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)

Answer 1

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

Answer 2

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

Answer 3

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.

Answer 4

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).