Java: How can I modify the size of a JPanel in runtime?

Question

I want to modify the size of the JPanel while the program is running, for example with a menu item. How can I reach it? Or how should I modify my program to get acces to the live JPanel in the content pane?

Edit: If I make a contentpane for a gamefield that's 400x400 in the start as default. But what if I want to ad an option in the menu to change the size to 500x500, but without losing all of the content already in progress by the player.

JFrame class:

package me.test;

import javax.swing.JFrame;


public class Main extends JFrame {
    private static final long serialVersionUID = 1L;

public static void main(String[] args) {
    // TODO Auto-generated method stub
    new Main();
}

public Main() {

    setContentPane(new MyJPanel());

    pack();
    setDefaultCloseOperation(EXIT_ON_CLOSE);
    setLocation(100,50);
    setResizable(false);
    setVisible(true);


    }

}

My modified JPanel:

package me.test;

import java.awt.Dimension;

import javax.swing.JPanel;

public class MyJPanel extends JPanel {

    public MyJPanel() {
        setPreferredSize(new Dimension(400,400));
    }
}

Answer 1

Have a look over this code for some ideas. It can change sizes according to actions placed either in a toolbar in the component itself, or menu items of the frame.

Important points of the code are:

• Set a preferred size on the component in question.
• Get a reference to the top level container.
• Finally pack() the top level container.

import java.awt.*;
import java.awt.event.ActionEvent;
import javax.swing.*;
import javax.swing.border.EmptyBorder;

public class GameSize {

    // the GUI as seen by the user (without frame)
    private JPanel gui = new JPanel(new GridBagLayout());
    Action small = new AbstractAction("Small") {

        @Override
        public void actionPerformed(ActionEvent e) {
            setSize(400, 100);
        }
    };
    Action large = new AbstractAction("Large") {

        @Override
        public void actionPerformed(ActionEvent e) {
            setSize(600, 200);
        }
    };

    private final void setSize(int w, int h) {
        gui.setPreferredSize(new Dimension(w, h));
        Container c = gui.getTopLevelAncestor();
        if (c instanceof JFrame) {
            JFrame f = (JFrame) c;
            f.pack();
        }
    }

    GameSize() {
        JToolBar tb = new JToolBar("Size");
        for (Action action : getActions()) {
            tb.add(action);
        }
        gui.add(tb);

        gui.setPreferredSize(new Dimension(200, 100));
        gui.setBackground(Color.RED);
    }

    /*
     * @return the Actions
     */
    public Action[] getActions() {
        Action[] actions = {small, large};

        return actions;
    }

    /**
     * @return the gui
     */
    public JPanel getGui() {
        return gui;
    }

    public static void main(String[] args) {
        Runnable r = new Runnable() {

            @Override
            public void run() {
                GameSize gs = new GameSize();

                JFrame f = new JFrame("Demo");
                f.add(gs.getGui());
                // Ensures JVM closes after frame(s) closed and
                // all non-daemon threads are finished
                f.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
                // See http://stackoverflow.com/a/7143398/418556 for demo.
                f.setLocationByPlatform(true);

                JMenuBar menuBar = new JMenuBar();
                f.setJMenuBar(menuBar);
                JMenu size = new JMenu("Size");
                menuBar.add(size);
                for (Action action : gs.getActions()) {
                    size.add(action);
                }

                // ensures the frame is the minimum size it needs to be
                // in order display the components within it
                f.pack();
                // should be done last, to avoid flickering, moving,
                // resizing artifacts.
                f.setVisible(true);
            }
        };
        // Swing GUIs should be created and updated on the EDT
        // http://docs.oracle.com/javase/tutorial/uiswing/concurrency
        SwingUtilities.invokeLater(r);
    }
}

Answer 2

Playing around....

import java.awt.Dimension;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JFrame;
import javax.swing.JMenu;
import javax.swing.JMenuBar;
import javax.swing.JMenuItem;
import javax.swing.JPanel;


public class PanelResize extends JFrame 
{
    private static final long serialVersionUID = 1L;

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

    private JPanel panel;

    public PanelResize() {

        panel = new MyJPanel();
        getContentPane().add(panel);

        JMenuBar menuBar = new JMenuBar();
        JMenu menu = new JMenu("Resize");
        JMenuItem menuItem = new JMenuItem("Resize");
        menuItem.addActionListener(new ActionListener()
        {
            @Override
            public void actionPerformed(ActionEvent e)
            {
                panel.setPreferredSize(new Dimension(500,500));
                pack();
            }
        });
        menu.add(menuItem);
        menuBar.add(menu);
        this.setJMenuBar(menuBar);

        pack();
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        setLocation(100,50);
        setResizable(false);
        setVisible(true);
    }

}


 class MyJPanel extends JPanel {

    public MyJPanel() {
        setPreferredSize(new Dimension(400,400));
    }
}