[ 
https://issues.apache.org/jira/browse/NETBEANS-407?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16374352#comment-16374352
 ] 

Eirik Bakke commented on NETBEANS-407:
--------------------------------------

> Your code doesn't actually check if k might be null or not.
That's correct. But the general existing policy of the null checking warning is 
to only warn if there some positive indication that k may be null, e.g. a 
"@Nullable" annotation on the return type on getFoo(), or where the compiler 
can see that k might be null due to an earlier assignment. The checker should 
not (and currently does not, except for this bug) give a warning simply because 
every method _might_ return null.

> If you add the if and the warning goes away, then everything should be fine, 
> since instanceof also implies != null.
Sorry, I was a little unclear here. The warning goes away even if the method 
invocation on k is outside the if statement. As in the following variation of 
the code:

{code:java}
public class TestClass {
  private Number getFoo() {
    return 3;
  }

  public void testFoo() {
    Number k = getFoo();
    if (k instanceof Integer) {
      System.out.println("It's an Integer (and thus not null)");
    }
    // No warning here now.
    System.out.println(k.doubleValue());
  }
}
{code}



> "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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