Should a retrieval method return 'null' or throw an exception when it can't produce the return value? [closed]

Answer 1

Here are a couple more suggestions.

If returning a collection, avoid returning null, return an empty collection which makes enumeration easier to deal with without a null check first.

Several .NET API's use the pattern of a thrownOnError parameter which gives the caller the choice as whether it is really an exceptional situation or not if the object is not found. Type.GetType is an example of this. Another common pattern with BCL is the TryGet pattern where a boolean is returned and the value is passed via an output parameter.

You could also consider the Null Object pattern in some circumstances which can either be a default or a version with no behaviour. The key is avoid null checks throughout the code base. See here for more information http://geekswithblogs.net/dsellers/archive/2006/09/08/90656.aspx

Answer 2

Unfortunately JDK is inconsistent, if you trying access non existing key in resource bundle, you get not found exception and when you request value from map you get null if it doesn't exists. So I would change winner answer to the following, if found value can be null, then raise exception when it isn't found, otherwise return null. So follow to the rule with one exception, if you need to know why value isn't found then always raise exception, or..

Answer 3

referring only to the case where null is not considered an exceptional behavior i am definitely for the try method, it is clear, no need to "read the book" or "look before you leap" as was said here

so basically:

bool TryFindObject(RequestParam request, out ResponseParam response)

and this means that the user's code will also be clear

...
if(TryFindObject(request, out response)
{
  handleSuccess(response)
}
else
{
  handleFailure()
}
...

Answer 4

Generally it should return null. The code calling the method should decide whether to throw an exception or to attempt something else.

Answer 5

it depends if your language and code promotes: LBYL (look before you leap) or EAFP (easier to ask forgiveness than permission)

LBYL says you should check for values (so return a null)
EAFP says to just try the operation and see if it fails (throw an exception)

though I agree with above.. exceptions should be used for exceptional/error conditions, and returning a null is best when using checks.

---

EAFP vs. LBYL in Python:
http://mail.python.org/pipermail/python-list/2003-May/205182.html (Web Archive)

Answer 6

Exceptions should be exceptional. Return null if it is valid to return a null.