Using Collections API to Shuffle
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05-07-2019 - |
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?
Solution
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.
OTHER TIPS
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.