Removing Elements During an ArrayList Traversal

A warehouse list units holds the number of units left of each product: 8, 3, 2, 12, and 4. Any product with fewer than 5 units goes on a reorder sheet and comes off this list.

for (int i = 0; i < units.size(); i++)
{
    if (units.get(i) < 5)
    {
        units.remove(i);
    }
}

The pass with i at 1 removes the 3. The 2 behind it shifts down to index 1, but i goes on to 2, so the 2 is never examined.

The loop stops when i reaches 4, since the size is 3, and the list ends as 8, 2, and 12. The loop throws no exception, because the condition calls size() before each pass and the size drops with each removal. So i never runs past the last index, though the list it leaves is wrong.

After a removal at index i, the next unexamined element is at index i, so the index shouldn't advance on that pass. A while loop can advance i only on passes that remove nothing.

int i = 0;
while (i < units.size())
{
    if (units.get(i) < 5)
    {
        units.remove(i);
    }
    else
    {
        i++;
    }
}

This loop examines all five values and leaves 8 and 12.

The other way is to traverse from the last index down to 0. A removal shifts only the elements after it, which a backward traversal has examined already.

Removing or adding inside an enhanced for loop over the same list changes its size while the loop is running. That can throw a ConcurrentModificationException. Reading the elements in an enhanced loop is safe.