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Java

I have two Jframe which is Login and register. I want to link the Login Jbutton to view the main page which is only a class java not a Jframe. How to link the Jframe with the class java??

What I have tried:

I have try

JFrame mP = new JFrame("Other Windows");
mP.setVisible(true);
mP.pack();
mP.setLocationRelativeTo(null);
mP.setDefaultCloseOperation(JFrame.MAXIMIZED_BOTH);