Learning Java, 4th Edition

Pop-Up Menus

One of Swing’s nifty components is JPopupMenu, a context menu that appears at the mouse location when you press the appropriate mouse button or keystroke. (On a two-button mouse, clicking the right mouse button invokes a pop-up menu. On a single-button Mac, you Command-click.) Which button you press depends on the platform you’re using; fortunately, from the code’s point of view you don’t have to care—Swing figures it out for you.

The care and feeding of JPopupMenu is basically the same as any other menu. You use a different constructor—JPopupMenu()—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 attach it to a JMenuBar. Instead, just pop up the menu whenever and wherever you need it. Prior to Java 5.0, this process is a little cumbersome; you have to register to receive the appropriate mouse events, check them to see if they are the pop-up trigger and then pop the menu manually. With Java 5.0, the process is simplified by having components manage their own pop-up menus.

First, we’ll show an example of explicit pop-up handling. The following example, PopupColorMenu, contains three buttons. You can use a JPopupMenu to set the color of each button or the background frame itself, depending on where you click the mouse.

    //file: PopUpColorMenu.java
    import java.awt.*;
    import java.awt.event.*;
    import javax.swing.*;

    public class PopUpColorMenu implements ActionListener
    {
      Component selectedComponent;

      public ...