Java List.contains() question

Question

According to the documentation, List.contains can throw NullPointerException in this scenario:

"if the specified element is null and this list does not support null elements (optional)."

I was just trying to think of a List implementation that doesn't allow nulls though, and I'm not aware of any. For example, I can have ArrayList<Double>, but it allows nulls.

    List<Double> list = new ArrayList<Double>();
    if (list.contains(null)) { // this won't throw NPE            
    }

So is the documentation here referring to custom implementations of this interface, or are there some native JAVA collection classes that extend List that don't allow null elements? I realize the exception is optional, I was just trying to think of a real world case where this could occur.

Answer 1

It's referring to custom implementations, until the day that one of the Java List implementations disallows null, and then it will be referring to that too.

Answer 2

--- Post Edited in response to comments ---

Basically the reason it's there is to remind you to catch the NullPointerExcepton because the designers of the List interface envisioned lists that might report any null access as an error.

--- Original Post Follows ---

The ones provided by the standard Java libraries support null, but there's no restriction on you creating a class that implements java.util.List which doesn't support nulls.

If the list doesn't support null, then checking for a null is equivalent to an error, so the exception might make sense depending on who implemented the List. That's why the interface has to mention the checked exception; because, if it didn't, then you couldn't throw a NullPointerException from the subclass should you have wanted to make sure that nobody every touched the list with a null.

Answer 3

Not all implementations of List<...> allow for elements to be null.

An example is http://java.sun.com/j2se/1.5.0/docs/api/javax/management/relation/RoleList.html#add%28javax.management.relation.Role%29

This documentation prepares you for such an encounter, encouraging you to check the documentation of whatever list you're working with to see if it's a concern, or to err on the side of caution if you are unable to check it. Tracking down NPE's is not fun. Knowing documentation (provided good documentation exists) can save a lot of headaches.

Answer 4

Guava's ImmutableList disallows null, but it returns false on contains(null).

Answer 5

A CheckedList would be an example of a List in the standard API that does not support nulls.