list.java

List<Pair<Integer, Integer>> indices = new ArrayList<>();
// indeices will be list containing a Pair
// Pair first and second will be indecies of the numbers whose some will be "b"

int[] a = {1, 3, 4, 5, 2}; // your list
int b = 5;  // number you want to compare
for (int i = 0; i < a.length; i++) {
  for (int j = i + 1; j < a.length; j++) {
    if (b == a[i] + a[j]) {
      indices.add(Pair.create(i, j));
    }
  }
}