Should a retrieval method return 'null' or

Question

I have a method that is supposed to return an object if it is found.

If it is not found, should I:

1. return null
2. throw an exception
3. other

Answer 1

That really depends on if you expect to find the object, or not. If you follow the school of thought that exceptions should be used for indicating something, well, err, exceptional has occured then:

• Object found; return object
• Object not-found; throw exception

Otherwise, return null.

Answer 2

I just wanted to recapitulate the options mentioned before, throwing some new ones in:

1. return null
2. throw an Exception
3. use the null object pattern
4. provide a boolean parameter to you method, so the caller can chose if he wants you to throw an exception
5. provide an extra parameter, so the caller can set a value which he gets back if no value is found

Or you might combine these options:

Provide several overloaded versions of your getter, so the caller can decide which way to go. In most cases, only the first one has an implementation of the search algorithm, and the other ones just wrap around the first one:

Object findObjectOrNull(String key);
Object findObjectOrThrow(String key) throws SomeException;
Object findObjectOrCreate(String key, SomeClass dataNeededToCreateNewObject);
Object findObjectOrDefault(String key, Object defaultReturnValue);

Even if you choose to provide only one implementation, you might want to use a naming convention like that to clarify your contract, and it helps you should you ever decide to add other implementations as well.

You should not overuse it, but it may be helpfull, espeacially when writing a helper Class which you will use in hundreds of different applications with many different error handling conventions.

Answer 3

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 4

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

Answer 5

Exceptions are related to Design by Contract.

The interface of an objects is actually a contract between two objects, the caller must meet the contract or else the receiver may just fail with an exception. There are two possible contracts

1) all input the method is valid, in which case you must return null when the object is not found.

2) only some input is valid, ie that which results in a found object. In which case you MUST offer a second method that allows the caller to determine if its input will be correct. For example

is_present(key)
find(key) throws Exception

IF and ONLY IF you provide both methods of the 2nd contract, you are allowed to throw an exception is nothing is found!

Answer 6

Prefer returning null --

If the caller uses it without checking, the exception happens right there anyway.

If the caller doesn't really use it, don't tax him a try/catch block