Question

If I have a rarely used collection in some class which may be instantiated many times, I may sometimes resort to the following “idiom” in order to save unnecessary object creations:

List<Object> list = null;

void add(Object object) {
    if (list == null)
        list = new ArrayList<Object>();

    list.add(object);
}

// somewhere else
if (list != null)
    for (Object object : list)
         ;

Now I was wondering if I couldn’t eliminate those null checks using Collections.emptyList(), however then I would have to alter the if check in add() like so:

if (list == Collections.<Object>emptyList())
    list = new ArrayList<Object>();

Is there a better way to handle this other than just allocating a new empty collection every time?

EDIT: just to be clear, I would like to use Collections.emptyList(), but the above check in add() is really really ugly… I was wondering if there’s a better way to do it or even a whole other way of handling this.

Answer 1

in order to save unnecessary object creations

That’s a really bad idea which will litter your code with == null checks and other handling of corner cases (and presumably end up in null pointer exceptions anyway)!

Now I was wondering if I couldn’t eliminate those null checks using Collections.emptyList()

No, not really. emptyList() returns an empty list. You could do

if (list.equals(Collections.<Object>emptyList()))

but that will still throw a NullPointerException if list == null, so it’s still not what you’re after.

My recommendation: Always initialize the list to new ArrayList<Object>, or, if you for instance want to return an empty list from a method, use Collections.emptyList() instead. (This returns the same instance every time, so no unnecessary object creation there either.)

And then use .isEmpty() to check if a collection is empty or not.