Compact shuffling code in Java

Question

I need to generate an array of integers in random order repeatedly in Java. I came up with the following ugly snippet (which runs in a bigger loop). What's a more efficient or compact way to do the same?

    ArrayList<Integer> t = new ArrayList<>();
    int[] d = new int[elts];
    for (int i = 0; i < elts; i++) {
        t.add(i);
    }
    Collections.shuffle(t);
    for (int i = 0; i < elts; i++) {
        d[i] = t.get(i);
    }

http://stackoverflow.com/questions/20058366/shuffle-a-list-of-integers-with-java-8-streams-api — hoat4, Jan 20 '17 at 20:17
http://stackoverflow.com/questions/1519736/random-shuffling-of-an-array — Stuart Marks, Jan 21 '17 at 23:15

score 3 · Accepted Answer · answered Jan 20 '17 at 20:12

3

Use the Java 8 Stream API:

    List<Integer> list = IntStream.range(0, elts).boxed().collect(toList());
    Collections.shuffle(list);
    int[] d = list.stream().mapToInt(i -> i).toArray();

answered Jan 20 '17 at 20:12

hoat4

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Vlad Bochenin · Answer 2 · 2017-01-20T23:46:43.843

The same with Java 8 Stream API but using collectAndThen()

IntStream.range(0, elts).  // range from 0 to elts
      boxed().  // from int to Integer
      collect(collectingAndThen(toList(), integers -> { //collect to list  
          Collections.shuffle(integers);  //shuffle integer
          return integers.toArray(new Integer[integers.size()]); // convert list to array
      }));

and with StreamEx

    int[] ints = IntStreamEx.range(0, elts)
            .sorted((o1, o2) -> random.nextInt())
            .toArray();

score 1 · Answer 3 · answered Jan 20 '17 at 20:13

1

Will work well in Java 8

List<Integer> range = IntStream.range(0, elts).boxed().collect(Collectors.toList());
Collections.shuffle(range);
Integer[] arr = new Integer[elts];
range.toArray(arr);

answered Jan 20 '17 at 20:13

ares

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Duplicate, the last two line can be merged, the question has int[] and not Integer[], and use zero sized arrays with toArray (https://shipilev.net/blog/2016/arrays-wisdom-ancients/). – hoat4 Jan 21 '17 at 07:52

Compact shuffling code in Java

3 Answers3