Is returning null bad design?

Question

I've heard some voices saying that checking for a returned null value from methods is bad design. I would like to hear some reasons for this.

pseudocode:

variable x = object.method()
if (x is null) do something

Answer 1

The rationale behind not returning null is that you do not have to check for it and hence your code does not need to follow a different path based on the return value. You might want to check out the Null Object Pattern which provides more information on this.

For example, if I were to define a method in Java that returned a Collection I would typically prefer to return an empty collection (i.e. Collections.emptyList()) rather than null as it means my client code is cleaner; e.g.

Collection<? extends Item> c = getItems(); // Will never return null.

for (Item item : c) { // Will not enter the loop if c is empty.
  // Process item.
}

... which is cleaner than:

Collection<? extends Item> c = getItems(); // Could potentially return null.

// Two possible code paths now so harder to test.
if (c != null) {
  for (Item item : c) {
    // Process item.
  }
}

Answer 2

Here's the reason.

In Clean Code by Robert Martin he writes that returning null is bad design when you can instead return, say, empty array. Since expected result is an array, why not? It'll enable you to iterate over result without any extra conditions. If it's an integer, maybe 0 will suffice, if it's a hash, empty hash. etc.

The premise is to not force calling code to immediately handle issues. Calling code may not want to concern itself with them. That's also why in many cases exceptions is better than nil.

Answer 3

Good uses of returning null:

• If null is a valid functional result, for example: FindFirstObjectThatNeedsProcessing() can return null if not found and the caller should check accordingly.

Bad uses: Trying to replace or hide exceptional situations such as:

• catch(...) and return null
• API dependency initialization failed
• Out of disk space
• Invalid input parameters (programming error, inputs must be sanitized by the caller)
• etc

In those cases throwing an exception is more adequate since:

• A null return value provides no meaningful error info
• The immediate caller most likely cannot handle the error condition
• There is no guarantee that the caller is checking for null results

However, Exceptions should not be used to handle normal program operation conditions such as:

• Invalid username/password (or any user-provided inputs)
• Breaking loops or as non-local gotos

Answer 4

Yes, returning NULL is a terrible design, in object-oriented world. In a nutshell, NULL usage leads to:

• ad-hoc error handling (instead of exceptions)
• ambiguous semantic
• slow instead of fast failing
• computer thinking instead of object thinking
• mutable and incomplete objects

Check this blog post for a detailed explanation: http://www.yegor256.com/2014/05/13/why-null-is-bad.html. More in my book Elegant Objects, Section 4.1.

Answer 5

Who says this is bad design?

Checking for nulls is a common practice, even encouraged, otherwise you run the risk of NullReferenceExceptions everywhere. Its better to handle the error gracefully than throw exceptions when you don't need to.

Answer 6

Based on what you've said so far, I think there's not enough information.

Returning null from a CreateWidget()method seems bad.

Returning null from a FindFooInBar() method seems fine.

Answer 7

Its inventor says it is a billion dollar mistake!

Answer 8

It depends on the language you're using. If you're in a language like C# where the idiomatic way of indicating the lack of a value is to return null, then returning null is a good design if you don't have a value. Alternatively, in languages such as Haskell which idiomatically use the Maybe monad for this case, then returning null would be a bad design (if it were even possible).

Answer 9

If you read all the answers it becomes clear the answer to this question depends on the kind of method.

Firstly, when something exceptional happens (IOproblem etc), logically exceptions are thrown. When exactly something is exceptional is probably something for a different topic..

Whenever a method is expected to possibly have no results there are two categories:

• If it is possible to return a neutral value, do so.
  Empty enumrables, strings etc are good examples
• If such a neutral value does not exist, null should be returned.
  As mentioned, the method is assumed to possibly have no result, so it is not exceptional, hence should not throw an exception. A neutral value is not possible (for example: 0 is not especially a neutral result, depending on the program)

Untill we have an official way to denote that a function can or cannot return null, I try to have a naming convention to denote so.
Just like you have the TrySomething() convention for methods that are expected to fail, I often name my methods SafeSomething() when the method returns a neutral result instead of null.

I'm not fully ok with the name yet, but couldn't come up with anything better. So I'm running with that for now.

Answer 10

I have a convention in this area that served me well

For single item queries:

• Create... returns a new instance, or throws
• Get... returns an expected existing instance, or throws
• GetOrCreate... returns an existing instance, or new instance if none exists, or throws
• Find... returns an existing instance, if it exists, or null

For collection queries:

• Get... always returns a collection, which is empty if no matching[1] items are found

[1] given some criteria, explicit or implicit, given in the function name or as parameters.

Answer 11

It's not necessarily a bad design - as with so many design decisions, it depends.

If the result of the method is something that would not have a good result in normal use, returning null is fine:

object x = GetObjectFromCache();   // return null if it's not in the cache

If there really should always be a non-null result, then it might be better to throw an exception:

try {
   Controller c = GetController();    // the controller object is central to 
                                      //   the application. If we don't get one, 
                                      //   we're fubar

   // it's likely that it's OK to not have the try/catch since you won't 
   // be able to really handle the problem here
}
catch /* ... */ {
}

Answer 12

Exceptions are for exceptional circumstances.

If your function is intended to find an attribute associated with a given object, and that object does has no such attribute, it may be appropriate to return null. If the object does not exist, throwing an exception may be more appropriate. If the function is meant to return a list of attributes, and there are none to return, returning an empty list makes sense - you're returning all zero attributes.

Answer 13

It's fine to return null if doing so is meaningful in some way:

public String getEmployeeName(int id){ ..}

In a case like this it's meaningful to return null if the id doesn't correspond to an existing entity, as it allows you to distinguish the case where no match was found from a legitimate error.

