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Why would you decide to make every screen a class? How are the screens related and how are they different? Draw out the relationship among them to get an idea.
Why would you decide to make just one JPanel and hide components? How many components would you have to hide? How hard would that be to maintain in comparison?
Why would you decide to make just one class? What is the goal of the program? Does the solution to the problem suggest more than one class?
Ask yourself a bunch of questions until one choice is a sure win.
If no choice becomes a sure win, pick the one you think is pretty.