Question

I have a JButton that I would like to expand to fill the size of a JPanel that is holding it. I’ve tried doing this a few different ways, with no luck. Here are some attempts:

1. Manual size setting as recommended here – size of the button did not change.

   panel  = new JPanel(new CardLayout());
   button = new JButton();
   button.setPreferredSize(new Dimension(BUTTON_SIZE, BUTTON_SIZE)); 
   panel.add(button);
   

2. Trying to use a BorderLayout to expand the button as hinted at here. Size did not change.

   panel  = new JPanel(new BorderLayout());
   button = new JButton();
   panel.add(button, BorderLayout.CENTER);
   

I’m probably doing something incorrectly, so any help is much appreciated.

EDIT

Here is a summary of what resolved it. There are 2 things that worked

1. Removing the LayoutManager (in the call to new JPanel), which according to Oracle defaults to BorderLayout.
2. Adding the dimensions to the call as in the accepted answer (i.e. new BorderLayout(0,0)).

Answer 1

Your second attempt should be working, but that does not mean your entire top level container will be filled by the JPanel that contains your JButton. When there are no other components in a BorderLayout container, the center component will expand to fill the entire container.

http://docs.oracle.com/javase/tutorial/uiswing/layout/border.html

If the window is enlarged, the center area gets as much of the available space as possible.

Can you post an image of what you’re seeing when you run your app? Here is a working example below. One thing worth noticing is that every container all the way up to the top JFrame is also using BorderLayout. It’s possible in your attempt that some upper level container is restricting the size of your JPanel, and therefore also the JButton inside.

frame = new JFrame();
frame.setBounds(100, 100, 450, 300);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

JPanel panel = new JPanel();
panel.setLayout(new BorderLayout(0, 0));
JButton button = new JButton("I'm a button!");
panel.add(button);

frame.getContentPane().add(panel, BorderLayout.CENTER);