I would like to know when NullPointerException comes in java. any help would be appreciated.
When you try to use a reference variable or expression that does not contain a reference to an Object - ie contains null. This may be because it hasn't been initialised, or it may be the result of calling a method that returns null following an error or other condition such as end-of-file.
A NPE will also occur if an object is defined (and also initialized) inside one method,
and you try to access it from another method.
Solution: Define the object outside of the method (to make it global).
@hfx642: No, the situation you describe would not compile. The variable would have no scope outside the method in which it was declared. It would complain "Cannot find symbol"
@hfx642: To add to what ezzaral said, variable is not available is correct , but then you may pass it as a parameter , even when you pass that as a parameter, since u say it has been initialized, it will not throw a NPE,unless you specifically initialize it to a NULL.
Also, use of global variables is not at all advised. There may be instances where you may need a few of them , but should not be used everywhere. Passing the variables/references and using them is the most judicious way.
Yes... You are right... Let me think... Oh, yeah...
If you define an object in a method, then use it in a listener,
the listener can't access the object.
eg. Using a JComboBox to alter the text in a JLabel.
If the JLabel is not globally defined, the listener can't alter it.
If the JLabel is not globally defined, the listener can't ... refer to it in its source code. You'll get a compiler "Cannot find symbol". It won't get to execute, so still no NPE.
Alright... Just ignore all of my posts for the moment.
I know there is SOMETHING about this, when I have an object declared AND initialized and I still get a NPE, if I declare it globally, the NPE error goes away.
I just can't think of it at the moment.