In Swing, menu bars, menu panes, and menu items are components, just like all other Swing components. JMenuBar is a container designed to hold JMenu objects. JMenu is a container designed to hold JMenuItem objects and other JMenu objects (as submenus). Working with menus is not exactly the same as working with other types of components, however, and Example 3-1 shows a simple example of creating pull-down and pop-up menus.
import java.awt.*; import java.awt.event.*; import javax.swing.*; public class MenuDemo { public static void main(String[] args) { // Create a window for this demo JFrame frame = new JFrame("Menu Demo"); JPanel panel = new JPanel(); frame.getContentPane().add(panel, "Center"); // Create an action listener for the menu items we will create // The MenuItemActionListener class is defined below ActionListener listener = new MenuItemActionListener(panel); // Create some menu panes, and fill them with menu items // The menuItem() method is important. It is defined below. JMenu file = new JMenu("File"); file.setMnemonic('F'); file.add(menuItem("New", listener, "new", 'N', KeyEvent.VK_N)); file.add(menuItem("Open...", listener, "open", 'O', KeyEvent.VK_O)); file.add(menuItem("Save", listener, "save", 'S', KeyEvent.VK_S)); file.add(menuItem("Save As...", listener, "saveas", 'A', KeyEvent.VK_A)); JMenu edit = new JMenu("Edit"); edit.setMnemonic('E'); edit.add(menuItem("Cut", listener, "cut", 0, KeyEvent.VK_X)); edit.add(menuItem("Copy", listener, "copy", 'C', KeyEvent.VK_C)); edit.add(menuItem("Paste", listener, "paste", 0, KeyEvent.VK_V)); // Create a menu bar and add these panes to it. JMenuBar menubar = new JMenuBar(); menubar.add(file); menubar.add(edit); // Add menu bar to the main window. Note special method to add menu bars. frame.setJMenuBar(menubar); // Now create a popup menu and add the some stuff to it final JPopupMenu popup = new JPopupMenu(); popup.add(menuItem("Open...", listener, "open", 0, 0)); popup.addSeparator(); // Add a separator between items JMenu colors = new JMenu("Colors"); // Create a submenu popup.add(colors); // and add it to the popup menu // Now fill the submenu with mutually exclusive radio buttons ButtonGroup colorgroup = new ButtonGroup(); colors.add(radioItem("Red", listener, "color(red)", colorgroup)); colors.add(radioItem("Green", listener, "color(green)", colorgroup)); colors.add(radioItem("Blue", listener, "color(blue)", colorgroup)); // Arrange to display the popup menu when the user clicks in the window panel.addMouseListener(new MouseAdapter() { public void mousePressed(MouseEvent e) { // Check whether this is the right type of event to pop up a popup // menu on this platform. Usually checks for right button down. if (e.isPopupTrigger()) popup.show((Component)e.getSource(), e.getX(), e.getY()); } }); // Finally, make our main window appear frame.setSize(450, 300); frame.setVisible(true); } // A convenience method for creating menu items public static JMenuItem menuItem(String label, ActionListener listener, String command, int mnemonic, int acceleratorKey) { JMenuItem item = new JMenuItem(label); item.addActionListener(listener); item.setActionCommand(command); if (mnemonic != 0) item.setMnemonic((char) mnemonic); if (acceleratorKey != 0) item.setAccelerator(KeyStroke.getKeyStroke(acceleratorKey, java.awt.Event.CTRL_MASK)); return item; } // A convenience method for creating radio button menu items public static JMenuItem radioItem(String label, ActionListener listener, String command, ButtonGroup mutExGroup) { JMenuItem item = new JRadioButtonMenuItem(label); item.addActionListener(listener); item.setActionCommand(command); mutExGroup.add(item); return item; } // An event listener class used with the menu items created above // For this demo, it just displays a dialog box when an item is selected public static class MenuItemActionListener implements ActionListener { Component parent; public MenuItemActionListener(Component parent) { this.parent = parent; } public void actionPerformed(ActionEvent e) { JMenuItem item = (JMenuItem) e.getSource(); String cmd = item.getActionCommand(); JOptionPane.showMessageDialog(parent, cmd + " was selected."); } } }
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