Iterator null collection

Question

It's quite common that I have to check null before iterate when not sure the collection reference is null or not. Sample:

Collection<Object> collection = ...
...
if(collection != null)//troublesome
    for(Object o : collection)

Of course, I know empty collection is much better than null, but in some cases client code cannot control the nullable collection from other modules (for instance, return value from 3rd party code). So I wrote a utility method:

public static <T> Iterable<T> nullableIterable(Iterable<T> it){
    return it != null ? it : Collections.<T>emptySet();
}

In client code, no need to check null any more:

for(Object o : nullableIterable(collection))
...

Do you think nullableIterable() is reasonable? Any advice? Any concern? Thanks!

Answer 1

That looks good. I personally do that too. You will always get developers who would disagree with this as it is kind of defensive programming. Imagine you have a workflow or a class that is not supposed to return null. This means that getting a null from it is a bug which your code will hide as it will turn the null to an empty collection and the bug will never surface.

If you are for example writing APIs that do not support null collections then you should avoid this. If client code gives you a null collection where you do not support it, you should throw an IllegalArgumentException to let client code know that there is something wrong with the provided collection. Something like:

public void myApiNoSupportForNull(Collection<Object> collection){
   // Pre condition
   if(collection == null) 
     throw new IllegalArgumentException("This API does not support null collections!");
   //...
}