Effective java Item 43 states return empty arrays or collections, not nulls. In practice you should return the same immutable empty collection every time you return a collection. There are a few ways to handle the exception to the rule when you encounter methods that should return a collection but instead return null. Check out return empty list, return empty map, return empty set, return empty list iterator, return empty sorted set, return empty sorted map and return empty iterator when having to deal with other collection types.
Straight up Java
Exception to the rule
The goal is to handle the null to empty collection early in the chain. If you are coding the method that returns an enumeration, there is no reason it should return null. When dealing with legacy code you have to deal with a null so here are a few options when that occurs: