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null pointer exception

terry oliver

Joined: Jan 04, 2008
Posts: 22
help can anyone help it does compile and thows the exception at runtime.

private void okButton_actionPerformed(ActionEvent e)
System.out.println("\nokButton_actionPerformed(ActionEvent e) called.");

System.out.println("adjused steel = "+nt.getSteel());
Ernest Friedman-Hill
author and iconoclast

Joined: Jul 08, 2003
Posts: 24199

A NullPointerException happens when something directly to the left of a "." or "[" operator is null at the time the line of code executes. Here, that could be the variables "nt" or "newSteelSize". Carefully check the code elsewhere in your application that's supposed to set those to some value.

One beginner mistake that's very likely to be the cause of this: make sure that in your class's constructor, you're not accidentally declaring local variables with the same names as these members. You should be writing

nt = ...


Whatever nt = ...

The stack trace should tell you exactly which line the error occurred on; it's probably that "setSteel" line, since both variables are used there.

[Jess in Action][AskingGoodQuestions]
Bruno Giminiani

Joined: May 15, 2008
Posts: 1
Hi Terry, there is a list of top ten error by java programmers.
"Null Pointer" is the FIRST. See the link:

Bruno Giminiani
I agree. Here's the link:
subject: null pointer exception
It's not a secret anymore!