Does the List subList() method prevent garbage collection of the rest of the list?

Question

I'm curious to find out how the latest JVMs would handle garbage collecting memory reserved by the following method.

public List<Player> getHallOfFame() {
  ArrayList<Player> listToSort = new ArrayList<Player>(map.values());
  Collections.sort(listToSort, comparator);

  return listToSort.subList(0, 5);
}

At worst I can imagine the JVM keeping the entire contents of listToSort in memory as long as there remain references to the sublist. Does anyone know if that is actually the case? I'm particularly interested in links that can prove this one way or the other for specific JVMs.

Answer 1

Yes, subList is only a "view" onto the existing list. All the data is really in the original list. From the documentation:

The returned list is backed by this list, so non-structural changes in the returned list are reflected in this list, and vice-versa.

So yes, returning a sub-list will keep the original list from being garbage collected.

If you don't want that effect, you basically need to make a copy of the relevant sublist. For example:

return new ArrayList<Player>(listToSort.subList(0, 5));

Answer 2

subList creates a new instance of AbstractList.SubList, which keeps a reference to original list. So if you keep the variable returned by getHallOfFame, it will prevent the gc to clean listToSort.