Time complexity of contains(Object o), in an ArrayList of Objects

Question

As the title says, I was wondering what the time complexity of the contains() method of an ArrayList is.

Answer 1

If you look into the source code for ArrayList and check its contains method it looks as below:

public boolean contains(Object o) {
    return indexOf(o) >= 0;
}

contains delegates the check to the indexOf method. So, if we check the indexOf implementation it is as follows:

public int indexOf(Object o) {
    if (o == null) {
        for (int i = 0; i < size; i++)
            if (elementData[i]==null)
                return i;
    } else {
        for (int i = 0; i < size; i++)
            if (o.equals(elementData[i]))
                return i;
    }
    return -1;
}

As it can be seen from the code, in order to find an index of a given element, one, in the worst case, must iterate through the whole array. As a size of the array grows and so does the search time by an element. Hence, the time complexity of contains method is O(n), where n is the number of elements in the list.