Generating an array of random numbers without duplicates [duplicate]

Answer 1

As far as I understand what you are doing, a better way would/could be:

List<Integer> myList = new ArrayList<Integer>();
for ( int i = 0; i < expectedNumberOfUniques; i++){
  Integer a = generateRandom();
  while ( myList.contains(a)){
    a = generateRandom();
  }
  myList.add(a);
}

A few remarks, though: the more elements in myList, the less efficiënt it might be less efficiënt.

But: as soon as you limit it to 'no duplicates allowed', we are no longer talking about 'random' numbers.

Answer 2

Since you said without any libraries you could reimplement the Collections.shuffle(...) method for arrays.

int [] randoms = new int[5];

// creating an array containing the numbers 1-10
int [] shuffleArray = new int[10];
for (int i = 1; i <= 10; i++) {
    shuffleArray[i-1] = i;
}

// shuffling that array
Random random = new Random();
for (int i = 0; i < 10; i++) {
    int j = random.nextInt(10);
    int tmp = shuffleArray[i];
    shuffleArray[i] = shuffleArray[j];
    shuffleArray[j] = tmp;
}

// assigning the first 5 values to the random array
for (int i = 0; i < randoms.length; i++) {
    randoms[i] = shuffleArray[i];
}

Although i have to remark that Random needs an import too (needs a library too)