What is the second argument doing in this function?

Arrays.sort(courses, (a, b) -> a[1] - b[1]);
PriorityQueue<Integer> queue = new PriorityQueue<>((a, b) -> b - a);

I tried looking at the docs but cannot understand what the second argument is responsible for. I know that .sort(array, 1, 4) means sort from index 1 to 3. But in this case, the arrow with -> a[1] - b[1] and ((a, b) -> b - a) is what I am unable to understand.

>Solution :

The arrow represents a lambda function that compares any two arguments a and b. Before lambdas were introduced (in Java 8), the sort would look like this:

Arrays.sort(courses, new Comparator<int[]>() {
    @Override
    public int compare(int[] a, int[] b) {
        return a[1] - b[1];
    }
});

I’m taking a guess at the type of a but feel free to substitute int[] with any numeric array.