AppletsSlide 2Applet SupportWhat an applet isThe genealogy of JAppletThe simplest possible appletThe simplest reasonable appletApplet methodsWhy an applet workspublic void init ( )start( ), stop( ) and destroy( )Methods are called in this orderpublic void paint(Graphics g)repaint( )update( )Sample Graphics methodsPainting at the right time is hardOther useful JApplet methodsApplets are not magic!Structure of an HTML pageHTMLThe <applet> tag<param name="arraysize" value="10">The EndJan 14, 2019Applets2AppletsAn applet is a program that is typically embedded in a Web page and can be run from a browserYou need special HTML in the Web page to tell the browser about the appletYou don’t need to supply a main method; the browser does thatWhen you write an applet, you are writing only part of a program You supply certain methods that the browser callsFor security reasons, applets run in a sandbox: they have no access to the client’s file system3Applet SupportMost modern browsers support applets if they have the appropriate pluginYou need a version of the plugin at least as new as the version of Java you are trying to runThe plugin is usually installed automatically when you install a new version of JavaThe best support isn't a browser, but the standalone program appletviewerappletviewer uses only the applets in a Web page, and ignores everything elseIf you want to reach the widest audience, you should use Java 1.1, and AWT instead of SwingThis is because Microsoft no longer supports Java4What an applet isYou write an applet by extending the class JApplet JApplet is just a class like any other; you can even use it in applications if you wantWhen you write an applet, you are only writing part of a program; the browser supplies the main methodOnce you understand how applets work, you can write a program that function either as an applet or as an application—just write a main method that calls the right methods at the right timeSuch programs have the ugly name “appletcations”5The genealogy of JAppletjava.lang.Object | +----java.awt.Component | +----java.awt.Container | +----java.awt.Panel | +----java.applet.Applet | +----javax.swing.JApplet6The simplest possible appletimport javax.swing.JApplet;public class TrivialApplet extends JApplet { }<applet code="TrivialApplet.class" width="150" height="100"></applet>TrivialApplet.javaTrivialApplet.html7The simplest reasonable appletimport java.awt.*;import javax.swing.JApplet;public class HelloWorld extends JApplet { public void paint(Graphics g) { g.drawString("Hello World!", 30, 30); }}8Applet methodspublic void init ()public void start ()public void stop ()public void destroy ()public void paint (Graphics)Also:public void repaint()public void update (Graphics)public void showStatus(String)public String getParameter(String)9Why an applet worksYou write an applet by extending the class JAppletApplet defines methods init( ), start( ), stop( ), paint(Graphics), destroy( )These methods do nothing--they are stubsYou make the applet do something by overriding these methodsWhen you create an applet in BlueJ, it automatically creates sample versions of these methods for you10public void init ( )init() is the first method to executeinit() is an ideal place to initialize variablesinit() is the best place to define the GUI Components (buttons, text fields, checkboxes, etc.), lay them out, and add listeners to themAlmost every applet you ever write will have an init( ) method11 start( ), stop( ) and destroy( )start() and stop( ) are used when the Applet is doing time-consuming calculations that you don’t want to continue when the page is not in frontpublic void start() is called:Right after init( )Each time the page is loaded and restarted public void stop( ) is called:When the browser leaves the pageJust before destroy( )public void destroy( ) is called after stop( )Use destroy() to explicitly release system resources (like threads)System resources are usually released automatically12Methods are called in this orderinit and destroy are only called once eachstart and stop are called whenever the browser enters and leaves the pagedo some work is code called by your listenerspaint is called when the applet needs to be repaintedinit()start()stop()destroy()do some work13public void paint(Graphics g)Needed if you do any drawing or painting other than just using standard GUI ComponentsAny painting you want to do should be done here, or in a method you call from herePainting that you do in other methods may or may not happenNever call paint(Graphics), call repaint( )14repaint( )Call repaint( ) when you have changed something and want your changes to show up on the screenYou do not need to call repaint() when something in Java’s own components (Buttons, TextFields, etc.)You do need to call repaint() after drawing commands (drawRect(...), fillRect(...), drawString(...), etc.)repaint( ) is a request--it might not happenWhen you call repaint( ), Java schedules a call to update(Graphics g)15update( )When you call repaint( ), Java schedules a call to update(Graphics g)Here's what update does: public void update(Graphics g) { // Fills applet with background color, then paint(g);}16Sample Graphics methodsA Graphics is something you can paint ong.drawRect(x, y, width, height);g.fillRect(x, y, width, height);g.drawOval(x, y, width, height);g.fillOval(x, y, width, height);g.setColor(Color.red);g.drawString(“Hello”, 20, 20);Hello17Painting at the right time is hardWhen you modify common components (JButtons, JLabels, JTextFields, etc.), Java keeps the screen display up to dateWhen you paint on a Graphics object, you have to make your changes appear on the screenTo help ensure your changes appear on screen, follow these rules:Rule #1: Never call paint(Graphics g), call repaint( )Rule #2: Do all your painting in paint, or in a method that is called from paintRule #3: If you paint on any Graphics other than the Applet’s, call its update method from the Applet’s paint methodRule #4. Do your painting in a separate ThreadThese rules aren't perfect, but they should helpIf you follow these rules and the screen still doesn’t change, I probably won’t be able to find the problem, either :-(18Other useful JApplet
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