3 & b=2 which means that here, a has the new value and b is the existing value from the queue

Similarly, can you tell me if the below lambda expression will work similarly or is it the other way around ?

PriorityQueue<Integer> heap = new PriorityQueue<Integer>((a,b) -> b-a);

I know the question could have been written in much simpler manner but I just couldn't manage to do it at the moment.

Addtionally, these two print the same values ie decreasing order 4,3,2,1

     PriorityQueue<Integer> heap2 = new PriorityQueue<Integer>(new Comparator<Integer>() {
       @Override
        public int compare(Integer a, Integer b) {
            // System.out.println(a+" "+b);
            return b-a;
        }
    });
    
    PriorityQueue<Integer> heap = new PriorityQueue<Integer>(
        (a,b) -> b-a
    );
    
    heap.add(1);
    heap.add(2);
    heap.add(3);
    heap.add(4);
    
    heap2.add(1);
    heap2.add(2);
    heap2.add(3);
    heap2.add(4);
    
    while (!heap.isEmpty()) {
        System.out.println(heap.poll()+" "+heap2.poll());
    }

Answer 1

Your two ways to define the Comparator are equivalent. Your lambda version actually expands to the other version after compiling, because Comparator is a functional interface.

Here's a third shorter way to define your Comparator , using a static factory method on the Comparator interface:

PriorityQueue<Integer> heap3 = new PriorityQueue<>(Comparator.reverseOrder());