Is it possible to distinguish which part was null from stack trace?

Question

When I have a method like:

public class Tttttt {

    public Thing getThingBy(String name) {
      return aaa.getThingBy(name);
    }
}

And then I see in logs NPE

Caused by java.lang.NullPointerException
   at xxx.yyy.Tttttt.getThingBy(SourceFile:666)

And the 666 fits the method definition, then can I say whether name was null or aaa was null?

you may guess, but there is no sure shot way of knowing unless you print all variables. — Rishi Goel, Oct 17 '16 at 07:17
If `name` was `null`, you would have the `getThingBy` call in the stack trace. — Arnaud, Oct 17 '16 at 07:17
@RishiGoel That is not correct. In this case clearly it is `aaa` that was null. If it was `name`, the call to `aaa.getThingBy()` would have succeeded and would appear on the stack trace. No guessing. — user207421, Oct 17 '16 at 07:18
@MarianPaździoch Even if they are named both the same, there would be another line in the Stacktrace if it were `name` that is null. — Fildor, Oct 17 '16 at 07:21
Calling in an objectmethod where object is null leads to a NPE (NullPointerException). Check if its instantiated somewhere with "new" or with an annotation like @Inject. — Daniel Gschösser, Oct 17 '16 at 07:21
Tempted to vote as a duplicate of this. http://stackoverflow.com/questions/3988788/what-is-a-stack-trace-and-how-can-i-use-it-to-debug-my-application-errors — OneCricketeer, Oct 17 '16 at 07:23
@ChiefTwoPencils ... because `aaa` is the only dereferenced object + no wrapper-to-primitive unboxing here. At least I believe the line 666 code is posted. — Lyubomyr Shaydariv, Oct 17 '16 at 07:30

score 2 · Accepted Answer · answered Oct 17 '16 at 07:24

2

In this case aaa is null.

name also can be null but it will be passed to the getThingBy as a parameter and you'd have more entries in your stack trace if the exception was thrown down there.

answered Oct 17 '16 at 07:24

Ivan

3,781
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20

OneCricketeer · Answer 2 · 2016-10-17T07:34:13.523

can I say whether name was null or aaa was null?

This is a poor example... aaa is most certainly null. Passing a null parameter into that method doesn't throw any exception. Even if the method did call itself, you'd see that in the stacktrace.

Say it did get called again, and you used aaa.someMethod(name.length()) on the exact same line. Then you might still be able to determine the variable based on the class type and method mentioned in the stacktrace, as for this question at xxx.yyy.Tttttt.getThingBy compared to Attempt to invoke virtual method 'int java.lang.String.length()' on a null object reference

Is it possible to distinguish which part was null from stack trace?

2 Answers2