How to sort an IntStream in reverse order

Question

I'm reading in numbers from a .txt file using BufferedReader. I want to reverse the order of elements in this steam so that when they are collected they will be arranged from the highest to the lowest. I don't want to sort after the array has been built because I have no idea how many elements might be in it, I only need the highest N elements.

in = new BufferedReader(reader);
                int[] arr = in.lines()
                        .mapToInt(Integer::parseInt)
                        .sorted()
                        .limit((long) N)
                        .toArray();

Answer 1

Try negating the values before sorting and negating (back to normal) after sorting:

in = new BufferedReader(reader);
int[] arr = in.lines()
              .mapToInt(Integer::parseInt)
              .map(i -> -i).sorted().map(i -> -i)
              .limit((long) N)
              .toArray();

Answer 2

Because the reverse order is not the natural order, sorted() can't be used to sort in reverse order. If you avoid the IntStream, using a Stream<Integer> instead, then you can use a Collections.reverseOrder() to sort the stream in a reverse to the natural order. Then you can call mapToInt and convert to int[] at the end.

int[] arr = in.lines()
            .map(Integer::valueOf)  // Extract Integer, not int
            .sorted(Collections.reverseOrder())  // On Stream<Integer>
            .limit(N)
            .mapToInt(i -> i)       // map Integer to int
            .toArray();

Answer 3

when using Instream, you are actually dealing with primitive, and your hands are tight (you are limited to natural ordering and you can't define custom comparator. You have two solutions:

• stick with the primitive stream and come up with hacks like the one proposed by @normanrz

• or you can convert to Integer (box) and use variety of solution like the one bellow (but be advised this boxing and unboxing might cause performance problems).

  int[] sortedArray = IntStream.of(costs).boxed()
                                .sorted(Collections.reverseOrder())
                                .mapToInt(value -> value.intValue()).toArray();