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Editorial Team
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Asked: 2026-06-01T17:52:28+00:00

Situation: a main form calls a modal jDialog with textboxes in which parameters are

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Situation: a main form calls a modal jDialog with textboxes in which parameters are filled in by the user to create or modify an instance of a certain class, call it ClassA.

When the dialog needs to modify an existing instance, it is passed in the constructor as a parameter. Otherwise the jDialog will create a new instance of ClassA.

Problem: the mainform needs to access that new instance, and I think it is unclean code to pass the entire main form as a parameter, and let the dialog push the new instance into it by a method call, because that way a perfectly re-usable stand-alone dialog becomes only usable with one single main form that needs a certain classname and method to receive the new instance.

It is much more logical to make the main form get the new instance from the jdialog after the OK button is clicked, by calling a getClassAInstance() method (which could be called also when an existing instance was being modified). The method is called after the “setVisible(true)” method on a new instance of the jdialog in question. The dialog appears, the thread of the main form will sleep until the dialog is closed (because it is modal). The OK button calls the dispose() method of the jDialog, then the very next statement is the getClassAInstance() call on the jDialog by the mainform.

Here’s the same thing in code..

ClassAInstanceMakerDialog imd = new ClassAInstanceMakerDialog(this, true);
imd.setVisible(true);
//imd.dispose(); after OK button click
System.out.println(imd.getClassAInstance()); //return a new ClassA instance

//output: whatever ClassA.toString() should return, works fine

Question: I’ve tried it and it seems to work perfectly fine. But, is it a good code? Is there any danger of the getClassAInstance() method returning “null”, because the garbage collector collected the ClassA instance after the jDialog was disposed and before the main form could complete the call?

Please excuse me if I didn’t make myself clear, I’m not a native English speaker. If you would rather see some code, let me know…

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1 Answer

  1. Editorial Team

    I think it’s perfectly legal to access the member variable of your dialog instance that holds the ClassA instance, the dialog instance will not be garbage collected until it goes out of scope, not just because you called dispose on it.

    I’d give slight preference to a solution where you define an event handler interface with a signature of

    someThingHappened(ClassA toThisObject), make your mainform or anything that might be interested that ClassA thing implement that interface make it possible to add listeners to the dialog before making it visible.

    That way, you would loosen the coupling between the dialog and the main form a little.

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