1. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Advanced Java Programming CSE 7345/5345/ NTU 531 Session 7 Welcome Back!!!

2. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp claurent@engr.smu.edu Chantale Laurent-Rice Welcome Back!!! trice75447@aol.com Office Hours: by appt 3:30pm-4:30pm SIC 353

3. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Short review Packages In Java, each class is defined in a single file called ClassName.java. A package is a name for a set of related classes, Java uses package names to locate classes at compile- time and at runtime. Packages can be seen as some kind of libraries in which the classes are grouped. The difference is that packages provide an additional scope. Each class file must belong to a package: Example: Package dpack; Package java.awt.event;

4. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Packages con’t If no package is given, the class is placed in a default package with no name. A compilation unit (a file) declares one or more classes. It is a file that may contain several class declarations. It is used as an input for the java Compiler (javac) which outputs a series of class files (bytecodes). There is a class file for each class in the compilation unit. At the most one class is declared public. Example: Source code: ClassName.java hello.java Bytecode:className.class hello.class

5. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Packages con’t To use the classes, one or more modules in a program can import the entire package with the declaration, such as import graphics.*; The asterisk indicates to the compiler that it should import all classes in the graphics package.

6. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Standard packages Java.applet-provides the means to create applet Java.awt- provides class-based GUI with windows Java.awt.*- have subpackages such as java.awt,color, java.awt.font, and java.awt.image Java.beans- provides basic elements for JavaBeans Java.io- provides I/O Java.lang- wrapper classes such as: Char, int, Double, String and StringBuffer java.math- math programmers Java.net- network, socket handler, internet utility classes Java.rmi- Remote Method Invocation provide support for distributed code controlled by remote interface Java.security- implements security, encryption keys, and certificates Java.sql- implements of Structured Query Language database fields types and methods Java.text- provide parsers and formatters Java.util- Java Application and applet (Random class, Collection, list, Set) packages container class. Java.util.jar- subpackage w/I java.util. Java.util.zip- subpackage for.zip file compression format.

7. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Packages con’t To import just one class: import graphics.Rectangle; It is not correct to import the package itself: import graphics; Some packages provide one or more subpackages, such as java.util.jar, a subpackage of java.util. Importing a package does not import any subpackages. You must do that explicitly using statements like these: Import java.utils.*; Import java.utils.jar.*;

8. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Small Quiz What happen if you tried to compile and run the following code? 1.public class EqualsTest{ 2.public static void main(String[] args){ 3.Long L = new Long( 7 ); 4.if( L.equals( 7L)) 5.System.out.println(“Equal”); else System.out.println(“Not Equal”); 6. } 7.} a.The program would compile and print “Equal” b.The program would compile and print “Not Equal”. c.The compiler would object to line 4. d.A runtime cast error would occur at line 4.

9. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Small Quiz C. qualsTest.java:5: cannot resolve symbol symbol : method equals (long) location: class java.lang.Long if( L.equals( 7L)) ^ 1 error The compiler knows that the equals method takes an Object rather than a primitive as input. Because the compiler does not compile answer a, b, d are incorrect.

10. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Small Quiz What happen if you tried to compile and run the following code? 1.public class EqualsTest{ 2.public static void main(String[] args){ 3. Object A = new Long ( 7 ); 4.Long L = new Long( 7 ); 5.if( A.equals( L )) System.out.println(“Equal”); 6.else System.out.println(“Not Equal”); 7. } 8.} a.The program would compile and print “Equal” b.The program would compile and print “Not Equal”. c.The compiler would object to line 5. d.A runtime cast error would occur at line 5.

11. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Small Quiz a. Because the Long object created in line 3 does not lose its identity when cast to Object A, so the equals method knows the class is correct and compares the values. Answer c, d, do not occur because this is the correct form for comparing objects with the equals method. Therefore, they are incorrect.

12. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp What compile-time error is generated for the following program? class D3 implements B { public void display() { System.out.println("D3"); } class InterfaceRefVar{ public static void main(String[] args){ B b; b = new D1(); b.display(); b = new D2(); b.display(); b = new D3(); b.display(); } interface B{ void display(); } class D1{ } class D2 implements B{ public void display() { System.out.println("D2"); } interface

13. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp interface The line “b = new D1();” generates an error because the types are incompatible for assignment. Class D1 does not implements interface B. C:\InterfaceRefVar.java:23: incompatible types found : D1 required: B b = new D1(); ^ 1 error

14. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Chapter 10 Getting Started with GUI Programming GUI Class Hierarchy Frames –Creating frames, centering frames, adding components to frames Layout Managers –FlowLayout, GridLayout, BorderLayout Drawing on Panels –The paintComponent method Using Colors, Fonts, and Font Metrics Drawing Geometric Figures –Lines, Rectangles, Ovals, Arcs, and Polygons Event-Driven Programming –Event Source, Listener, Listener Interface

15. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp AWT/JFC AWT was a powerful toolkit when introduced –it was the original driving force behind Java’s popularity –is today’s standard, a limited implementation –not designed to provide a serious, main UI for the needs of many programmers. Now Swing, which has about four times the number of user interface (UI) components as the AWT –Swing is part of the standard Java distribution

16. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp AWT (Optional)

17. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Question? Is JFC and Swing the same?

18. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp JFC Many programmer that that JFC and Swing are the same thing, but that’s not so; the JFC contains Swing and quite a number of other items.

19. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp What’s in JFC? Swing- The large UI package Cut and paste- Clipboard support Accessibility features- Aimed at users with disabilities. The desktop Colors features- First introduced in Java 1.1 Java2D- Improved color, image, and text support Printing- Originally enabled in Java 1.1

20. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp What’s in Swing? ClassDescription AbstractButtonAbstract superclass for Swing buttons ButtonGroupEncapsulates a mutually exclusive set of buttons ImageIconEncapsulates an icon JAppletThe Swing version of Applet JButtonThe Swing push button class JCheckBoxThe Swing check box class JComboBoxEncapsulates a combo box JLabelThe Swing version of a label JRadioButtonThe Swing version of a radio button JScrollPaneEncapsulates a scrollable window JTableEncapsulates a table-based control JTextFieldThe Swing version of a text field JTreeEncapsulates a tree-based control

21. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp GUI Class Hierarchy (Swing)

22. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp JComponent

23. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp JApplets Fundamental to Swing is the JApplet class, which extends Applet. Applets that use Swing must be subclasses of JApplet. JApplet is rich with functionality that is not found in Applet. For Example: JApplet support various “panes”, such as the content pane, the glass pane, and the root pane.

24. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Frames Frame is a window that is not contained inside another window. Frame is the basis to contain other user interface components in Java GUI applications. The Frame class can be used to create windows. For Swing GUI programs, use JFrame class to create widows.

25. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp UI Components

26. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Creating Frames Run import javax.swing.*; public class MyFrame { public static void main(String[] args) { JFrame frame = new JFrame("Test Frame"); frame.setSize(400, 300); frame.setVisible(true); frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE); } NOTE: You must have JDK 1.3 or higher to run the slides.

27. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Centering Frames By default, a frame is displayed in the upper-left corner of the screen. To display a frame at a specified location, you can use the setLocation(x, y) method in the JFrame class. This method places the upper-left corner of a frame at location (x, y).

28. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Centering Frames, cont. Run

29. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Adding Components into a Frame // Add a button into the frame frame.getContentPane().add( new JButton("OK")); Run

30. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp NOTE The content pane is a subclass of Container. The statement in the preceding slide can be replaced by the following two lines: Container container = frame.getContentPane(); container.add(new JButton("OK")); You may wonder how a Container object is created. It is created when a JFrame object is created. A JFrame object uses the content pane to hold components in the frame.

31. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Layout Managers Java’s layout managers provide a level of abstraction to automatically map your user interface on all window systems. The UI components are placed in containers. Each container has a layout manager to arrange the UI components within the container.

32. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Kinds of Layout Managers FlowLayout (Chapter 10) GridLayout (Chapter 10) BorderLayout (Chapter 10) CardLayout (Chapter 12) GridBagLayout (Chapter 12)

33. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Example 10.1 Testing the FlowLayout Manager The components are arranged in the container from left to right in the order in which they were added. When one row becomes filled, a new row is started. Run

34. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp FlowLayout Constructors public FlowLayout(int align, int hGap, int vGap) Constructs a new FlowLayout with a specified alignment, horizontal gap, and vertical gap. The gaps are the distances in pixel between components. public FlowLayout(int alignment) Constructs a new FlowLayout with a specified alignment and a default gap of five pixels for both horizontal and vertical. public FlowLayout() Constructs a new FlowLayout with a default center alignment and a default gap of five pixels for both horizontal and vertical.

35. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Example 10.2 Testing the GridLayout Manager The GridLayout manager arranges components in a grid (matrix) formation with the number of rows and columns defined by the constructor. The components are placed in the grid from left to right starting with the first row, then the second, and so on. Run

36. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp GridLayout Constructors public GridLayout(int rows, int columns) Constructs a new GridLayout with the specified number of rows and columns. public GridLayout(int rows, int columns, int hGap, int vGap) Constructs a new GridLayout with the specified number of rows and columns, along with specified horizontal and vertical gaps between components.

37. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Example 10.3 Testing the BorderLayout Manager The BorderLayout manager divides the container into five areas: East, South, West, North, and Center. Components are added to a BorderLayout by using the add method. Run add(Component, constraint), where constraint is BorderLayout.EAST, BorderLayout.SOUTH, BorderLayout.WEST, BorderLayout.NORTH, or BorderLayout.CENTER.

38. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Example 10.3, cont. Run

39. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Using Panels as Containers Panels act as smaller containers for grouping user interface components. It is recommended that you place the user interface components in panels and place the panels in a frame. You can also place panels in a panel.

40. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Example 10.4 Testing Panel This example uses panels to organize components. The program creates a user interface for a Microwave oven. Run

41. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Drawing on Panels JPanel can be used to draw graphics (including text) and enable user interaction. To draw in a panel, you create a new class that extends JPanel and override the paintComponent method to tell the panel how to draw things. You can then display strings, draw geometric shapes, and view images on the panel.

42. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Drawing on Panels, cont. public class DrawMessage extends JPanel { /** Main method */ public static void main(String[] args) { JFrame frame = new JFrame("DrawMessage"); frame.getContentPane().add(new DrawMessage()); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); frame.setSize(300, 200); frame.setVisible(true); } /** Paint the message */ public void paintComponent(Graphics g) { super.paintComponent(g); g.drawString("Welcome to Java!", 40, 40); }

43. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Drawing on Panels, cont.

44. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp NOTE The Graphics class is an abstract class for displaying figures and images on the screen on different platforms. The Graphics class is implemented on the native platform in the JVM. When you use the paintComponent method to draw things on a graphics context g, this g is an instance of a concrete subclass of the abstract Graphics class for the specific platform. The Graphics class encapsulates the platform details and enables you to draw things uniformly without concerning specific platforms.

45. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp NOTE Whenever a component is displayed, a Graphics object is created for the component. The Swing components use the paintComponent method to draw things. The paintComponent method is automatically invoked to paint the graphics context when the component is first displayed or whenever the component needs to be redisplayed. Invoking super.paintComponent(g) is necessary to ensure that the viewing area is cleared before a new drawing is displayed.

46. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp NOTE To draw things, normally you create a subclass of JPanel and override its paintComponent method to tell the system how to draw. In fact, you can draw things on any GUI component.

47. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp The Color Class You can set colors for GUI components by using the java.awt.Color class. Colors are made of red, green, and blue components, each of which is represented by a byte value that describes its intensity, ranging from 0 (darkest shade) to 255 (lightest shade). This is known as the RGB model. Color c = new Color(r, g, b); r, g, and b specify a color by its red, green, and blue components. Example: Color c = new Color(228, 100, 255);

48. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Setting Colors You can use the following methods to set the component’s background and foreground colors: setBackground(Color c) setForeground(Color c) Example: setBackground(Color.yellow); setForeground(Color.red);

49. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp The Font Class Font myFont = Font(name, style, size); Example: Font myFont = new Font("SansSerif ", Font.BOLD, 16); Font myFont = new Font("Serif", Font.BOLD+Font.ITALIC, 12);

50. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Finding All Available Font Names GraphicsEnvironment e = GraphicsEnvironment.getLocalGraphicsEnvironment(); String[] fontnames = e.getAvailableFontFamilyNames(); for (int i = 0; i < fontnames.length; i++) System.out.println(fontnames[i]);

51. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Setting Fonts public void paint(Graphics g) { Font myFont = new Font("Times", Font.BOLD, 16); g.setFont(myFont); g.drawString("Welcome to Java", 20, 40); //set a new font g.setFont(new Font("Courier", Font.BOLD+Font.ITALIC, 12)); g.drawString("Welcome to Java", 20, 70); }

52. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp The FontMetrics Class public void paint(Graphics g) { g.getFontMetrics(Font f); or g.getFontMetrics(); }

53. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Get FontMetrics public int getAscent() public int getDescent() public int getLeading() public int getHeight() public int stringWidth(String str)

54. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Example 10.5 Using FontMetrics Objective: Display “Welcome to Java” in SansSerif 20-point bold, centered in the frame. Run

55. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp

56. Example 10.5, cont.

57. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Drawing Geometric Figures Drawing Lines Drawing Rectangles Drawing Ovals Drawing Arcs Drawing Polygons

58. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Drawing Lines drawLine(x1, y1, x2, y2);

59. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Drawing Rectangles drawRect(x, y, w, h); fillRect(x, y, w, h);

60. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Drawing Rounded Rectangles drawRoundRect(x, y, w, h, aw, ah); fillRoundRect(x, y, w, h, aw, ah);

61. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Drawing Ovals drawOval(x, y, w, h); fillOval(x, y, w, h);

62. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Drawing Arcs drawArc(x, y, w, h, angle1, angle2); fillArc(x, y, w, h, angle1, angle2);

63. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Drawing Polygons int[] x = {40, 70, 60, 45, 20}; int[] y = {20, 40, 80, 45, 60}; g.drawPolygon(x, y, x.length); g.fillPolygon(x, y, x.length);

64. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Example 10.6 Drawing a Clock Objective: Use drawing and trigonometric methods to draw a clock showing the specified hour, minute, and second in a frame. Run

65. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Drawing Clock xEnd = xCenter + handLength  sin(  ) yEnd = yCenter - handLength  cos(  ) Since there are sixty seconds in one minute, the angle for the second hand is second  (2  /60)

66. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Drawing Clock, cont. xEnd = xCenter + handLength  sin(  ) yEnd = yCenter - handLength  cos(  ) The position of the minute hand is determined by the minute and second. The exact minute value comined with seconds is minute + second/60. For example, if the time is 3 minutes and 30 seconds. The total minutes are 3.5. Since there are sixty minutes in one hour, the angle for the minute hand is (minute + second/60)  (2  /60)

67. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Drawing Clock, cont. xEnd = xCenter + handLength  sin(  ) yEnd = yCenter - handLength  cos(  ) Since one circle is divided into twelve hours, the angle for the hour hand is (hour + minute/60 + second/(60  60)))  (2  /12)

68. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Event-Driven Programming Procedural programming is executed in procedural order. In event-driven programming, code is executed upon activation of events.

69. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Events An event can be defined as a type of signal to the program that something has happened. The event is generated by external user actions such as mouse movements, mouse button clicks, and keystrokes, or by the operating system, such as a timer.

70. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Event Information id : A number that identifies the event. target : The source component upon which the event occurred. arg : Additional information about the source components. x, y coordinates : The mouse pointer location when a mouse movement event occurred. clickCount : The number of consecutive clicks for the mouse events. For other events, it is zero. when : The time stamp of the event. key : The key that was pressed or released.

71. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Event Classes

72. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Selected User Actions SourceEvent Type User ActionObjectGenerated Clicked on a button JButtonActionEvent Changed text JTextComponentTextEvent Double-clicked on a list item JListActionEvent Selected or deselected an item JListItemEvent with a single click Selected or deselected an item JComboBoxItemEvent

73. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp The Delegation Model

74. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Selected Event Handlers Event ClassListener InterfaceListener Methods (Handlers) ActionEventActionListeneractionPerformed(ActionEvent) ItemEventItemListeneritemStateChanged(ItemEvent) WindowEventWindowListenerwindowClosing(WindowEvent) windowOpened(WindowEvent) windowIconified(WindowEvent) windowDeiconified(WindowEvent) windowClosed(WindowEvent) windowActivated(WindowEvent) windowDeactivated(WindowEvent) ContainerEventContainerListenercomponentAdded(ContainerEvent) componentRemoved(ContainerEvent)

75. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Example 10.7 Handling Simple Action Events Objective: Display two buttons OK and Cancel in the window. A message is displayed on the console to indicate which button is clicked, when a button is clicked. Run

76. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Example 10.8 Handling Window Events Run  Objective: Demonstrate handling the window events. Any subclass of the Window class can generate the following window events: window opened, closing, closed, activated, deactivated, iconified, and deiconified. This program creates a frame, listens to the window events, and displays a message to indicate the occurring event.

77. Liang, Oreilly, Herbert Schildt, Joseph O’Neil, Steven Holzner, IBM Corp Example 10.9 Multiple Listeners for a Single Source Run  Objective: This example modifies Example 10.7 to add a new listener for each button. The two buttons OK and Cancel use the frame class as the listner. This example creates a new listener class as an additional listener for the action events on the buttons. When a button is clicked, both listeners respond to the action event.