Why is my JPanel hogging up the JFrame?

Question

I am relatively new to programming, so I am sorry if this question is stupid. I am creating a Java program that involves one JButton inside a JPanel, and the JPanel is in a JFrame. Another button is outside the JPanel but still in the JFrame. I set the layout to a BoxLayout. My problem is that the the panel, which I made black, is taking up the whole JFrame except for where the second button is. How do I make the JPanel so it is only taking up the area right around the first button?

public class alt {
    JFrame frame = new JFrame();
    JPanel panel = new JPanel();
    JButton button1 = new JButton("button 1");
    JButton button2 = new JButton("button 2");
    public alt(){
        frame.setVisible(true);
        frame.getContentPane().setLayout(new BoxLayout(frame.getContentPane(), BoxLayout.Y_AXIS));
        panel.setBackground(Color.black);
        frame.setTitle("test");
        frame.setExtendedState(java.awt.Frame.MAXIMIZED_BOTH);
        panel.add(button1);
        frame.add(panel);
        frame.add(button2);
        button2.setAlignmentX(Component.CENTER_ALIGNMENT);
    }
}

Answer 1

The reason why your panel takes up the bulk of the frame's content pane lies in the way the BoxLayout manager works with the minimum, preferred, and maximum values of components. It takes the maximum value of a component into account. And since the maximum value of a JPanel is huge, it takes all the space available. The solution is to change the maximum value of a panel. However, this is bad practice. I do not recommend to use the BoxLayout manager -- it is very weak and leads to poor code.

I recommend to use either the MigLayout manager or the GroupLayout manager.

I provide three solutions: a corrected BoxLayout solution, a MigLayout solution, and a GroupLayout solution.

BoxLayout solution

We determine the maximum size of the button and change the panel's size to be a bit larger than the button's.

package com.zetcode;

import java.awt.Color;
import java.awt.Component;
import java.awt.Dimension;
import java.awt.EventQueue;
import javax.swing.BorderFactory;
import javax.swing.Box;
import javax.swing.BoxLayout;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;


public class BoxLayoutPanel extends JFrame {

    public BoxLayoutPanel() {

        initUI();
    }

    private void initUI() {

        JPanel cpane = (JPanel) getContentPane();
        cpane.setBorder(BorderFactory.createEmptyBorder(15, 15, 15, 15));
        cpane.setLayout(new BoxLayout(cpane, 
                BoxLayout.Y_AXIS));        

        JPanel pnl = new JPanel();

        JButton btn1 = new JButton("Button 1");
        JButton btn2 = new JButton("Button 2");

        Dimension dm = btn1.getMaximumSize();
        dm.height += 15;
        dm.width += 15;

        pnl.setMaximumSize(dm);

        pnl.setBackground(Color.black);
        add(pnl);
        add(Box.createVerticalStrut(10));
        pnl.add(btn1);
        btn2.setAlignmentX(Component.CENTER_ALIGNMENT);
        add(btn2);

        setTitle("BoxLayout solution");
        pack();
        setLocationRelativeTo(null);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                BoxLayoutPanel ex = new BoxLayoutPanel();
                ex.setVisible(true);
            }
        });   
    }
}

This is not a clean solution. Generally, we should avoid calling the getMaximumSize() and the setMaximumSize() in the application code -- this is the layout manager's job. Also in three occasions, we use fixed pixel widths: when we define an empty border, a vertical strut, and a maximum panel's size. This code is however not portable. Pixel widths change when the resolution of the screen changes. This is a shortcoming of the BoxLayout manager.

MigLayout solution

This solution is much cleaner and more portable. MigLayout is a third-party manager, so we need to download additional libraries.

package com.zetcode;

import java.awt.Color;
import java.awt.EventQueue;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import net.miginfocom.swing.MigLayout;


public class MigLayoutPanel extends JFrame {

    public MigLayoutPanel(){

        initUI();

        setTitle("MigLayout solution");
        setLocationRelativeTo(null);       
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }

    private void initUI() {

        JPanel main = new JPanel(new MigLayout("center"));

        JPanel pnl2 = new JPanel();

        JButton btn1 = new JButton("Button 1");
        JButton btn2 = new JButton("Button 2");

        pnl2.setBackground(Color.black);

        pnl2.add(btn1);
        main.add(pnl2, "wrap");
        main.add(btn2, "alignx center");

        add(main);

        pack();
    }


    public static void main(String[] args){

        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                MigLayoutPanel ex = new MigLayoutPanel();
                ex.setVisible(true);
            }
        });
    }
}

GroupLayout solution

GroupLayout is a built-in layout manager. With MigLayout, they are the most portable and flexible layout managers.

package com.zetcode;

import java.awt.Color;
import java.awt.Container;
import java.awt.EventQueue;
import javax.swing.GroupLayout;
import static javax.swing.GroupLayout.Alignment.CENTER;
import static javax.swing.GroupLayout.DEFAULT_SIZE;
import static javax.swing.GroupLayout.PREFERRED_SIZE;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import static javax.swing.LayoutStyle.ComponentPlacement.RELATED;


public class GroupLayoutPanel extends JFrame {

    public GroupLayoutPanel(){

        initUI();

        setTitle("GroupLayout solution");
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setLocationRelativeTo(null);        
    }

    private void initUI() {

        Container pane = getContentPane();
        GroupLayout gl = new GroupLayout(pane);
        pane.setLayout(gl); 

        JPanel pnl = new JPanel();

        JButton btn1 = new JButton("Button 1");
        pnl.add(btn1);

        JButton btn2 = new JButton("Button 2");

        pnl.setBackground(Color.black);

        gl.setAutoCreateGaps(true);

        gl.setHorizontalGroup(gl.createSequentialGroup()
                .addContainerGap(DEFAULT_SIZE, Integer.MAX_VALUE)
                .addGroup(gl.createParallelGroup(CENTER)
                        .addComponent(pnl, DEFAULT_SIZE, DEFAULT_SIZE, 
                                PREFERRED_SIZE)
                        .addComponent(btn2))
                .addContainerGap(DEFAULT_SIZE, Integer.MAX_VALUE)
        );

        gl.setVerticalGroup(gl.createSequentialGroup()
                .addContainerGap()
                .addComponent(pnl, DEFAULT_SIZE, DEFAULT_SIZE, 
                        PREFERRED_SIZE)
                .addPreferredGap(RELATED)
                .addComponent(btn2)
                .addContainerGap()
        );

        pack();
    }


    public static void main(String[] args){

        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                GroupLayoutPanel ex = new GroupLayoutPanel();
                ex.setVisible(true);
            }
        });
    }
}

Answer 2

You could make use of a different layout manager, one which gives you more control over deciding how space is allocated and filling is handled, for example, GridBagLayout...

import java.awt.Color;
import java.awt.EventQueue;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class SampleLayout {

    public static void main(String[] args) {
        new SampleLayout();
    }

    public SampleLayout() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                }

                JPanel panel = new JPanel();
                JButton button1 = new JButton("button 1");
                JButton button2 = new JButton("button 2");

                panel.setBackground(Color.black);
                panel.add(button1);

                JFrame frame = new JFrame("Testing");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setLayout(new GridBagLayout());
                GridBagConstraints gbc = new GridBagConstraints();
                gbc.gridwidth = GridBagConstraints.REMAINDER;
                frame.add(panel, gbc);
                frame.add(button2, gbc);
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

}

Take a look at Laying Out Components Within a Container for more details