How can I center a JButton in a JPanel using the default FlowLayout?

Question

I'm trying to center a JButton in a JPanel using the default FlowLayout. Here is my code:

import javax.swing.*;
import java.awt.*;
public class CoinFlip {
    public static void main(String[] args) {
        JFrame frame = new JFrame("Coin Flip");
        frame.setSize(500, 500);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        JButton coinFlipButton = new JButton("Flip Coin");
        coinFlipButton.setSize(100, 100);
        JPanel coinFlipPanel = new JPanel();
        coinFlipPanel.setSize(500, 100);
        coinFlipPanel.setBackground(Color.GRAY);
        coinFlipPanel.add(coinFlipButton);
        JTextArea coinFlipResults = new JTextArea();
        coinFlipResults.setSize(500, 200);
        frame.getContentPane().add(coinFlipPanel);
        frame.getContentPane().add(coinFlipResults);
        frame.setVisible(true);
    }
}

How can I center the JButton (coinFlipButton) in the JPanel (coinFlipPanel)? I've tried using FlowLayout.CENTER but that makes the button increase in size so that it takes up all the space.

This code doesn't really seem to make sense. If you're adding two components to the JFrame like this, you're adding them both to the same area and the JFrame should only display one (because its layout, BorderLayout, works like that). And trying to center a FlowLayout doesn't make sense to me personally. Try to tell us what you want to do and we'll try to come up with something that works. — markspace, Jul 29 '21 at 20:49
Your question is incomplete. You have two components and don't specify the requirements for each of the components. Do you want the components on the same line or displayed vertically or some other arrangement? Also, don't use setSize(...). It does nothing. The layout manager will determine the size of each component based on the rules of the layout manager. — camickr, Jul 29 '21 at 22:46
See [this answer](https://stackoverflow.com/a/7181197/418556) for four ways to center a component. I generally don't want the component stretched to the size of the container (panel) so avoid the two on the left. I prefer `GridBagLayout` (top right) to `BoxLayout`.. **General Tips:** 1) See [Should I avoid the use of set(Preferred|Maximum|Minimum)Size methods in Java Swing?](http://stackoverflow.com/q/7229226/418556) (Yes.) That advice also applies to `setSize()`. There are a variety of ways to make a button larger. .. — Andrew Thompson, Jul 30 '21 at 01:37
.. 2) Provide ASCII art or a simple drawing of the *intended* layout of the GUI at minimum size, and if resizable, with more width and height - to show how the extra space should be used. — Andrew Thompson, Jul 30 '21 at 01:40

markspace · Answer 1 · 2021-07-29T23:43:42.700

I double checked with the docs and FlowLayout does have an option to center its components, so mea culpa on that. But otherwise you just kind of have to read the tutorials on the various components and their layouts, and experiment a bit.

One very important point is that jframe.getContentPane().add() for a default JFrame uses a layout that only admits one single component. If you want more than one, you have to use a second component like a JPanel with a Grid or Flow layout. By calling that method twice you're just replacing the previous component. Please go read the tutorial on JFrames and BorderLayout.

Anyway I made a quick program that does something like what you want. I added your text area to a ScrollPane because that's another way of dealing with the main content pane of a JFrame. And when I added the JPanel with the button I added it to the SOUTH part of the BorderLayout, because that's how you display more than one component with BorderLayout. (Tutorial again.)

public class JFrameBorderWithButton {
   public static void main( String[] args ) {
      SwingUtilities.invokeLater( JFrameBorderWithButton::newGui );
   }
   private static void newGui() {
      JFrame frame = new JFrame();
      
      JTextArea text = new JTextArea(5, 40);
      JScrollPane scroll = new JScrollPane( text );
      frame.add( scroll );
      JPanel panel = new JPanel( new FlowLayout( FlowLayout.CENTER ) );
      JButton button = new JButton( "Press Me" );
      panel.add(  button );
      frame.add( panel, BorderLayout.SOUTH );
      
      frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
      frame.pack();
      frame.setLocationRelativeTo( null );
      frame.setVisible( true );
   }
}

*"FlowLayout does have an option to center its components"* Horizontally, yes. For both horizontal **and** vertical centering, use a `GridBagLayout`. — Andrew Thompson, Jul 30 '21 at 01:35
Thanks, I'm not really familiar with GridBag, didn't know that. — markspace, Jul 30 '21 at 01:53

score 0 · Answer 2 · answered Jul 29 '21 at 21:20

In my experience, the simple layouts are rarely worthwhile. use something like GridBagLayout

public static void main(String [] args) throws IOException {
    JFrame frame = new JFrame("Coin Flip");
    JTextArea textArea = new JTextArea();
    JScrollPane textScroll = new JScrollPane(textArea);
    Action flipAction = new AbstractAction("Flip Coin!") {
        private Random random = new Random();
        
        @Override
        public void actionPerformed(ActionEvent e) {
            if (!textArea.getText().isEmpty()) {
                textArea.append(System.lineSeparator());
            }
            
            if (random.nextBoolean()) {
                textArea.append("Heads!");
            } else {
                textArea.append("Tails!");
            }
        }
    };
            
    JButton flipButton = new JButton(flipAction);
    
    frame.setLayout(new GridBagLayout());
    frame.setPreferredSize(new Dimension(175, 500));
    frame.add(flipButton, new GridBagConstraints(0, 0, 1, 1, 1.0, 0.0, GridBagConstraints.CENTER, GridBagConstraints.HORIZONTAL, new Insets(4, 4, 4, 4), 0, 0));
    frame.add(textScroll, new GridBagConstraints(0, 1, 1, 3, 1.0, 1.0, GridBagConstraints.CENTER, GridBagConstraints.BOTH, new Insets(4, 4, 4, 4), 0, 0));
    frame.pack();
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setLocationRelativeTo(null);
    frame.setVisible(true);
}

*simple layouts are rarely worthwhile* - would tend to disagree. They are handy when used properly. For example a FlowLayout for a tool bar or a GridLayout for a grid. A GridBagLayout can be overly complex especially when you use code like the above. Most people don't know the order of all the 9 parameters and what they mean. In fact the "gridheight" of 3, does nothing, since you can just randomly assign a component to take up 3 cells. Also, OP wants the button centered, so using a fill of horizontal defaults the purpose — camickr, Jul 29 '21 at 23:58
*"the simple layouts are rarely worthwhile. use something like GridBagLayout"* This sounds as though you're not familiar with *combining* layouts. Combining them is the simplest way to create complex, robust GUIs. See [this answer](https://stackoverflow.com/questions/5621338/how-to-add-jtable-in-jpanel-with-null-layout/5630271#5630271) for one example. — Andrew Thompson, Jul 30 '21 at 02:08
I've been writing swing for near 15 years. Combining layouts is a sign you don't know how to use more complex layouts. GridBagLayout and MiGLayout are by far them more useful Layouts. The only time I've seen embedded layouts necessary is when adding headers and footers to a screen. — Ryan, Jul 30 '21 at 14:12

How can I center a JButton in a JPanel using the default FlowLayout?

2 Answers2