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Introduction to Java Applets

Learn about applets in Java, how they can be transported over the internet, and run in a web browser that supports Java. Explore the methods and functionalities provided by the Applet class.

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Introduction to Java Applets

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  1. Applets Applets are small java programs that can be transported over the internet from one computer to another and run using the Applet viewer or any web browser that supports java. All applets are subclasses of Applet. Thus, all applets must import java.applet. Applets must also import java.awt. (AWT stands for the Abstract Window Toolkit)Since all applets run in a window, it is necessary to include support for that window. Execution of an applet does not begin at main( ). Instead, execution of an applet is started and controlled with an entirely different mechanism.Once an applet has been compiled, it is included in an HTML file using the APPLET tag.The applet will be executed by a Java-enabled web browser when it encounters the APPLET tag within the HTML file. The applet tag in the html file <applet code="MyApplet" width=200 height=60> </applet>

  2. The Applet Class Applet provides all necessary support for applet execution, such as starting and stopping. It also provides methods that load and display images, and methods that load and play audio clips. Applet extends the AWT class Panel. In turn, Panel extends Container, which extends Component. These classes provide support for Java's window-based, graphical interface. Thus, Applet provides all of the necessary support for window-based activities. applets override a set of methods by which the browser or applet viewer interfaces to the applet and controls its execution. Four of these methods—init( ), start( ), stop( ), and destroy( )—are defined by Applet. Another, paint( ), is defined by the AWT Component class.

  3. When an applet begins, the AWT calls the following methods, in this sequence: init( ) start( ) paint( ) When an applet is terminated, the following sequence of method calls takes place: stop( ) destroy( ) init( ) The init( ) method is the first method to be called. This is whereyou should initialize variables. This method is called only once during the run time of your applet. start( ) The start( ) method is called after init( ). It is also called to restart an applet after it has been stopped. Whereas init( ) is called once—the first time an applet is loaded— start( )is called each time an applet's HTML document is displayed onscreen. So, if a user leaves a web page and comes back, the applet resumes execution at start( ).

  4. paint( ) The paint( ) method is called each time your applet's output must be redrawn. the applet window may be minimized and then restored. paint( ) is also called when the applet begins execution. Whatever the cause, whenever the applet must redraw its output,paint( ) is called. The paint( ) method has one parameter of type Graphics. This parameter will contain the graphics context, which describes the graphics environment in which the applet is running. This context is used whenever output to the applet isrequired. stop( ) The stop( ) method is called when a web browser leaves the HTML document containing the applet—when it goes to another page.

  5. destroy( ) The destroy( ) method is called when the environment determines that your applet needs to be removed completely from memory. At this point, you should free up any resources the applet may be using. The stop( ) method is always called before destroy( ). Simple Applet Display Methods void drawString(String message, int x, int y) message is the string to be output beginning at x,y. In a Java window, the upperleft corner is location 0,0. void setBackground(Color newColor) void setForeground(Color newColor) To set the background and the forground color. Color.black Color.magenta Color.blue Color.orange Color.cyan Color.pink Color.darkGray Color.red Color.gray C olor.white Color.green Color.yellow Color.lightGray

  6. Color getBackground( ) Color getForeground( ) To obtain the current settings for the background and foreground colors Ex: import java.awt.*; import java.applet.*; /* <applet code="Sample" width=300 height=50> </applet> */ public class Sample extends Applet { String msg; public void init() {setBackground(Color.cyan); setForeground(Color.red); msg = "Inside init( ) —“; }

  7. public void start() { msg += " Inside start( ) —"; } public void paint(Graphics g) { msg += " Inside paint( )."; g.drawString(msg, 10, 30); } } repaint() method • Whenever our applet needs to update the information displayed in itswindow, we can call the repaint( ) method which calls the paint() method

  8. import java.awt.*; import java.applet.*; /*<applet code="SimpleBanner" width=300 height=50> </applet>*/ public class SimpleBanner extends Applet implements Runnable { String msg = " A Simple Moving Banner."; Thread t = null; int state; boolean stopFlag; public void init() { setBackground(Color.cyan); setForeground(Color.red); }

  9. public void start() { t = new Thread(this); stopFlag = false; t.start(); } public void run() { char ch; for( ; ; ) { try { repaint(); Thread.sleep(250); ch = msg.charAt(0); msg = msg.substring(1, msg.length()); msg += ch; if(stopFlag) break; } catch(InterruptedException e) {} } }

  10. public void stop() { stopFlag = true; t = null; } public void paint(Graphics g) { g.drawString(msg, 50, 30); }} Passing parameters to applet The PARAM tag allows you to specify applet-specific arguments in an HTML page.Applets access their attributes with the getParameter( ) method. < PARAM NAME = AttributeName VALUE = AttributeValue> < PARAM NAME = AttributeName2 VALUE = AttributeValue> . . .

