Question

I have created a method to randomize the list:

public <T> List<T> randomize(List<T> list) {
    LinkedList<T> randomizedList = new LinkedList<>();
    Random random = new Random(list.size());
    for (int i = random.nextInt(); i < list.size(); i++) {
        randomizedList.add(list.get(i));
    }
    return randomizedList;
}

The list that I am passing to this method contains for e.g. five elements. When I am creating Random random = new Random(list.size()); I expect so when I call random.nextInt() what it will return me the random integer which will be an index of the list element.

But when I call random.nextInt(); instead of returning the number from the interval [0, 4] (which I expect to be returned) it returns me the value of for e.g.: -349120689. Which gives me an java.lang.IndexOutOfBoundsException: Index: -349120689, Size: 5.

Why is this happening and how to solve this?

Answer 1

1. new Random(list.size()); this sets the seed of the Random number generator to list.size(); I suggest changing to new Random() (which will give you a seed based on the current time of the system). Preferably though, you’d want to reuse the same Random object at all times.

2. random.nextInt(); here is where you’d want to put random.nextInt(list.size()); which will give you a number from 0 to list.size() – 1.

3. Even with the above changes, your code would just give you a sublist of the list starting at a random index and going until the end. Use Collections.shuffle(list) instead.

To do a real shuffling, you would need to “remember” which elements
you have inserted or not. In pseudo-code, you could do the
following:

• Copy the original list to a new one, let’s call it “orig”
• Create a new, empty, list, let’s call it “result”
• As long as the orig list has elements, get an element at random index and add it to the result list. Remove the chosen element from orig list.
• Return result list.