Graphical User Interfaces - Building Java Programs: A Back to Basics Approach - page 833

8 JOptionPane.showMessageDialog( null ,
9 "You clicked the button!");
10 }
11 }

Now that we've written this class, we can attach a MessageListener object to
any button or other component of interest, and it will respond to action events on that
component. For example, if we attach a MessageListener to a button, an option
pane will pop up whenever that button is clicked. To attach the listener, we'll use a
method called addActionListener that is found in several Swing components and
that accepts a parameter of type ActionListener .
For example, we can add the following line to the ComponentsExample program
we developed earlier to attach the listener to the first button:

// attach a listener to handle events on this button
button1.addActionListener(new MessageListener());

Here is the result when the program is executed and the user clicks on the button:

Note that a listener only responds to events on components to which it is added. If
the user clicks the “Click me!” button, nothing will happen. If we want the same event
to occur when the user clicks this button, we must add a MessageListener to it. If
we want a different response to occur on a click of this button, we can write a second
class that implements ActionListener and attach one of its objects to the button.
To summarize, follow these steps to handle an event in Java:

• Write a class that implements ActionListener .

• Place the code to handle the event into its actionPerformed method.

• Attach an object of your listener class to the component of interest using its

addActionListener method.

14.2 Laying Out Components

Earlier, we used a FlowLayout to position the components in our frame. In this sec-
tion, we will explore several different layout manager objects that can be used to
position components in a variety of ways.