How to generate random non-same numbers?

Question

Android

Hello everyone)
We need a random number generator, but so that the numbers do not repeat ..
I do this:

Random random = new Random();

    ..
    ArrayList<Integer> rtn = getFalseRandom(5);

        for(int i=0;i<rtn.size();i++){
            Log.d("TAG", "LOL "+rtn.get(i).toString());
        }
    ...

    public ArrayList<Integer> getFalseRandom(int count){
        ArrayList<Integer> rtn = new ArrayList<Integer>();
        for(int i=0;i<3;i++) { // 3 потому что нужно 3 числа
            int n = getContainsRandom(rtn, count)+1;
            rtn.add(n);
        }
        return rtn;
    }

    public int getContainsRandom(ArrayList<Integer> list, int count){
        int n = random.nextInt(count);
        if(!list.contains(n)) {
            return n;
        }
        return getContainsRandom(list,count);
    }

But it does not work as it should, there are still the same numbers ...
Please tell me why

Answer 1

At you linear complexity (search in an array) plus a recursion on such simple operation.

Set<Integer> used = new HashSet<>();
...
public int getContainsRandom( Set<Integer> used ){
    int i;
    do{
        i = random.nextInt(...);
    while(used.contains(i));
    used.put(i);
    return i;
}