choose unique random numbers with specific range

Question

my problem is I want my program to make four unique random choices in range of numbers between 0 to 3 I tried to do it in random class but I could not, , if you could help by co

Answer 1

You're effectively looking for a random permutation of the integers from 0 to n-1.

You could put the numbers from 0 to n-1 into an ArrayList, then call Collections.shuffle() on that list, and then fetch the numbers from the list one by one:

    final int n = 4;
    final ArrayList arr = new ArrayList(n); 
    for (int i = 0; i < n; i++) {
        arr.add(i);
    }
    Collections.shuffle(arr);
    for (Integer val : arr) {
        System.out.println(val);
    }

Collectons.shuffle() guarantees that all permutations occur with equal likelihood.

If you wish, you could encapsulate this into an Iterable:

    public class ChooseUnique implements Iterable {

        private final ArrayList arr;

        public ChooseUnique(int n) {
            arr = new ArrayList(n);
            for (int i = 0; i < n; i++) {
                arr.add(i);
            }
            Collections.shuffle(arr);
        }

        public Iterator iterator() {
            return arr.iterator();
        }
    }

When you iterate over an instance of this class, it produces a random permutation:

    ChooseUnique ch = new ChooseUnique(4);
    for (int val : ch) {
        System.out.println(val);
    }

On one particular run, this printed out 1 0 2 3.