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Right.
One was mkay that extends JFrame

The other was:
JFrame jtfMainFrame;
...
jtfMainFrame = new JFrame("Which Button Demo");

Do you now see where there are two windows being created?

What title is shown in the window with the components? Do a Find in the source for that String to see where it is used. Or look for a statement that has: new JFrame(

The IDE should not make any difference. Your friend's code must have been different.

Did you see two windows after the change I suggested?
Look at the code. There are two JFrame objects being...

Try changing the setVisible parameter to true and see what happens.

What happens when you execute the program?