  11. import java.awt.*; import java.applet.*; /* <applet code="ParamDemo" width=300 height=80> <param name=fontName value=Courier> <param name=fontSize value=14> </applet> */ public class ParamDemo extends Applet { String fontName; int fontSize;

  12. public void start() { String param; fontName = getParameter("fontName"); if(fontName == null) fontName = "Not Found"; param = getParameter("fontSize"); try { if(param != null) fontSize = Integer.parseInt(param); else fontSize = 0; } catch(NumberFormatException e) { fontSize = -1;} Font f=new Font(fontName,Font.BOLD,fontsize); this.setFont(f);}

  13. public void paint(Graphics g) { g.drawString("Font name: " + fontName, 0, 10); g.drawString("Font size: " + fontSize, 0, 26); }} The HTML APPLET Tag < APPLET [CODEBASE = codebaseURL] CODE = appletFile [ALT = alternateText] [NAME = appletInstanceName] WIDTH = pixels HEIGHT = pixels [ALIGN = alignment] [VSPACE = pixels] [HSPACE = pixels] >

  14. [< PARAM NAME = AttributeName VALUE = AttributeValue>] [< PARAM NAME = AttributeName2 VALUE = AttributeValue>] . . . </APPLET> • CODEBASE CODEBASE specifies the directory that will be searched for the applet's executable class file (specified by the CODE tag). The HTML document's URL directory is used as the CODEBASE if this attribute is not specified. CODE CODE is a required attribute that gives the name of the file containing your applet‘s scompiled .class file.

  15. ALT The ALT tag is an optional attribute used to specify a short text message that should be displayed if the browser understands the APPLET tag but can't currently run Java applets. NAME NAME is an optional attribute used to specify a name for the applet instance. Applets must be named in order for other applets on the same page to find them by name and communicate with them. WIDTH and HEIGHT WIDTH and HEIGHT are required attributes that give the size (in pixels) of the applet display area. ALIGN • ALIGN is an optional attribute that specifies the alignment of the applet. LEFT, RIGHT, TOP,BOTTOM, MIDDLE

  16. VSPACE and HSPACE These attributes are optional. VSPACE specifies the space, in pixels, above and below the applet. HSPACE specifies the space, in pixels, on each side of the applet. • PARAM NAME and VALUE The PARAM tag allows you to specify applet-specific arguments in an HTML page.Applets access their attributes with the getParameter( ) method. getDocumentBase( ) • returns the directory holding the HTML file that started the applet getCodeBase( ) Returns directory from which the applet's class file was loaded

  17. Loading an Image To load an image use the the getImage( ) method defined by the Applet class. It has the following forms: Image getImage(URL url) • returns an Image object that encapsulates the image found at the location specified by url. Image getImage(URL url, String imageName) • returns an Image object that encapsulates the image found at the location specified by url and having the name specified by imageName. • Displaying an Image • We can display an image by using drawImage( ), which is a member of the Graphics class. Syntax: boolean drawImage(Image imgObj, int left, int top, ImageObserver imgOb)

  18. This displays the image passed in imgObj with its upper-left corner specified by left and top. imgOb is a reference to a class that implements the ImageObserver interface. (note:This interface is implemented by all AWT components). /* <applet code="Load" width=248 height=146> </applet>*/ import java.awt.*; import java.net.URL; import java.applet.*; public class Load extends Applet { Image img; public void init() { URL u=null; try{ u=new URL("file:/c:/s3mca21/viv.jpg"); }catch(MalformedURLException e){System.out.println(e);}

