Collections.emptyList() instead of null check?

user_634020 0
Viewed 77143

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.

empty-list null collections java
9 Answers

There is an emptyIfNull method in package org.apache.commons.collections4;. It returns an empty list if the one provided is null.

List<Object> list = CollectionUtils.emptyIfNull(list);
user_56126290

Here's a variation on using optional as @Stas suggested, but also using the isEmpty immutable collection as originally requested in the question:

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

This approach is also the closest thing I can find to the nice ability in Javascript to use an empty array if the collection is null.

For example:

// no need to indent everything inside a null check of myObjects
for (MyObj myObj : Optional.ofNullable(myObjects).orElse(Collections.emptyList())){
    // do stuff with myObj
}
user_6974800
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