Get a random subset from a result set in java

Question

I have a set of 100 object. How can i get a subset of 5 objects from this set ?

I'm doing this for now but it only returns me one object

          int size = memberSet.size();
      Set<Member> randomSet = new HashSet<Member>();
            int item = new Random().nextInt(size);
            int i = 0;
            for(Member mbr : memberSet)
            {
                if (i == item){
                    randomSet.add(mbr);
            }
                i = i + 1;
            }

Dup? http://stackoverflow.com/questions/124671/picking-a-random-element-from-a-set — , Aug 25 '11 at 13:37
see http://stackoverflow.com/questions/136474/best-way-to-pick-a-random-subset-from-a-collection — NPE, Aug 25 '11 at 13:38
Seems like a duplicate but the provided link will allow you to return only one random object. User could want a solution that returns a set of N objects. But the question is a bit unclear. -> @aix yes that looks like a match :D — Johann du Toit, Aug 25 '11 at 13:40
Sorry for my english, but yes, I want to get more than one item in my random subset and none of the questions you link me answers my question. — Kévin_Bransard, Aug 25 '11 at 13:47

dacwe · Accepted Answer · 2011-08-25T13:55:20.620

18

List<Member> list = new LinkedList<Member>(memberSet);
Collections.shuffle(list);
Set<Member> randomSet = new HashSet<Member>(list.subList(0, 5));

Full example:

public static void main(String... args) {

    Set<Member> memberSet = new HashSet<Member>();
    for (int i = 0; i < 100; i++)
        memberSet.add(new Member(i));

    List<Member> list = new LinkedList<Member>(memberSet);
    Collections.shuffle(list);
    Set<Member> randomSet = new HashSet<Member>(list.subList(0, 5));

    System.out.println(randomSet);
}

static class Member {
    final int value;
    public Member(int value) {
        this.value = value; 
    }
    @Override
    public String toString() {
        return "" + value;
    }
}

edited Aug 25 '11 at 13:55

answered Aug 25 '11 at 13:40

dacwe

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Thanks, that's exactly what I needed. Clear and simple ! – Kévin_Bransard Aug 25 '11 at 14:15
4

Just a small detail: it would be more efficient to use `ArrayList` instead of `LinkedList`. Shuffling a `LinkedList` will force Java to create a temporary array, shuffle it, and put things back in the `LinkedList`. While with `ArrayList`, it would be done in place. – Helder Pereira Jul 21 '17 at 06:37

Get a random subset from a result set in java

1 Answers1