  19. img = getImage(u); } public void paint(Graphics g) { g.drawImage(img, 0, 0, this); } } Playing Audio Files using Applet Create an audioclip object by using the getAudioClip() method defined in the Applet class . syntax. AudioClip getAudioClip(URL url) AudioClip getAudioClip(URL url,String clipName) Then play the AudioClip object using the Play() method of the AudioClip class Also we can stop the audioclip by using the stop method of the AudioClip class

  20. /* <applet code="Load" width=248 height=146> </applet>*/ import java.awt.*; import java.applet.*; import java.net.URL; public class Load extends Applet { AudioClip g=null; URL u=null; public void init() { try{ u=new URL("file:/c:/nisha/die.au");}catch(MalformedURLException e){System.out.println(e);} g = getAudioClip(u); }public void paint(Graphics g1) { g.play();} public void stop(){ g.stop() } }

  21. Working with Graphics The AWT supports a rich set of graphics methods. All graphics are drawn relative to a window. The origin of each window is at the top-left corner and is 0,0. Coordinates are specified in pixels. All output to a window takes place through a graphics context. Drawing Lines Void drawLine(int startX, int startY, int endX, int endY) Draw a line that begins at startX,startY and ends at endX,endY. Drawing Rectangles The drawRect( ) and fillRect( ) methods display an outlined and filled rectangle,respectively. void drawRect(int top, int left, int width, int height) void fillRect(int top, int left, int width, int height)

  22. To draw a rounded rectangle, use drawRoundRect( ) or fillRoundRect( ), void drawRoundRect(int top, int left, int width, int height, int xDiam, int yDiam) void fillRoundRect(int top, int left, int width, int height, int xDiam, int yDiam) A rounded rectangle has rounded corners. The upper-left corner of the rectangle is top,left. The dimensions of the rectangle are specified by width and height. The diameterof the rounding arc along the X axis is specified by xDiam. The diameter of the rounding arc along the Y axis is specified by yDiam.

  23. Drawing Ellipses and Circles To draw an ellipse, use drawOval( ). To fill an ellipse, use fillOval( void drawOval(int top, int left, int width, int height) void fillOval(int top, int left, int width, int height) The ellipse is drawn within a bounding rectangle whose upper-left corner is specified by top,left and whose width and height are specified by width and height. To draw a circle, specify a square as the bounding rectangle. • Drawing Arcs Arcs can be drawn with drawArc( ) and fillArc( ) void drawArc(int top, int left, int width, int height, int startAngle,int sweepAngle) void fillArc(int top, int left, int width, int height, int startAngle, int sweepAngle)

  24. The arc is bounded by the rectangle whose upper-left corner is specified by top,left and whose width and height are specified by width and height. The arc is drawn from startAngle through the angular distance specified by sweepAngle. • Drawing Polygons • It is possible to draw arbitrarily shaped figures using drawPolygon( ) and fillPolygon( ) void drawPolygon(int x[ ], int y[ ], int numPoints) void fillPolygon(int x[ ], int y[ ], int numPoints) • The polygon's endpoints are specified by the coordinate pairs contained within the x and y arrays. The number of points defined by x and y is specified by numPoints.

  25. Event Handling Event is an object that specifies the change of state in the source.it is generated whenever an action takes place like a mouse button is clicked or text is modified.when action takes place an event is generated .the generated event and all the information about it such as time of its occurrence ,type of event etc, are sent to the appropriate event handling code provided within the program. Event Handling It is based on the concept of source and listener . Event source : is an object that generates a particular kind of event.An event is generated when the internal state of an event source is changed. EventListener:is an object which receives notification when an event occurs.

  26. The source generates an event and sends it to one or more listeners.On receiving the event ,listener processes the event and returns it.The source has a registered list of listeners which will receive the events as they occur.only the listeners that have been registered actually receive the notification when a specific event is generated. • Click the button EventGenerated notifies the listener Listenerhandle it Object Obj of ActionEvent isgenerated actionPerformed(ActionEvent e) method of ActionListener is executed Button Click the button Button

  27. The general form of method to register a listener is: Public void addTypeListener(TypeListenereventListener) The general form of method to unregister a listener is: Public void removeTypeListener(TypeListenereventListener) EventHandling Java provides various classes and interfaces to handle the generated events Event classes Event classes encapsulates all types of events occuring in the system.thesupercalss of all these event is the EventObjectclass.EventObject class belongs to java.util package.

