The Null Object Pattern is another simple one, yet one that will pay big dividends for minimal effort.  It boils down to a simple rule: ''__Don't pass NULLs__''.

Sure, there are cases where NULL values are useful.
 * In the database, they represent "no value" which is a different thing than a zero or an empty string.
 * As a return value from a search method, they make a convenient "not found" indicator.
 * As a value for an instance variable, they can indicate "not yet initialized."

There are all fine.  Just don't pass them around.

What do you use instead?

For a collection it's really easy.  Just return an empty collection.  The client code can iterate over that empty collection without any problems.  If it needs to do something special for the empty condition, it can check the size of the collection.

For other situations, you can create an instance of the object, or a special class, that can receive method calls without blowing up, but that do approximately nothing.  If you're checking to see which user has logged into the application but the user hasn't logged in yet, don't return a NULL for the user.  Return an instance of !UnknownUser, instead.

Michael Feather's [[http://www.artima.com/weblogs/viewpost.jsp?thread=168511|Offensive Coding]] is perhaps the best short description of why this pattern is such a no-brainer.