ArrayList, double contains

Question

private static boolean moreThanOnce(ArrayList<Integer> list, int number) {
    if (list.contains(number)) {
        return true;
    }
    return false;
}

How do I use list.contains to check if the number was found in the list more than once? I could make a method with for loop, but I want to know if it's possible to do using .contains. Thanks for help!

Answer 1

You can't do it just with contains. That just tests if the item is anywhere in the list.

You can do it with indexOf, though. There are two overloads of indexOf, one of which allows you to set a position in the list to start searching from. So: after you find one, start from one after that position:

int pos = list.indexOf(number);
if (pos < 0) return false;
return list.indexOf(number, pos + 1) >= 0;

Or replace the last line with:

return list.lastIndexOf(number) != pos;

If you want a more concise way (although this iterates the whole list twice in the case that it's not found once):

return list.indexOf(number) != list.lastIndexOf(number);

Answer 2

You can use Streams:

private static boolean moreThanOnce(ArrayList<Integer> list, int number) {
    return list.stream()
               .filter(i -> i.equals (number))
               .limit(2) // this guarantees that you would stop iterating over the
                         // elements of the Stream once you find more than one element
                         // equal to number
               .count() > 1;
}