ADVANCED GUIS AND GRAPHICS - Java Programming: From Problem Analysis to Program Design - page 814

private JButton exitB, appendB;
exitB = new JButton("Exit");
appendB = new JButton("Append");

Container pane = getContentPane();

pane.add(headingL);
pane.add(lineTF);
pane.add(whiteBoardTA);
pane.add(appendB);
pane.add(exitB);
We will specify the sizes and locations of the GUI components when we write the
complete program.
In Chapter 6, action listener interfaces are implemented through inner classes. As
explained in Chapter 10, any class containing the application can directly implement
the interface ActionListener . The GUI programs of this chapter directly implement
the interfaces to handle events. Suppose WhiteBoard is the name of the class to imple-
ment the application to create the preceding GUI. Then, the heading of this class is:

public class WhiteBoard extends JFrame implements ActionListener

The method actionPerformed is included as a member of the class WhiteBoard .To
register the action listener with exitB , all you need to do is include the following
statement in the program:
exitB.addActionListener( this );

Of course, the necessary code is placed in the method actionPerformed .
Because action events are generated by the two buttons, the method actionPerformed
uses the methods getActionCommand and equals to identify the source of the event.
The definition of this method is:

public void actionPerformed(ActionEvent e)
{

if (e.getActionCommand().equals("Append")) //Line 1

whiteBoardTA.append(lineTF.getText()); //Line 2

else if (e.getActionCommand().equals("Exit")) //Line 3

System.exit(0); //Line 4

}

If the user clicks the Append button, the if statement in Line 1 evaluates to true .Inthiscase,
the statement in Line 2 retrieves the line of text from the text field object lineTF and appends
it to the text in the text area object whiteBoardTA . When the user clicks the Exit button, the
if statement in Line 3 evaluates to true and the statement in Line 4 terminates the program.
The complete program listing follows:

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;