How to pop items from a collection in Java?

Question

Is there a method in JDK or apache commons to "pop" a list of elements from a java.util.List? I mean, remove the list of elements and return it, like this method:

public Collection pop(Collection elementsToPop, Collection elements) {

  Collection popped = new ArrayList();

  for (Object object : elementsToPop) {
    if (elements.contains(object)) {
      elements.remove(object);
      popped.add(object);
    }
  }

  return popped;
}

Answer 1

If you're looking for a stack-like structure I suggest accepting a Deque (LinkedList is the most common implementation) instead of a Collection.

If you don't actually need to treat it as a stack, just get an iterator from the Collection and use the remove() method:

for (Iterator<SomeType> it = elements.iterator(); it.hasNext(); ) {
    SomeType e = it.next();
    it.remove();
    popped.add(e);
}

Do note that remove is an optional operation, and some implementations may throw an UnsupportedOperationException (for example, the iterator returned by a Collection from Collections.unmodifiable...() will).

Edit: After looking more closely at your question, I think you just need this:

elements.removeAll(elementsToRemove);

If your main point is you need to know exactly which elements were actually popped, I think you're stuck with your original code.

Answer 2

There is no such method in the standard JDK-provided methods. Apache Commons provides the ListUtils.subtract() method.

Edit: As other answerers have noted, your use of the term pop is nonstandard. Usually,

The pop operation removes an item from the top of [a stack]

Wikipedia has a nice description of stacks.