Using Collections API to Shuffle

Question

I am getting very frustrated because I cannot seem to figure out why Collections shuffling is not working properly.

Lets say that I am trying to shuffle the 'randomizer' array.

    int[] randomizer = new int[] {200,300,212,111,6,2332}; 
    Collections.shuffle(Arrays.asList(randomizer));

For some reason the elements stay sorted exactly the same whether or not I call the shuffle method. Any ideas? Thanks in advance.

Answer 1

Arrays.asList cannot be used with arrays of primitives. Use this instead:

Integer[] randomizer = new Integer[] {200,300,212,111,6,2332}; 
Collections.shuffle(Arrays.asList(randomizer));

The same rule applies to most classes in the collections framework, in that you can't use primitive types.

The original code (with int[]) compiled fine, but did not work as intended, because of the behaviour of the variadic method asList: it just makes a one-element list, with the int array as its only member.

Answer 2

Chris' answer is correct.

As i said in a comment on Chris' answer, your underlying array will change appropriately unless the arraylist needs to grow, and the list creates a new one and copies items into it.

You may want to keep a reference to the list and iterate over that after the Arrays.asList call, and not iterate over the array after that, iterate over the List instead.