java - Generating unique random number is increasing complexity and might cause performance overhead? -

implementation:

private static list<integer> getrandomdistribution(list<string> unsortedlist, int max) {     random random = new random();     list<integer> indexcontainer = new arraylist<>();     (int = 0; < max; i++) {         int index = random.nextint(max);         // below don't like,          if (indexcontainer.contains(index)) {             i--;         } else {             indexcontainer.add(index);         }     }     return indexcontainer; } 

so saying that, until don't find required unique random number. continue loop, happen might keep looping long time increasing overhead.

problems:

• int index = random.next(max) should decide randomness, have maintain ordering. why have used list
• secondly, i-- stuck, because frankly don't implementation.

note: have maintain order within indexcontainer.

since generating permutation of numbers 0 max-1, make more sense populate list numbers 0 max-1 , call collections.shuffle(list).

random random = new random(); list<integer> indexcontainer = new arraylist<>(); (int = 0; < max; i++) {     indexcontainer.add(i); } collections.shuffle(indexcontainer, random);