Tobias Oelgarte commented on NETBEANS-407:

Your code doesn't actually check if _k_ might be _null_ or not. If _getFoo()_ 
would return _null_, then you would dereference _null_, because the line with 
"_k instanceof Integer_" does nothing to prevent it from happening. If you add 
the _if_ and the warning goes away, then everything should be fine, since 
_instanceof_ also implies _!= null_. If it would still happen, then it is a bug.

> "Dereferencing possible null pointer" after doing instanceof on variable 
> (false positive)
> -----------------------------------------------------------------------------------------
>                 Key: NETBEANS-407
>                 URL: https://issues.apache.org/jira/browse/NETBEANS-407
>             Project: NetBeans
>          Issue Type: Bug
>          Components: java - FindBugs, java - Hints
>    Affects Versions: 8.2, 9.0
>            Reporter: Eirik Bakke
>            Priority: Minor
> In the following example, the "boolean foo = k instanceof Integer" assignment 
> causes a spurious "Dereferencing possible null pointer" warning to occur on 
> the "k.doubleValue()" expression. I believe this did not happen in earlier 
> NetBeans versions (pre-8.2). Note that if the assignment to "boolean foo" is 
> replaced by an if statement ("if (k instanceof Integer)"), the warning goes 
> away.
> {code:java}
> public class TestClass {
>   private Number getFoo() {
>     return 3;
>   }
>   public void testFoo() {
>     Number k = getFoo();
>     // The warning only occurs if this assignment is present.
>     boolean foo = k instanceof Integer;
>     // Warning on .doubleValue() here: "Dereferencing possible null pointer"
>     System.out.println(k.doubleValue());
>   }
> }
> {code}
> This bug was previously described at 
> https://netbeans.org/bugzilla/show_bug.cgi?id=269324 .

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