  28. EventListener interfaces • The ActionListener Interface When an ActionEvent occurs the actionPerformed() method defined by the Actionlistener interface is invoked The general form of the actionPerformed method is void actionPerformed(ActionEvent e) e is the reference to an object of ActionEvent class • The AdjustmentListener interface • When an AdjustmentEvent occurs the adjsutmentValueChanged() method defined by the Adjustmentlistener interface is invoked The general form of the adjustmentValueChanged() method is Void adjustmentValueChanged(AdjustmentEvent e) Where e is the reference to an object of AdjustmentEvent class.

  29. ItemListenerInterface The method itemStateChanged() defined by the itemlistener interface is invoked when the state of an item is changed.The object of ItemEvent class is passed as method parameter. The syntax of the method is Void itemStateChanged(ItemEvent item) • KeyListener interface The methods defined in KeyListener interface are invoked when any key is pressed or released or a character is entered. The methods are Void keyPressed(KeyEvent e) Void keyReleased(KeyEvent e) Void keyTyped(KeyEvent e) If a key B is pressed and released three events are generated.

  30. When a key is pressed, a KEY_PRESSED event is generated. This results in a call to the keyPressed( ) event handler. When the key is released, a KEY_RELEASED event is generated and the keyReleased( ) handler is executed. If a character is generated by the keystroke, then a KEY_TYPED event is sent and the keyTyped( ) handler is invoked. There is one other requirement that your program must meet before it can process keyboard events: it must request input focus. To do this, call requestFocus( ), which is defined by Component. If you don't, then your program will not receive any keyboard events.

  31. The MouseListener Interface and MouseMotionListener Interface The method s defined by the MouseListener inteface are invoked when the mouse is clicked,pressed ,released and when the mouse enters or exits the component. The methods defined by the MouseMotionListener intereface are invoked when the mouse is dragged or moved from one position to another.The object Mouse Event is passed as method parameter. The methods are void mousePressed(MouseEvent e) void mouseReleased(MouseEvent e) void mouseClicked(MouseEvent e) void mouseEntered(MouseEvent e) Void mouseExited(MouseEvent e)

  32. The methods defined in MouseMotionListener interface is Void mouseMoved(MouseEvent e) Void mouseDragged(MouseEvent me) The TextListener interface The method textValueChanged() defined by the TextListener interface is invoked when the text inside the text area or textfield alters. The syntax of the method is Void textValueChanged(TextEvent e) The WindowListenerInterface The method s defined by the WindowListener inteface are invoked when the state of the window changes.The methods are

  33. public void windowOpened(WindowEvent e) invoked the first time the window is made visible. public void windowClosed(WindowEvent e) Invoked when a window has been closed public void windowClosing(WindowEvent e) Invoked when the user attempts to close the window public void windowActivated(WindowEvent e) Invoked when the Window is set to be the active Window. public void windowDeactivated(WindowEvent e) Invoked when a Window is no longer the active Window. public void windowIconified(WindowEvent e) Invoked when a window is changed from a normal to a minimized state. public void windowDeiconified(WindowEvent e) Invoked when a window is changed from a minimized to a normal state.

  34. /*<applet code=ButTextLabel width=300 height=400> </applet>*/ import java.awt.*; import java.awt.event.*; import java.applet.*; public class ButTextLabel extends Applet implements ActionListener { TextField t1; TextField t2; TextField t3; Button bAdd; Button bsub; Label l1; Label l2; public void init() { bAdd=new Button("add"); bsub=new Button("Sub");

  35. t1=new TextField(10); t2=new TextField(15); t3=new TextField(15); l1=new Label("Enter First Number"); l2=new Label("Enter Second Number"); this.add(l1); this.add(t1); this.add(l2); this.add(t2); this.add(bAdd); this.add(bsub); this.add(t3); bAdd.addActionListener(this); bsub.addActionListener(this); } public void actionPerformed(ActionEvent e) { int x=Integer.parseInt(t1.getText()); int y=Integer.parseInt(t2.getText()); if (e.getSource()==bsub) t3.setText(x-y+""); else t3.setText(x+y+""); } }

