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Exploring Java

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Creating GUI Components
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14.5 PopupMenu

One of the new components in Java 1.1 is PopupMenu: a menu that automatically appears when you press the appropriate mouse button inside of a component. Which button you press depends on the platform you're using; fortunately, you don't have to care.

The care and feeding of PopupMenu is basically the same as any other menu. You use a different constructor (PopupMenu()) to create it, but otherwise, you build a menu and add elements to it the same way. The big difference is that you don't need to attach it to MenuBar, and consequently don't need to worry about putting MenuBar in a Frame. Instead, you call add() to add the PopupMenu to any component.

The PopupColorMenu applet contains three buttons. You can use a PopupMenu to set the color of each button or the applet itself, depending on where you press the mouse. (Setting the color of the applet also sets the buttons' colors.) Figure 14.6 shows the applet in action; the user is preparing to change the color of the right-most button.

Figure 14.6: The PopupColorMenu applet

Figure 14.6
import java.awt.*;
import java.awt.event.*;
public class PopUpColorMenu extends java.applet.Applet 
implements ActionListener {
    PopupMenu colorMenu;
    Component selectedComponent;
    public void init() {        
        add( new Button("One") );
        add( new Button("Two") );
        add( new Button("Three") );
        
        colorMenu = new PopupMenu("Color");
        colorMenu.add( makeMenuItem("Red") );
        colorMenu.add( makeMenuItem("Green") );
        colorMenu.add( makeMenuItem("Blue") );
        addMouseListener( new MouseAdapter() {
            public void mousePressed(MouseEvent e) {
                if ( e.isPopupTrigger() ) {
                    selectedComponent = getComponentAt( e.getX(), e.getY() );
                    colorMenu.show(e.getComponent(), e.getX(), e.getY());
                }
            }
        } );
        add(colorMenu);
    }
    public void actionPerformed(ActionEvent e) {
        String color = e.getActionCommand();
        if ( color.equals("Red") )
            selectedComponent.setBackground( Color.red );
        else if ( color.equals("Green") )
            selectedComponent.setBackground( Color.green );
        else if ( color.equals("Blue") )
            selectedComponent.setBackground( Color.blue );
    }
    private MenuItem makeMenuItem(String label) {
        MenuItem item = new MenuItem(label);
        item.addActionListener( this );
        return item;
    }
}

Because the popup menu is triggered by mouse events, we need to register a MouseListener. In our call to addMouseListener(), we create an anonymous inner class based on the MouseAdapter. In this class, we override the mousePressed() method to display the popup menu when we get an appropriate event. How do we know what an "appropriate event" is? Fortunately, we don't need to worry about the specifics of our user's platform; we just need to call the event's isPopupTrigger() method. If this method returns true, we know the user has done whatever normally displays a popup menu on his system.

Once we know that the user wants to raise a popup menu, we figure out which component the mouse is over by calling getComponentAt(), with the coordinates of the mouse click (given by e.getX() and e.getY()). Then we display the popup menu by calling its show() method, again with the mouse coordinates as arguments.

If we wanted to provide different menus for different types of components or the background, we could add a test within the check for the popup trigger:

if ( e.isPopupTrigger() ) {
    selectedComponent = getComponentAt( e.getX(), e.getY() );
          
    if ( selectedComponent instanceof Button )
         colorMenu.show(e.getComponent(), 
                        e.getX(), e.getY());
    else if ( selectedComponent instanceof Applet )
            // show a menu for the background
    else if ( selectedComponent instanceof someOtherComponent )
            // show another menu
}

The only thing left is to handle the action events from the popup menu items. As in our earlier example, we use a helper method called makeMenuItem() to register the applet as an action listener for every item we add. The applet implements ActionListener and has the required actionPerformed() method. This method reads the action command from the event, which is equal to the selected menu item's label by default. It then sets the background color of the selected component appropriately.


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