Question

Assuming that arraylist is defined as ArrayList<String> arraylist, is arraylist.removeAll(arraylist) equivalent to arraylist.clear()?

If so, can I assume that the clear() method is more efficient for emptying the array list?

Are there any caveats in using arraylist.removeAll(arraylist) instead of arraylist.clear()?

Answer 1

The source code for clear():

public void clear() {
    modCount++;

    // Let gc do its work
    for (int i = 0; i < size; i++)
        elementData[i] = null;

    size = 0;
}

The source code for removeAll()(As defined in AbstractCollection):

public boolean removeAll(Collection<?> c) {
    boolean modified = false;
    Iterator<?> e = iterator();
    while (e.hasNext()) {
        if (c.contains(e.next())) {
            e.remove();
            modified = true;
        }
    }
    return modified;
}

clear() is much faster since it doesn’t have to deal with all those extra method calls.

And as Atrey points out, c.contains(..) increases the time complexity of removeAll to O(n²) as opposed to clear‘s O(n).