Automatically size JPanel inside JFrame

Question

I have a JPanel subclass on which I add buttons, labels, tables, etc. To show on screen it I use JFrame:

MainPanel mainPanel = new MainPanel(); //JPanel subclass

JFrame mainFrame = new JFrame();
mainFrame.setTitle("main window title");
mainFrame.getContentPane().add(mainPanel);
mainFrame.setLocation(100, 100);
mainFrame.pack();
mainFrame.setVisible(true);
mainFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

But when I size the window, size of panel don't change. How to make size of panel to be the same as the size of window even if it was resized?

Answer 1

You need to set a layout manager for the JFrame to use - This deals with how components are positioned. A useful one is the BorderLayout manager.

Simply adding the following line of code should fix your problems:

mainFrame.setLayout(new BorderLayout());

(Do this before adding components to the JFrame)

Answer 2

You can set a layout manager like BorderLayout and then define more specifically, where your panel should go:

MainPanel mainPanel = new MainPanel();
JFrame mainFrame = new JFrame();
mainFrame.setLayout(new BorderLayout());
mainFrame.add(mainPanel, BorderLayout.CENTER);
mainFrame.pack();
mainFrame.setVisible(true);

This puts the panel into the center area of the frame and lets it grow automatically when resizing the frame.