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JOptionPane

Maureen Charlton
Ranch Hand

Joined: Oct 04, 2004
Posts: 218
When using JOptionPane to get input from a user and you wish to write code to do something i.e. Println ("Program finished") when the user selects the Cancel button. What would you write?

Would you write:
dataInput = JOptionPane.showInputDialog(message1);
OK = dataInput.equals(null);

if (OK)
{
System.out.println("Cancel has been selected. Data Input finished.");
}//end if statement

When I did this I got an error Null Pointer Exception and I don't know why?
Could someone confirm whether using null as the validation criteria for the Cancel button being selected is alright? Or is there a better way of doing this?
Joel McNary
Bartender

Joined: Aug 20, 2001
Posts: 1824

don't use dataInput.equals(null), use dataInput == null.

The former assumes that the dataInput actually refers to an object and thus can call a method on that object. The latter checks to see if the reference is to an object or to null.


Piscis Babelis est parvus, flavus, et hiridicus, et est probabiliter insolitissima raritas in toto mundo.
Maureen Charlton
Ranch Hand

Joined: Oct 04, 2004
Posts: 218
Joel,

Thank you! It worked BUT I would like to understand a little more.

You use equals when it is an object; in other words, a creation of the class (the constructor using the new word) - it that the only time you use equals?
Dave Cryder
Greenhorn

Joined: Aug 11, 2004
Posts: 9
Maureen,

Yes. You can't reference .equals() on a reference that isn't actually referencing an object of the class.



You should check the state of the object by using "==" if there is a chance that the object wasn't initialized.

[ edited to break long line -ds ]
[ December 03, 2004: Message edited by: Dirk Schreckmann ]
 
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subject: JOptionPane