How do you find the exact variable of a null pointer exception?

Question

I've been at this one for a bit now. I've got a null pointer exception so I'm guessing somethings not been initialized.

AdminMessages.inbox1.setText(messageRsetArray[0]);

That's my code where it's targetting. But I can't find what's inside. It hasn't been initialized.

AdminMessages is a class which contains a JTextField called inbox1, messageRsetArray is an array which has taken variables from an array.

Also inbox1 is static. I couldn't get the getters and setter to work. I know it's bad practice though.

Exception in thread "main" java.lang.NullPointerException
 at project.AdminMessages.fillInboxGui(AdminMessages.java:587)
 at project.AdminMessages.<init>(AdminMessages.java:156)
 at project.AdminUser.createAdminMessages(AdminUser.java:31)
 at project.AdminUser.<init>(AdminUser.java:17)
 at project.AdminUser.main(AdminUser.java:45)

the line numbers arent the problem. its the point on the line where it goes off. — OVERTONE, Apr 26 '10 at 13:28
Why don't you just add tests above it? `if (messageRsetArray == null) throw new NullPointerException("messageRsetArray is null");` and so on? Way faster than waiting on StackOverflow:) — extraneon, Apr 26 '10 at 13:37
@ extraneon neat idea. never would have thought of that. then again. beginner here. wouldnt have heard of it if not for stack. — OVERTONE, Apr 26 '10 at 13:49
See [JEP 358 with JDK 14](https://stackoverflow.com/a/58908439/6309) — VonC, Nov 18 '19 at 05:29

Bozho · Answer 1 · 2010-04-26T12:59:11.620

7

inbox1 and/or messageRsetArray are null. You have to instantiate them using:

inbox1 = new JTextField();

and

messageRsetArray = new String[size];

In such cases simply check which of the variables can possibly be null at that location. If you cannot tell for sure, then split the expression so that there is only one "NPE-thrower" per line:

String text = messageRsetArray[0];
AdminMessages.inbox1.setText(text);

And you will have your definitive answer in the next stacktrace.

edited Apr 26 '10 at 12:59

answered Apr 26 '10 at 12:50

Bozho

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It would also be possible that messageRsetArray is null. – M. Jessup Apr 26 '10 at 12:51
That's probably it yeah. If you can't get getters and setters to work you may well forget to create your instances :) – extraneon Apr 26 '10 at 12:52
I don't see a reason. +1 to counter that. – BalusC Apr 26 '10 at 13:06
you were right about it being defined wrong. good news is it runs fine with no exception, still not getting them too output into the Jlist but thats another story. thanks anyway. – OVERTONE Apr 26 '10 at 13:39

score 6 · Accepted Answer · answered Apr 26 '10 at 13:03

To the best of my knowledge what you want cannot be easily done (ANY "dot" is a candidate for the exception).

A slight rewrite of the source code may be the easiest way to get what you need.

AdminMessages.inbox1.setText(messageRsetArray[0]);

into

inbox1 = Adminmessages.inbox1;
text = messageRSetArray[0];
inbox1.setText(text);

This leaves you with only one "dot" operation per line, and makes it obvious which one is broken.

You may also want to help the person to see the stacktrace with a

if (AdminMessages.inbox1 == null) {
   throw new IllegalArgumentException("AdminMessages.inbox1 == null");
}

(I like this for method arguments. When inside code you may want NullPointerExceptions instead).

broke up the code and found it. it was the Resultset defined wrong. i had two initialized twice. thanks — OVERTONE, Apr 26 '10 at 13:40

score 5 · Answer 3 · answered Apr 26 '10 at 12:47

5

set a debugger breakpoint and print out the values of the various candidates.
make some local variables so that only one thing can be null at a time.

answered Apr 26 '10 at 12:47

bmargulies

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If all else fails / is unavailable there is always verbose logger.trace(...) – M. Jessup Apr 26 '10 at 12:51
@extraneon a stack trace doesn't help tell you with a statement that has three possible null values in one Java statement. – bmargulies Apr 26 '10 at 13:01
you guys must be freakin rich. i have no idea what half this stuff means. thanks very much for the help though – OVERTONE Apr 26 '10 at 13:50

score 0 · Answer 4 · answered Apr 26 '10 at 12:49

0

The quickest and easiest way: use a debugger.

And in general:

Try to avoid null values by setting everything to useful default values if possible
Try to avoid chaining method calls and property accesses like that

answered Apr 26 '10 at 12:49

Michael Borgwardt

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Debugger seems like overkill a stack trace is all that is needed. – dominic Apr 26 '10 at 12:56
@denis: if you have multiple dereferencings on one line, which is exactly what this question is about, then a stacktrace is NOT all that is needed. – Michael Borgwardt Apr 26 '10 at 13:09
@denis - a stack trace won't tell you which of several potential null instances on that line are the issue, hence the suggestion of a debugger. – Chris Knight Apr 26 '10 at 13:19
The question was rather "how do you find the exact variable of a null pointer exception" not "fix my null pointer". The stack trace can tell you where the exception was thrown but not the exact variable. Hence, the debugger. – Apr 26 '10 at 13:37
found it anyway without the debugger. just broke the code into smaller bits. thanks for the help though – OVERTONE Apr 26 '10 at 13:41

How do you find the exact variable of a null pointer exception?

4 Answers4