Java Programming, Second Edition

1. Java Programming, Second Edition Chapter Nine Applets

2. In this chapter, you will: • Write an HTML document to host an applet • Understand simple applets • Use Labels with simple AWT applets • Write a simple Swing applet and use a JLabel • Add JTextField and JButton Components to Swing applets

3. Learn about event-driven programming • Add output to a Swing applet • Understand the Swing applet life cycle • Create a more sophisticated interactive Swing applet • Use the setLocation() and setEnabled() methods

4. To Write a Java Application: • Write the application in the Java programming language, and then save it with a .java file extension • Compile the application into bytecode using the javac command. The bytecode is stored in a file with a .class file extension • Use the java command to interpret and execute the .class file

5. Writing an HTML Document to Host an Applet • Applets- Programs that are called from within another application • You run applets within • a page on the Internet • an intranet • or a local computer from within another program called Applet Viewer • To view an applet, it must be called from within another document written in HTML

6. Writing an HTML Document to Host an Applet To create an applet: • Write the applet in the Java programming language, and save it with a .java file extension • Compile the applet into bytecode using the javac command • Write an HTML document that includes a statement to call your compiled Java class • Load the HTML document into a Web browser or run the AppletViewer program

7. Writing an HTML Document to Host an Applet • Applets are popular because users can execute them using a Web browser • Web browser- A program that allows you to display HTML documents on your computer screen • Internet Explorer • Netscape Navigator

8. Writing an HTML Document to Host an Applet • Code to run an applet from within an HTML document • <applet> • </applet> • Applet tag attributes • CODE = is followed by the name of the compiled applet you are calling • WIDTH = is followed by the width of the applet on the screen • HEIGHT = is followed by the height of the applet on the screen

9. Applets • The WIDTH and HEIGHT attributes are measured in pixels • Pixels- Picture elements, or tiny dots that make up the image on your video monitor

10. Understanding Simple Applets To write an applet you must also: • Include import statements to ensure that necessary classes are available • Learn to use some Windows components and applet methods • Learn to use the keyword extends

12. Understanding Simple Applets • Component- A class that defines any object that you want to display • Container- A class that is used to define a component that can contain other components

13. Understanding Simple Applets • Most AWT applets contain 2 import statements • import java.applet.*; • import java.awt.*; • java.applet- Contains a class named Applet • Every applet you create is based on Applet • java.awt- The Abstract Windows Toolkit, or AWT

14. Understanding Simple Applets • Most Swing applets contain 2 import statements • import javax.swing.*; • import java.awt.*; • javax.swing- A package that contains classes that define GUI components (Swing components)

15. Understanding Simple Applets • Swing classes- part of a more general set of GUI programming capabilities that are known as the Java Foundation Classes, or JFC • JFC includes Swing component classes and selected classes from the java.awt package

16. AWT and Swing Applets AWT and Swing applets • Begin the same way as Java applications • Must also include • extends Applet • extends JApplet • The extends keyword indicates the applet will build upon Applet and JApplet

17. Applets • Four methods in every applet • public void init() • public void start() • public void stop() • public void destroy() • Java can create these for you

18. Using Labels with Simple AWT Applets • The java.awt package contains commonly used Windows components • Labels • Menus • Buttons • Label- Built-in class that holds text that you can display within an applet

19. Using Labels with Simple AWT Applets • Label class contains fields that indicate font and alignment • You can assign some text to a label with the setText() method • Use the add() method to add a component to an applet window

20. Writing a Simple Swing Applet and Using a JLabel • JLabel- Built-in class that holds text that you can display within an applet • The counterpart to the AWT Label

21. Writing a Simple Swing Applet and Using a JLabel • Available constructors include: • JLabel() creates a JLabel instance with no image and an empty string for the title • JLabel(Icon image) creates a JLabel instance with the specified image • JLabel(Icon image, int horizontalAlignment) creates a JLabel instance with the specified image and horizontal alignment • JLabel(String text) creates a JLabel instance with the specified text • JLabel(String text, Icon icon, int horizontalAlignment) creates a JLabel instance with the specified text, image, and horizontal alignment • JLabel(String text, int horizontalAlignment) creates a JLabel instance with the specified text and horizontal alignment

22. Writing a Simple Swing Applet and Using a JLabel • AWT components are added directly to the Applet • Swing components must use a content pane • The content pane is an object of the Container class • A container can be created using the getContentPane() method

