Question

I have a problem with random and arrays. I have a list empty list and I want to add 8 some random values, but I don’t want to some values was the same. To get some random value I use this code:

        for (int i = 0; i < 8; i++) {
            round = random.nextInt(31);
            while (temp.get(round).equals(mix)) {
                round = random.nextInt(31);
            }
            mix.add(temp.get(round));
        }

temp is list with 32 objects and mix is my list where I want to add 8 random values. But when I random some values sometimes I get the same values. How I can get random values but without duplicates?

Answer 1

While others have described ways to implement a new solution, I think yours can be fixed by changing a single line:

while (temp.get(round).equals(mix))

I guess the condition is supposed to check whether the element of temp at position round is already in mix. This should do:

while (mix.contains(temp.get(round))

The problem with the first line is that equals is defined to compare an object to any other kind of object, and thus compares an element of temp, which is probably not a collection itself, to mix, which is. Thus, the condition is always false. Collection.contains is the method to use, and you need to use the one of mix. (As others have also pointed out, a Set is best suited to a case where contains is applied often, but for 8 cases, as you need, it does not matter much whether mix is a List instead).

[edit]: By the way: since the upper bound in random.nextInt(int) is excluded, you should only ever get the final element of 32-size temp in your mix if you use nextInt(32).