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

Answer 1

As long as it's supposed to return a reference to the object, returning a NULL should be good.

However, if it's returning the whole bloody thing (like in C++ if you do: 'return blah;' rather than 'return &blah;' (or 'blah' is a pointer), then you can't return a NULL, because it's not of type 'object'. In that case, throwing an exception, or returning a blank object that doesn't have a success flag set is how I would approach the problem.

Answer 2

If null never indicates an error then just return null.

If null is always an error then throw an exception.

If null is sometimes an exception then code two routines. One routine throws an exception and the other is a boolean test routine that returns the object in an output parameter and the routine returns a false if the object was not found.

It's hard to misuse a Try routine. It's real easy to forget to check for null.

So when null is an error you just write

object o = FindObject();

When the null isn't an error you can code something like

if (TryFindObject(out object o)
  // Do something with o
else
  // o was not found

Answer 3

I agree with what seems to be the consensus here (return null if "not found" is a normal possible outcome, or throw an exception if the semantics of the situation require that the object always be found).

There is, however, a third possibility that might make sense depending on your particular situation. Your method could return a default object of some sort in the "not found" condition, allowing calling code to be assured that it will always receive a valid object without the need for null checking or exception catching.

Answer 4

If it's important for client code to know the difference between found and not found and this is supposed to be a routine behavior, then it's best to return null. Client code can then decide what to do.

Answer 5

Depends on what it means that the object is not found.

If it's a normal state of affairs, then return null. This is just something that might happen once in an while, and the callers should check for it.

If it's an error, then throw an exception, the callers should decide what to do with the error condition of missing object.

Ultimately either would work, although most people generally consider it good practice to only use Exceptions when something, well, Exceptional has happened.

Answer 6

Or return an Option

An option is basically a container class that forces the client to handle booth cases. Scala has this concept, look up it's API.

Then you have methods like T getOrElse(T valueIfNull) on this object thet either return the found object, or an allternative the client specifieces.