Solution1

public Set<Integer> pickDistinctNumbers(int k, int[] numbers){
	if(numbers == null || numbers.length == 0) return null;
	int len = numebrs.length;
	if(k <= 0 || k > len) return null; // ask about these situations. this is just an example

	Set<Integer> result = new HashSet<Integer>(); // Sets.newHashSet() if you use Google Guava
    int size = 0;
    while(size < k){
        int picked = (int) (Math.random() * len);
        if(result.add(numbers[picked])) size++;
    }
    return result;
}

Solution2

public Set<Integer> pickDistinctNumbers(int k, int[] numbers){
    if(numbers == null || numbers.length == 0) return null;
	  int len = numebrs.length;
	  if(k <= 0 || k > len) return null; // ask about these situations. this is just an example
	  
	  Set<Integer> result = new HashSet<Integer>(); // Sets.newHashSet() if you use Google Guava
	  int size = 0;
	  while(size < k){
	      int picked = (int) (Math.random() * len);
	      result.add(numbers[picked]);
	      size++;
	      numbers[picked] = numbers[len-1];
	      len--;
	  }
	  return result;
}