Priority Queue of an array of integers in java

Question

I want to compare the second element of the following array:

int[][] intervals = new int[][]{new int[]{0, 30},new int[]{5, 10},new int[]{15, 20}};

My priority queue with custom comparator:

PriorityQueue<int[]> heap = new PriorityQueue(intervals.length, (a, b) -> a[1] - b[1]);

But I get the below 2 errors:

Line 8: error: array required, but Object found
        PriorityQueue<Integer[]> heap = new PriorityQueue(intervals.length, (a, b) -> a[1] - b[1]);
                                                                                       ^
Line 8: error: array required, but Object found
        PriorityQueue<Integer[]> heap = new PriorityQueue(intervals.length, (a, b) -> a[1] - b[1]);
                                                                                             

Oriole

Does this answer your question? What is a raw type and why shouldn't we use it? – Tula

In short, you shouldn't use a raw type here. new PriorityQueue<>(intervals.length, (a, b) -> a[1] - b[1]) works – Tula

Answer 1

You could use Integer.compare

PriorityQueue<int[]> heap = new PriorityQueue<>(intervals.length, (a,b) -> Integer.compare(a[1],b[1]));

Flump

Answer 2

You should cast a to int array first.

PriorityQueue<int[]> heap = new PriorityQueue(intervals.length, (a, b) -> ((int[])a)[1] - ((int[])b)[1]);

Wyattwyche

This answer is incorrect. See stackoverflow.com/questions/2728793 – Golem

Answer 3

This is because you are using a parameterized class rawly. Add <> after PriorityQueue:

PriorityQueue<int[]> heap = new PriorityQueue<>(intervals.length, (a, b) -> a[1] - b[1]);

Mirtamirth

This answer is incorrect. See stackoverflow.com/questions/2728793 – Golem

Answer 4

Well, in your example, you have a 2D array; however, in your code snippet, you're assuming that a single-dimensional array should be kept in the Queue. So, which is your question? are you sure you want to maintain the Queue of 2D arrays? that doesn't mean, that you would have a Queue of pairs/tuples, that means, that each element in that queue, will be a discrete 2D array object. And in this case, probably each array will have more than one element.. and that means, that you should most likely iterate through them.. but if you're sure what you want with this code to be achieved and it's correct, then you can compare first elements like this:

    PriorityQueue<int[][]> heap = new PriorityQueue<int[][]>((a, b) -> {
        if (a[0][0] > b[0][0]) {
            return 1; //change according to your logic
        } else if (a[0][0] < b[0][0]) {
            return -1; //change according to your logic
        } else {
            return 0; //change according to your logic
        }
    });

Nunciata

Well yes ... but (a, b) -> Integer.compare(a[0][0], b[0][0]) would be more concise. – Golem

Answer 5

You can use:

PriorityQueue<int[]> pq = new PriorityQueue<>(intervals.length,
                             Comparator.comparingInt(interval -> interval[1]));

Masseter

Answer 6

Your code is missing one vital syntax. You can add <> or <int[]> to the RHS of the assignment statement and it will work. So the line statement will become:

PriorityQueue<int[]> heap = new PriorityQueue<int[]>(intervals.length, (a, b) -> a[1] - b[1]);

Irs