Java swing BoxLayout center doesn't work

Question

I have a JFrame that has JPanel inside. Inside JPanel there are 2 buttons. JPanel has a BoxLayout. I need buttons to be displayed horizontally in the center of the window. Here is my code:

I just create two buttons, set their alignment to the center (tried all the ways I knew) and add them to the panel horizontally.

public class UserInterface extends JFrame  {
   public UserInterface()  {
                setup();
   }
   private void setup()  {
         ...
         panel=new UserInterfacePanel();
         add(panel);
   }
}

class UserInterfacePanel extends JPanel {
         private JToggleButton startButton;
         private JToggleButton stopButton;

         public UserInterfacePanel()  {
             setup();
         }

         private void setup()  {
            setLayout(new BoxLayout(this, BoxLayout.X_AXIS));

            setupButtons();
            setupButtonsActions();
            add(startButton);
            add(stopButton);
        }

        private void setupButtons()  {
            ...
            startButton.setHorizontalAlignment(JButton.CENTER);
            stopButton.setHorizontalAlignment(JButton.CENTER);
            startButton.setAlignmentX(CENTER_ALIGNMENT);
            stopButton.setAlignmentX(CENTER_ALIGNMENT);
        }
}

And yet it doesn't work. Why doesn't it and how to fix it?

Answer 1

buttons are still located at the left side of the window.

If you want the components center horizontally on the line you need to use "glue"

add(Box.createHorizontalGlue());
add(startButton);
add(stopButton);
add(Box.createHorizontalGlue());

Answer 2

You're setting the x alignment of the buttons but you're box layout lays out along the x axis -- doesn't make sense. Either set the y alignment and use the x axis for the box layout direction, or set the x alignment and use the y axis for the box layout direction.

e.g.,

private void setup() {
    setLayout(new BoxLayout(this, BoxLayout.Y_AXIS));  // **** change ****

    setupButtons();
    // setupButtonsActions();
    add(startButton);
    add(stopButton);
}

private void setupButtons() {
    // ...
    startButton.setHorizontalAlignment(JButton.CENTER);
    stopButton.setHorizontalAlignment(JButton.CENTER);
    startButton.setAlignmentX(CENTER_ALIGNMENT);
    stopButton.setAlignmentX(CENTER_ALIGNMENT);
}

or

private void setup() {
    setLayout(new BoxLayout(this, BoxLayout.X_AXIS));  

    setupButtons();
    // setupButtonsActions();
    add(startButton);
    add(stopButton);
}

private void setupButtons() {
    // ...
    startButton.setHorizontalAlignment(JButton.CENTER);
    stopButton.setHorizontalAlignment(JButton.CENTER);
    startButton.setAlignmentY(CENTER_ALIGNMENT);  // **** change ****
    stopButton.setAlignmentY(CENTER_ALIGNMENT);  // **** change ****
}

To center all, use a different layout, such as a GridBagLayout:

import java.awt.Dimension;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.Insets;    
import javax.swing.*;

@SuppressWarnings("serial")
public class UserInterfacePanel extends JPanel {
    private static final Insets INSETS = new Insets(3, 3, 3, 3);
    private static final int PREF_W = 800;
    private static final int PREF_H = 650;
    private JToggleButton startButton = new JToggleButton("Start");
    private JToggleButton stopButton = new JToggleButton("Long Texted Title");

    public UserInterfacePanel() {
        setup();
    }

    private void setup() {
        setLayout(new GridBagLayout());  // **** change

        add(startButton, createGbc(0, 0));
        add(stopButton, createGbc(1, 0));
    }


    private GridBagConstraints createGbc(int x, int y) {
        GridBagConstraints gbc = new GridBagConstraints();
        gbc.gridx = x;
        gbc.gridy = y;
        gbc.insets = INSETS;
        return gbc;
    }

    @Override
    public Dimension getPreferredSize() {
        if (isPreferredSizeSet()) {
            return super.getPreferredSize();
        }
        return new Dimension(PREF_W, PREF_H);
    }


    private static void createAndShowGui() {
        UserInterfacePanel mainPanel = new UserInterfacePanel();

        JFrame frame = new JFrame("UserInterfacePanel");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().add(mainPanel);
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> createAndShowGui());
    }
}