[algorithm] Algorithm to return all combinations of k elements from n

There is no need for collection manipulations. The problem is almost the same as cycling over K nested loops but you have to be careful with the indexes and bounds (ignoring Java and OOP stuff):

 public class CombinationsGen {
    private final int n;
    private final int k;
    private int[] buf;

    public CombinationsGen(int n, int k) {
        this.n = n;
        this.k = k;
    }

    public void combine(Consumer<int[]> consumer) {
        buf = new int[k];
        rec(0, 0, consumer);
    }

    private void rec(int index, int next, Consumer<int[]> consumer) {
        int max = n - index;

        if (index == k - 1) {
            for (int i = 0; i < max && next < n; i++) {
                buf[index] = next;
                next++;
                consumer.accept(buf);
            }
        } else {
            for (int i = 0; i < max && next + index < n; i++) {
                buf[index] = next;
                next++;
                rec(index + 1, next, consumer);
            }
        }
    }
}

Use like so:

 CombinationsGen gen = new CombinationsGen(5, 2);

 AtomicInteger total = new AtomicInteger();
 gen.combine(arr -> {
     System.out.println(Arrays.toString(arr));
     total.incrementAndGet();
 });
 System.out.println(total);

Get expected results:

[0, 1]
[0, 2]
[0, 3]
[0, 4]
[1, 2]
[1, 3]
[1, 4]
[2, 3]
[2, 4]
[3, 4]
10

Finally, map the indexes to whatever set of data you may have.