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Java - generate unique random numbers

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Created by:

Inferio

328

There is couple of ways to generate unique random numbers in java.

1. List with Collections.shuffle

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List<Integer> list = new ArrayList<>();
for (int i = 1; i <= 10; i++) {
    list.add(i);
}
Collections.shuffle(list);
​
// [8, 9, 7, 2, 10, 5, 3, 6, 1, 4]
System.out.println(list);

Collections.shuffle(list) - java docs:
Randomly permutes the specified list using a default source of randomness.
All permutations occur with approximately equal likelihood.

2. HashSet with random numbers between 1 and 20

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Set<Integer> unique = new HashSet<>();
​
while (unique.size() != 10) {
    int randInt = ThreadLocalRandom.current().nextInt(1, 20);
    unique.add(randInt);
}
​
// [16, 1, 17, 18, 6, 9, 10, 13, 14, 15]
System.out.println(unique.toString());

We generate new unique numbers until HashSet has expected size.

3. Java 8 - Random + IntStream + distinct + limit

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List<Integer> list = ThreadLocalRandom.current()
        .ints(1, 20)
        .boxed()
        .distinct()
        .limit(10)
        .collect(Collectors.toList());
​
// [3, 17, 9, 15, 4, 19, 18, 16, 14, 8]
System.out.println(list);