  36. Write an applet program to implement a simple calculator /*<applet code=Mouse width=300 height=400> </applet>*/ import java.awt.*; import java.awt.event.*; import java.applet.*; public class Mouse extends Applet implements MouseListener { int x=0,y=0; String msg=""; public void init() { this.addMouseListener(this); } public void paint(Graphics g) { g.drawString(msg,x,y); }

  37. public void mousePressed(MouseEvent e) { x=e.getX(); y=e.getY(); msg ="MousePressed"; showStatus("Mousepressed at "+x +" "+y); } public void mouseReleased(MouseEvent e) { x=20; y=50; msg="MouseReleased"; repaint(); } public void mouseEntered(MouseEvent e) { x=20; y=60; msg="MouseEntered"; repaint(); }

  38. public void mouseExited(MouseEvent e) { x=20; y=70; msg="MouseExited"; repaint(); } public void mouseClicked(MouseEvent e) { x=e.getX(); y=e.getY(); msg="MouseClicked"; repaint(); } }

  39. /*<applet code=freedraw width=300 height=400> </applet>*/ import java.awt.*; import java.awt.event.*; import java.applet.*; public class freedraw extends Applet implements MouseMotionListener { Graphics g; public void init() { this.addMouseMotionListener(this); g=getGraphics(); }

  40. public void mouseMoved(MouseEvent e) { } public void mouseDragged(MouseEvent e) { int x=e.getX(); int y=e.getY(); g.drawString(".",x,y); } }

  41. Adapter Classes An adapter class provides an empty implementation of all methods in an event listener interface. Adapter classes are useful when you want to receive and process only some of the events that are handled by a particular event listener interface. You can define a new class to act as an event listener by extending one of the adapter classes and Implementing only those events in which you are interested. For example, the MouseMotionAdapter class has two methods, mouseDragged( ) and mouseMoved( ). The signatures of these empty methods defined in theMouseMotionListener interface. If we are interested in only mouse drag events, then simply extend MouseMotionAdapter and implement mouseDragged( ).

  42. Adapter Class Listener Interface KeyAdapterKeyListener MouseAdapterMouseListener MouseMotionAdapterMouseMotionListener WindowAdapter W indowListener /*<applet code=adapter width=300 height=400> </applet>*/ import java.applet.*; import java.awt.*; import java.awt.event.*; public class adapter extends Applet { intx,y; public void init() { this.addMouseListener(new myadapt(this));}

  43. public void paint(Graphics g) { g.drawString("MouseClicked",x,y); } } class myadapt extends MouseAdapter { adapter a; myadapt(adapter x) { this.a=x; } public void mouseClicked(MouseEvent e) { a.x=e.getX(); a.y=e.getY(); a.repaint(); a.showStatus("Mouseclicked at "+a.x+" "+a.y); } }

  44. Program for handling keyboard event import java.awt.*; import java.awt.event.*; import java.applet.*; /*<applet code=keyevent width=300 height=100> </applet>*/ public class keyevent extends Applet implements KeyListener { String msg = ""; int X = 10, Y = 20; public void init() { addKeyListener(this); requestFocus(); // request input focus } public void keyPressed(KeyEventke) { showStatus("Key Down"); }

  45. public void keyReleased(KeyEvent ke) { showStatus("Key Up"); } public void keyTyped(KeyEvent ke) { msg += ke.getKeyChar(); repaint(); } public void paint(Graphics g) { g.drawString(msg, X, Y); } } If you want to handle the special keys, such as the arrow or function keys, you need to respond to them within the keyPressed( ) handler. They are not available through keyTyped( ).

  46. import java.awt.*; import java.awt.event.*; import java.applet.*; /*<applet code=keyevent width=300 height=100> </applet> */ public class keyevent extends Applet implements KeyListener { TextField t1; public void init() { t1=new TextField(15); t1.addKeyListener(this); this.add(t1); //requestFocus(); // request input focus }

  47. public void keyPressed(KeyEvent ke) { } public void keyReleased(KeyEvent ke) { } public void keyTyped(KeyEvent ke) { showStatus(ke.getKeyChar()+""); ke.setKeyChar('a'); }}

  48. import java.awt.*; import java.awt.event.*; import java.applet.*; /*<applet code=keyevent1.java width=300 height=100> </applet>*/ public class keyevent1 extends Applet implements KeyListener { String msg = ""; int X = 10, Y = 20; public void init() { addKeyListener(this); requestFocus(); }

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