Proper way to handle Android Studio's NullPointerException lint warning

问题

I'm new to android/java programming and am confused how to properly deal with this warning.

Method invocation '' may produce 'Java.lang.NullPointerException'

Should I be ussing assert to remove the warning?

Or rather a runtime exception?

Any help would be appreciated.

回答1:

I doubt this question can be answered conclusively, as it's a matter of opinion. Or at least I believe so -- an opinion too. :)

I understand you want "0 warnings" (a very laudable goal) but there's probably not a "one size fits all" issue. That said...

Things I believe you should not do:

• Use assert. While you can add an assert statement, Dalvik ignores them. You can configure an emulator to use them if you want, but not a real device (see Can I use assert on Android devices?). So while it would possibly remove the warning, it's useless in practice.
• Have the method throw NullPointerException. This would be a bad idea, generally. In this case, since you're probably overriding onOptionsItemSelected(), it's not even possible.

Checking for (variable != null) is generally the best approach. What to do if it is, though, presents some other options.

• If it's a problem you can recover from, i.e. you can continue the application even though the searchView isn't there, just do so. For example, just return from the method. It's a good idea to log this situation though, so you can spot it while testing.
• Otherwise, if continuing isn't possible, throw an exception. You want to fail early, so that the problem can be easily detected. A reasonable exception for this case would be IllegalStateException (see Java equivalent to .NET System.InvalidOperationException). It basically indicates that this method was executed at an inappropriate time. Be careful though, that as a RuntimeException, these exceptions are unchecked, and hence will probably cause the app to crash.

回答2:

I started to use

@SuppressWarnings("ConstantConditions")

on simple methods where I'm sure that the id is not null.

回答3:

What @Herrbert74 suggested it surely working fine, but sometimes it's better to not add a @SuppressWarnings("ConstantConditions") to an entire method (if it's not trivial), a better approach could be to use //noinspection ConstantConditions on the warned line.

Those are my rules of thumb:

• Use @SuppressWarnings("ConstantConditions") when the method is simple

• Use //noinspection ConstantConditions when the method is complex and you need to remove the warning only on a specific line

回答4:

I personally prefer using try{ }catch{ } simply because it is more elegant. However, it does add a lot of bulk to your code, if you imagine putting every, possible, NULL value into a try catch (if they are not next to each other)

回答5:

As @matiash pointed out there is no one-size-fits-all solution.

For me a good compromise was to disable NullPointerException warning for all calls to findViewById() and keep it for other method calls. That way I take responsibility for checking the resource ids but still get a benefit of getting warnings if I make other mistakes.

To achieve this I added _ -> !null method contract with Android Studio quick fix menu.

The action generated a following file file at android/support/v7/app/annotations.xml in my project root.

<root>
  <item name='android.support.v7.app.AppCompatActivity android.view.View findViewById(int)'>
    <annotation name='org.jetbrains.annotations.Contract'>
      <val val="&quot;_ -&gt; !null&quot;" />
    </annotation>
  </item>
</root>

Update: Unfortunately it doesn't survive Android Studio restarts :-( External annotations are really useful so I hope I'll figure out a way to make Android Studio load them after restart.

回答6:

Yes. Using if (Object != null){} for validating is the proper way. try {} catch (NullPointerException) {} is the next solution which is preferred in this case.

If you want to get ride of it, throw an NullPointerException. Lint will ignore it in this case. public void myFunc() throws NullPointerException{}.

Anyway, good Coding always means validating everything for a possible problem while runtime. Validating != null is just fine and should always be used whenever it's possible null.

回答7:

I like the answer to this link.

Warning is not an Error. And the warning which you are talking about says "it may produce", don't say 'it must produce'. So choice is yours. Either add null check or not

So, If you are sure that findViewById in your code will never be cause of NPE, then don't add the null check.

来源：https://stackoverflow.com/questions/23371938/proper-way-to-handle-android-studios-nullpointerexception-lint-warning