Collections.emptyList() instead of null check?

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.

Answer 2

The suggested answers are absolutely correct, just small tip - in Java 8 you can use the new Optional class to handle the case where the list instance is null, in a more functional approach.

For example, something like this:

public static List<String> addElement(List<String> list, String toAdd) {        List<String> newList = Optional.ofNullable(list).orElse(new ArrayList<>());        newList.add(toAdd);        return newList; } 

Following a tip in the comments, it's better to replace new ArrayList<>() with Collections.emptyList() in order to prevent the creation of a new instance of an empty ArrayList

public static List<String> addElement(List<String> list, String toAdd) {    List<String> newList = Optional.ofNullable(list).orElse(Collections.emptyList());    newList.add(toAdd);    return newList; }