Popup Menus

You've already encountered the Choice class that implements a popup menu component. Choices are fixed in a particular location.

The PopupMenu class, on the other hand, is activated when the user holds the right mouse button or otherwise indicates that they want to pop up a menu. Typically this is used for context sensitive menus.

java.awt.PopupMenu is a subclass of of java.awt.Menu. For the most part you use it just like you use a regular menu. Items are added to it with the add(MenuItem mi) method, and user selections are responded to by installing an ActionListener on the MenuItem. For example, to build a popup menu with a number of URLs you might do this:

    PopupMenu pm = new PopupMenu();
    MenuItem mi = new MenuItem("http://java.sun.com/");
    mi.addActionListener(URLActionListener);
    pm.add(mi); 
    mi = new MenuItem("http://home.netscape.com/");
    mi.addActionListener(URLActionListener);
    pm.add(mi); 
    mi = new MenuItem("http://www.cafeaulait.org/course/");
    mi.addActionListener(URLActionListener);
    pm.add(mi); 
    MenuItem mi = new MenuItem("http://www.yahoo.com/");
    mi.addActionListener(URLActionListener);
    pm.add(mi); 

However PopupMenus don't belong to any particular MenuBar. Instead they're added to a component. For example, given a Frame f, you would install the PopupMenu pm in the frame by passing it to the frame's add() method, like so:

   f.add(pm);

(Is this the same add() method used to add components to a frame?)

The exact trigger for popping up the menu is platform dependent. For example, on Windows a PopupMenu is triggered on right mouse button up. However, in Motif a PopupMenu is triggered on right mouse button down. Regardless of the exact sequence of events leading to a menu popping up, when the user makes a selection, when a MenuItem is selected an ActionEvent is fired to any listeners registered for that item.

A PopupMenu can be deinstalled from a component by passing it to the component's remove() method like this:

   f.remove(pm);

No more than one PopupMenu can be installed in any given component. If a PopupMenu is installed in a container, then triggers over the container's components will also trigger the popup, provided the contained component has no popup of its own.