Null checks for values that can't possibly be null (Multimap.get, protocol buffer accessors)

[cushon]

Original issue created by cpovirk@google.com on 2013-02-12 at 02:49 AM

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I see this from time to time:

if (multimap.get(foo) == null) { ... }

But Multimap.get always returns a collection (possibly empty), never null:

http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Multimap.html#get(K)

A quick search shows that this call appears a few dozen times in Google's codebase.

Another place that I see such null checks is with protocol buffers:

if (proto.getFoo() == null) { ... }

But proto fields aren't null; they have default values.

You could catch even more of these by looking for isNullOrEmpty calls:

if (Strings.isNullOrEmpty(proto.getFoo())) { ... }

But isNullOrEmpty is less likely to be a bug, since it will still fire (as the coder apparently expects) for a default value of "".

Are there other such methods? No doubt, but those are the two that spring to mind. I wonder whether there's an annotation that could identify them? JSR305 (covered in issue 44) has @Nonnull, but I think I've seen it mainly with parameters, and there's no documentation to say whether it's supposed to be used with return types.

[cushon]

Original comment posted by cpovirk@google.com on 2013-02-12 at 02:51 AM

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(adding a comment so that I get an email so that I can reply internally)

[cushon]

Original comment posted by cpovirk@google.com on 2013-02-12 at 02:53 AM

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(Of course, there are probably other Multimap.get callers who assign the result to a variable and then check for null. I don't know how much harder that is to catch.)

[cushon]

Original comment posted by eaftan@google.com on 2013-06-14 at 06:24 PM

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Felix (flx) is working on this for protos.

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Status: Started