How to show if a method may return null

问题:

After posting this question and reading that one I realized that it is very important to know if a method is supposed to return null, or if this is considered an error condition and an exceptions should be thrown. There also is a nice discussion when to return ‘null’ or throw exception .

I'm writing a method and I already know if I want to return null or throw an exception, what is the best way to express my decision, in other words, to document my contract?

Some ways I can think of:

• Write it down in the specs / the documentation (will anyone read it?)
• Make it part of the method name (as I suggested here)
• assume that every method that throws an exception will not return null, and every one that does 'not' throw might return null.

I'm mainly talking about java, but it might apply to other languages, too: Why is there a formal way to express if exceptions will be thrown (the throws keywords) but no formal way to express if null might be returned?

Why isn't there something like that:

public notnull Object methodWhichCannotReturnNull(int i) throws Exception
{
    return null; // this would lead to a compiler error!
}

Summary and Conclusion

There are many ways to express the contract:

• If your IDE supports it (as IntelliJ), it's best to use an annotation like @NotNull because it is visible to the programmer and can be used for automated compile time checking. There's a plugin for Eclipse to add support for these, but it didn't work for me.
• If these are not an option, use custom Types like Option<T> or NotNull<T>, which add clarity and at least runtime checking.
• In any way, documenting the contract in the JavaDoc never hurts and sometimes even helps.
• Using method names to document the nullability of the return value was not proposed by anyone but me, and though it might be very verbose und not always useful, I still believe sometimes it has its advantages, too.

回答1:

A very good follow up question. I consider null a truly special value, and if a method may return null it must clearly document in the Javadoc when it does (@return some value ..., or null if ...). When coding I'm defensive, and assume a method may return null unless I'm convinced it can't (e.g., because the Javadoc said so.)

People realized that this is an issue, and a proposed solution is to use annotations to state the intention in a way it can be checked automatically. See JSR 305: Annotations for Software Defect Detection, JSR 308: Annotations on Java Types and JetBrain's Nullable How-To.

Your example might look like this, and refused by the IDE, the compiler or other code analysis tools.

@NotNull
public Object methodWhichCannotReturnNull(int i) throws Exception
{
    return null; // this would lead to a compiler error!
}

回答2:

You can use the Option type, which is very much like a list that has zero or one element. A return type of Option<Object> indicates that the method may return an Object, or it may return a special value of type None. This type is a replacement for the use of null with better type checks.

Example:

public Option<Integer> parseInt(String s) {
   try {
      return Option.some(Integer.parseInt(s));
   }
   catch (Exception e) {
      return Option.none();
   }
}

If you use this consistently, you can turn on IDE null-warnings, or just use grep for null which should not appear in your code at all if you use Option.none() everywhere you would normaly use a null literal.

Option comes standard with Scala, and it is called Maybe in Haskell. The link above is to a library called Functional Java that includes it. That version implements the Iterable interface, and has monadic methods that let you compose things nicely. For example, to provide a default value of 0 in case of None:

int x = optionalInt.orSome(0);

And you can replace this...

if (myString != null && !"".equals(myString))

...with this, if you have an Option<String>...

for (String s : myOptionString)

回答3:

Indeed: in our framework we have a 'non-null' pointer type, which may be returned to indicate that the method will always return a value.

I see three options:

1. wait for language support to express it (e.g. the C# ?! thing)
2. use Aspect Orientation to build your own language extensions to express it
3. use a custom type to express it
4. (but builds on developer cooperation) use a naming scheme to indicate it

回答4:

For Java, one can use the Javadoc description of a method to document the meaning of the returned value, including whether it can be null. As has been mentioned, annotations may also provide assistance here.

On the other hand, I admit that I don't see null as something to be feared. There are situations in which "nobody's home" is a meaningful condition (although the Null Object technique also has real value here).

It is certainly true that attempting a method invocation on a null value will cause an exception. But so will attempting to divide by zero. That doesn't mean that we need to go on a campaign to eliminate zeroes! It just means that we need to understand the contract on a method and do the right thing with the values that it returns.

回答5:

Have you had a look at Spec#?

回答6:

You could write your own annotation (Java) or attribute (C#) to indicate that the return value might be null. Nothing will automatically check it (although .NET 4.0 will have code contracts for this sort of thing) but it would at least act as documentation.

回答7:

There's some support for a @Nullable and @NotNull annotation in IntelliJ IDEA. There's also some talk about adding those annotations (or a similar feature) to Java 7. Unfortunately I don't know how far that got or if it's still on track at all.

回答8:

Maybe you could define a generic class named "NotNull", so that your method might be like:

public NotNull<Object> methodWhichCannotReturnNull(int i) throws Exception
{
   // the following would lead to a run-time error thown by the
   // NotNull constructor, if it's constructed with a null value
   return new NotNull<Object>(null);
}

This is still a run-time (not a compile-time) check, but:

• It's thrown in the implementation of the method (it's not a fault in the calling code)
• It's self-documenting (the caller knows he's geting NotNull<T> as a return type)

回答9:

At all costs, avoid relying on the JavaDocs. People only read them if the signature doesn't appear trivial and self-explanatory (Which is bad to begin with), and these who actually bother to read them are less likely to make a mistake with the nulls since they are currently being more careful.

回答10:

If you're using Java 5+, you can use a custom Annotation, e.g. @MayReturnNull

UPDATE

All coding philosophy aside (returning null, using exceptions, assertions, yada yada), I hope the above answers your question. Apart from primitives having default values, complex types may or may not be null, and your code needs to deal with it.

回答11:

Generally speaking, I would assume that a null return value is against the contract of the API by default. It is almost always possible to design your code such that a null value is never returned from your APIs during "normal" flow of execution. (For example, check foo.contains(obj) rather then calling foo.get(obj) and having a separate branch for null. Or, use the Null object pattern.

If you cannot design your API in such a way, I would clearly document when and why a null could be thrown--at least in the Javadoc, and possibly also using a custom @annotation such as several of the other answers have suggested.