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Trail: Creating a GUI with JFC/Swing
Lesson: Writing Event Listeners

## How to Write a Container Listener

> Container events are fired by a `Container` just after a component is added to or removed from the container. These events are for notification only -- no container listener need be present for components to be successfully added or removed.

> The following applet demonstrates container events. By clicking Add a button or Remove a button, you can add components to or remove them from a panel at the bottom of the applet. Each time a component is added to or removed from the panel, the panel fires a container event, and the panel's container listener is notified. The listener displays descriptive messages in the text area at the top of the applet.

> This is a picture of the applet's GUI. To run the applet, click the picture. The applet will appear in a new browser window.

> > Try this:
> > 1. Click the button labeled Add a button.
> >    You'll see a button appear near the bottom of the applet. The container listener (in this example, an instance of `ContainerEventDemo`) reacts to the resulting component-added event by displaying "JButton #1 was added to javax.swing.JPanel" at the top of the applet.
> > 2. Click the button labeled Remove a button.
> >    This removes the most recently added button from the panel, causing the container listener to receive a component-removed event.

> You can find the applet's code in `ContainerEventDemo.java`. Here is the applet's container event handling code:

> >     public class ContainerEventDemo ... implements ContainerListener ... {
> >         ...//where initialization occurs:
> >     	buttonPanel = new JPanel();
> >     	buttonPanel.addContainerListener(this);
> >         ...
> >         public void componentAdded(ContainerEvent e) {
> >     	displayMessage(" added to ", e);
> >         }

> >         public void componentRemoved(ContainerEvent e) {
> >     	displayMessage(" removed from ", e);
> >         }

> >         void displayMessage(String action, ContainerEvent e) {
> >     	display.append(((JButton)e.getChild()).getText()
> >     		       + " was"
> >     		       + action
> >     		       + e.getContainer().getClass().getName()
> >     		       + newline);
> >         }
> >         ...
> >     }

### The Container Event API

> The `ContainerListener` interface and its corresponding adapter class, `ContainerAdapter`, contain two methods:
> `void componentAdded(ContainerEvent)`
>   Called just after a component is added to the listened-to container.
> `void componentRemoved(ContainerEvent)`
>   Called just after a component is removed from the listened-to container.
> Each container event method has a single parameter: a `ContainerEvent` object. The `ContainerEvent` class defines two useful methods:
> `Component getChild()`
>   Returns the component whose addition or removal triggered this event.
> `Container getContainer()`
>   Returns the container that fired this event. You can use this instead of the `getSource` method.

### Examples that Use Container Listeners

> The following table lists the examples that use container listeners.

> Example | Where Described | Notes
> --- | --- | ---
> `ContainerEventDemo` | This section | Reports all container events that occur on a single panel to demonstrate the circumstances under which container events are fired.

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