Question

As the title mentions, if I create an ArrayList with initial capacity 500 and then clear it after some time will its capacity still be 500 ? or do I need to re-init it for that ?

Answer 1

Yes, it preserves its capacity. (at least, not in the implementation of the Oracle VM):

/**
 * Removes all of the elements from this list.  The list will
 * be empty after this call returns.
 */
public void clear() {
    modCount++;

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

    size = 0;
}

Just to be clear: an ArrayList is backed by an array (e.g. int[] for ArrayList) and that array is expanded whenever you go over capacity by creating a new array and copying things over. Clearing will not (as seen in the code) create a new smaller array, copy things there, and destroy the old big array.