23. Changing a JLabel’s Font • Font object- Holds typeface and size information • setFont() method requires a Font object argument • To construct a Font object you need 3 arguments • Typeface • Style • Point size

24. Changing a JLabel’s Font • To construct a Font object you need 3 arguments • Typeface • String representing a font • Common fonts are Arial, Courier, and New Times Roman • Is only a request • Style- applies an attribute to displayed text • Font.PLAIN • Font.BOLD • Font.ITALIC • Point size • Integer that represents 1/72 of an inch • Printed text is usually 10- or 12 points

25. Adding JTextField Components to Swing Applets • JTextField- Component into which a user can type a single line of text data • JText field can be constructed from • public JTextField() constructs a new JTextField • public JTextField(int numColumns) constructs a new empty JTextField with a specified number of columns • public JTextField(String text) constructs a new JTextField initialized with the specific text • public JTextField(String text, int columns) constructs a new JTextField with the specified text and columns

26. Other JTextField Methods • setText() method- Allows you to change the text in a JTextField that has already been created • getText() method- Allows you to retrieve the string of text in a JTextField

27. Other JTextField Methods • Keyboard focus- When the user clicks within the JTextField, the JTextField has focus, which means the next entries from the keyboard will be at that location • requestFocus() method- To have the insertion point appear automatically within the TextField without requiring the user to click in it first

28. Other JTextField Methods • Editable- The capacity for a field to accept keystrokes • setEditable() method- Used to change the editable status of a JTextField

29. Adding JButton Components to Swing Applets • JButton- Creates a button • JButton can be constructed from • public JButton() constructs a button with no set text • public JButton(Icon icon) creates a button with an icon of type Icon or ImageIcon • public JButton(String text) creates a button with the specific text • public JButton(String text, int columns) constructs a new JTextField with the specified text and columns

30. Adding JButton Components to Swing Applets • setLabel() method • To change a JButton’s label • readyJButton.setLabel(“Don’t press me again!”)

31. Adding Multiple Components to a JApplet • To add multiple components in a container use a layout manager • To control component positioning • Default behavior is to use a border layout • Border layouts • Flow layouts

32. Adding Multiple Components to a JApplet • Border layouts • Created by the BorderLayout class • Divide a container into 5 sections • North, South, East, West, and center • Created with the BorderLayout() or BorderLayout(int, int) methods

33. Adding Multiple Components to a JApplet • Flow Layouts • Places components in a row, and when a row is filled, it automatically spills components onto the next row • Default positioning of the row of components is centered in the container

34. Learning about Event-Driven Programming • Event- Occurs when someone using your applet takes action on a component • Procedural- Programmers dictate the order in which events occur • Event-driven programs- The user can initiate any number of events in any order • Source- Component on which an event is generated • Listener- Object that is interested in an event

35. Preparing Your Swing Applet to Accept Event Messages • Prepare your applet to accept mouse events by: • importing the java.awt.event package • adding the phrase implements ActionListener to the class header • ActionListener is an interface • Interface- A set of specifications for methods that you can use with event objects

36. Telling Your Swing Applet to Expect Events to Happen • addActionListener() method • To tell the applet to expect ActionEvents • aButton.addActionListener(this);

37. Telling Your Swing Applet How to Respond to Any Events That Happen • actionPerformed(ActionEvent e) method • When a JApplet has registered as a listener with a JButton, and a user clicks the JButton the actionPerformed method executes

38. Adding Output to a Swing Applet • You can add components to an applet using the add() method • You can also remove components from an applet using the remove() method • Remove(answer);

39. Understanding the Swing Applet Life Cycle • Override- When you write a method that has the same method header as an automatically provided method

40. Understanding the Swing Applet Life Cycle • start() method- Executes after the init() method • Executes every time the applet becomes active after it has been inactive • stop() method- When a user leaves a web page • You do not usually write your own stop() methods

41. Understanding the Swing Applet Life Cycle • destroy() method- When the user closes the browser or AppletViewer • You do not usually write your own destroy() methods

42. Using the setLocation() and setEnabled() Methods • setLocation() method- Allows you to place a component at a specific location within the AppletViewer window • X-axis- Horizontal position in a window • X-coordinate- Value increases as you travel from left to right across the window • Y-axis- Vertical position in the window • Y-coordinate- Value increases as you travel from top to bottom in the window

44. The setEnabled() Method • setEnabled() method- To make a component unavailable and, in turn, to make it available again • True if you want to enable a component • False if you want to disable a component • If (yLoc==280) pressButton.setEnabled(false);