People may think this is bad because it can be abused as a "special" return value that indicates an error condition, which is not so good, a bit like returning error codes from a function but confusing because the user has to check the return for null, instead of catching the appropriate exceptions, e.g.

public Integer getId(...){
   try{ ... ; return id; }
   catch(Exception e){ return null;}
}

Answer 14

For certain scenarios, you want to notice a failure as soon as it happens.

Checking against NULL and not asserting (for programmer errors) or throwing (for user or caller errors) in the failure case can mean that later crashes are harder to track down, because the original odd case wasn't found.

Moreover, ignoring errors can lead to security exploits. Perhaps the null-ness came from the fact that a buffer was overwritten or the like. Now, you are not crashing, which means the exploiter has a chance to execute in your code.

Answer 15

What alternatives do you see to returning null?

I see two cases:

• findAnItem( id ). What should this do if the item is not found

In this case we could: Return Null or throw a (checked) exception (or maybe create an item and return it)

• listItemsMatching (criteria) what should this return if nothing is found?

In this case we could return Null, return an empty list or throw an Exception.

I believe that return null may be less good than the alternatives becasue it requires the client to remember to check for null, programmers forget and code

x = find();
x.getField();  // bang null pointer exception

In Java, throwing a checked exception, RecordNotFoundException, allows the compiler to remind the client to deal with case.

I find that searches returning empty lists can be quite convenient - just populate the display with all the contents of the list, oh it's empty, the code "just works".

Answer 16

Make them call another method after the fact to figure out if the previous call was null. ;-) Hey, it was good enough for JDBC

Answer 17

Well, it sure depends of the purpose of the method ... Sometimes, a better choice would be to throw an exception. It all depends from case to case.

Answer 18

Sometimes, returning NULL is the right thing to do, but specifically when you're dealing with sequences of different sorts (arrays, lists, strings, what-have-you) it is probably better to return a zero-length sequence, as it leads to shorter and hopefully more understandable code, while not taking much more writing on API implementer's part.

Answer 19

The base idea behind this thread is to program defensively. That is, code against the unexpected. There is an array of different replies:

Adamski suggests looking at Null Object Pattern, with that reply being up voted for that suggestion.

Michael Valenty also suggests a naming convention to tell the developer what may be expected. ZeroConcept suggests a proper use of Exception, if that is the reason for the NULL. And others.

If we make the "rule" that we always want to do defensive programming then we can see that these suggestions are valid.

But we have 2 development scenarios.

Classes "authored" by a developer: The Author

Classes "consumed" by another(maybe) developer: the Developer

Regardless of whether a class returns NULL for methods with a return value or not, the Developer will need to test if the object is valid.

If the developer cannot do this, then that Class/method is not deterministic. That is, if the "method call" to get the object does not do what it "advertises" (eg getEmployee) it has broken the contract.

As an author of a class, I always want to be as kind and defensive ( and deterministic) when creating a method.

So given that either NULL or the NULL OBJECT (eg if(employee as NullEmployee.ISVALID)) needs to be checked and that may need to happen with a collection of Employees, then the null object approach is the better approach.

But I also like Michael Valenty's suggestion of naming the method that MUST return null eg getEmployeeOrNull.

An Author who throws an exception is removing the choice for the developer to test the object's validity, which is very bad on a collection of objects, and forces the developer into exception handling when branching their consuming code.

As a developer consuming the class, I hope the author gives me the ability to avoid or program for the null situation that their class/methods may be faced with.

So as a developer I would program defensively against NULL from a method. If the author has given me a contract that always returns a object (NULL OBJECT always does) and that object has a method/property by which to test the validity of the object, then I would use that method/property to continue using the object, else the object is not valid and I cannot use it.

Bottom line is that the Author of the Class/Methods must provide mechanisms that a Developer can use in their defensive programming. That is, a clearer intention of the method.

The Developer should always use defensive programming to test the validity of the objects returned from another class/method.

regards

GregJF

Answer 20

Other options to this, are: returning some value that indicates success or not (or type of an error), but if you just need boolean value that will indicate success / fail, returning null for failure, and an object for success wouldn't be less correct, then returning true/false and getting the object through parameter.
Other approach would to to use exception to indicates failures, but here - there are actually many more voices, that say this is a BAD practice (as using exceptions may be convenient but has many disadvantages).
So I personally don't see anything bad in returning null as indication that something went wrong, and checking it later (to actually know if you have succeeded or not). Also, blindly thinking that your method will not return NULL, and then base your code on it, may lead to other, sometimes hard to find, errors (although in most cases it will just crash your system :), as you will reference to 0x00000000 sooner or later).

Answer 21

Unintended null functions can arise during the development of a complex programs, and like dead code, such occurrences indicate serious flaws in program structures.

A null function or method is often used as the default behavior of a revectorable function or overrideable method in an object framework.

Null_function @wikipedia

Answer 22

If the code is something like:

command = get_something_to_do()

if command:  # if not Null
    command.execute()

If you have a dummy object whose execute() method does nothing, and you return that instead of Null in the appropriate cases, you don't have to check for the Null case and can instead just do:

get_something_to_do().execute()

So, here the issue is not between checking for NULL vs. an exception, but is instead between the caller having to handle special non-cases differently (in whatever way) or not.

Answer 23

For my use case I needed to return a Map from method and then looking for a specific key. But if I return an empty Map, then it will lead to NullPointerException and then it wont be much different returning null instead of an empty Map. But from Java8 onward we could use Optional. The above is the very reason Optional concept was introduced.

Answer 24

G'day,

Returning NULL when you are unable to create a new object is standard practise for many APIs.

Why the hell it's bad design I have no idea.

Edit: This is true of languages where you don't have exceptions such as C where it has been the convention for many years.

HTH

'Avahappy,