Does 'consider returning Null Object' apply to a DAO read method?

Question

I was looking for an answer to the question of what a DAO should return when a search ends up to be for an entity that does not exist.

There are some generic previous questions related to returning null, but I am looking for an answer specifically in the context of the DAO retrieve scenario.

To be clear, when I say Null Object I am referring to the Null Object pattern. In the Java space, Josh Bloch recommends an empty collection solution in the book Effective Java (See Item 43) which provides an very good introduction to the problem/solution.

Despite being aware of Bloch's advice when I first wrote this question I considered returning null appropriate in a read/retrieval scenario. I'm in good company making a mistake with nulls.

Answer 1

In "Effective Java" item 43 p. 201, Joshua Bloch says, "There is no reason ever to return null from an array- or collection-valued method." He recommends returning a zero-length array or an empty collection (Collections.emptyXxx) when there are legitimately no results. For one thing, it's a pain for the client to double-check everything:

if ( (retVal == null) || (retVal.size() < 1) ) {
    ...
}

Before returning null, ask yourself if the null return value is a result of a programming error on the part of the calling code. Such a check is generally done in the first few lines of a procedure as a defensive check for invalid input values and could legitimately result in throwing an unchecked exception instead of returning null. For my reasoning on this, please see the Checked vs. Unchecked Exceptions entry in my blog.

Scala and Haskell avoid returning null with an Option class. I don't immediately see how this is better than null (other than being type-safe just for the sake of it), but I thought I should mention it as another point of view:

val nameMaybe = request getParameter "name"
nameMaybe match {
  case Some(name) =>
    println(name.trim.toUppercase)
  case None =>
    println("No name value")
}

Bloch suggests that null is an appropriate return instead of an empty collection in C because array lengths are stored separately from arrays and there is no advantage to allocating an empty array. But a null is not the right answer all the time in all languages.

Answer 2

A search suggests that there will be multiple results which is an easy case - always return a collection. If there are no results, then the collection will simply have no items.

If there can be 0 or 1 result, it depends on the semantics of the search. Can there only be a 1 result because you are artificially limiting the search results to 1? I would go with a collection with 0 or 1 items, the limit could probably change at some point.

If there can truly only be 0 or 1 result, you are probably trying to "search" by an unique identifier or such. In normal application usage, this "search" will never fail. It is not really a search. Your application doesn't generate links with invalid ids or whatever.

However, in exceptional situations such as:

• A page with links has been loaded and been sitting idle for a while. Some of the resources behind the links have been deleted in the meanwhile. Now when you click such a link, the "search" would fail.
• The user manually "circumvents" the application and uses the address bar directly to enter arbitrary id numbers.
• ...

All of the above are exceptions. So it is very easy, throw an exception. Probably a NotFoundException that doesn't need to be caught anywhere that can just bubble to the top for an automatic 404 error page.