Applets Applets n n An applet is a

Applets

Applets n n An applet is a Panel that allows interaction with a Java program A applet is typically embedded in a Web page and can be run from a browser You need special HTML in the Web page to tell the browser about the applet For security reasons, applets run in a sandbox: they have no access to the client’s file system 2

Applet Support n n Most modern browsers support Java 1. 4 if they have the appropriate plugin In the PC labs, Internet Explorer 5. 5 has been updated, but Netscape has not The best support isn't a browser, but the standalone program appletviewer In general you should try to write applets that can be run with any browser 3

What an applet is n n You write an applet by extending the class Applet is just a class like any other; you can even use it in applications if you want When you write an applet, you are only writing part of a program The browser supplies the main method 4

The genealogy of Applet java. lang. Object | +----java. awt. Component | +----java. awt. Container | +----java. awt. Panel | +----java. applet. Applet 5

The simplest possible applet Trivial. Applet. java import java. applet. Applet; public class Trivial. Applet extends Applet { } Trivial. Applet. html <applet code="Trivial. Applet. class” width=150 height=100> </applet> 6

The simplest reasonable applet import java. awt. *; import java. applet. Applet; public class Hello. World extends Applet { public void paint( Graphics g ) { g. draw. String( "Hello World!", 30 ); } } 7

Applet methods public public Also: public void void init () start () stop () destroy () paint (Graphics) void repaint() void update (Graphics) void show. Status(String) String get. Parameter(String) 8

Why an applet works n n n You write an applet by extending the class Applet defines methods init( ), start( ), stop( ), paint(Graphics), destroy( ) These methods do nothing--they are stubs You make the applet do something by overriding these methods When you create an applet in Blue. J, it automatically creates sample versions of these methods for you 9

public void init ( ) n n init() is the first method to execute It is an ideal place to initialize variables If you are creating a GUI, init() is the best place to define the GUI Components (buttons, text fields, scrollbars, etc. ), lay them out, and add listeners to them Almost every applet you ever write will have an init( ) method 10

start( ), stop( ) and destroy( ) n n start() and stop( ) are used when the Applet is doing timeconsuming calculations that you don’t want to continue when the page is not in front public void start() is called: n n n public void stop( ) is called: n n n Right after init( ) Each time the page is loaded and restarted When the browser leaves the page Just before destroy( ) public void destroy( ) is called after stop( ) n n Use destroy() to explicitly release system resources (like threads) System resources are usually released automatically 11

Methods are called in this order init() start() do some work stop() destroy() n n init and destroy are only called once each start and stop are called whenever the browser enters and leaves the page do some work is code called by your listeners paint is called when the applet needs to be repainted 12

public void paint(Graphics g) n n Needed if you do any drawing or painting other than just using standard GUI Components Any painting you want to do should be done here, or in a method you call from here Painting that you do in other methods may or may not happen Never call paint(Graphics), call repaint( ) 13

repaint( ) n Call repaint( ) when you have changed something and want your changes to show up on the screen n n You do not need to call repaint() when something in Java’s own components (Buttons, Text. Fields, etc. ) You do need to call repaint() after drawing commands (draw. Rect(. . . ), fill. Rect(. . . ), draw. String(. . . ), etc. ) repaint( ) is a request--it might not happen When you call repaint( ), Java schedules a call to update(Graphics g) 14

update( ) n n 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); } 15

Sample Graphics methods n A Graphics is something you can paint on g. draw. String(“Hello”, 20); Hello g. draw. Rect(x, y, width, height); g. fill. Rect(x, y, width, height); g. draw. Oval(x, y, width, height); g. fill. Oval(x, y, width, height); g. set. Color(Color. red); 16

Painting at the right time is hard n n n When you modify common components (Buttons, Labels, Text. Fields, etc. ), Java keeps the screen display up to date When you paint on a Graphics object, you have to make your changes appear on the screen To help ensure your changes appear on screen, follow these rules: n n n 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 paint Rule #3: If you paint on any Graphics other than the Applet’s, call its update method from the Applet’s paint method Rule #4. Do your painting in a separate Thread These rules aren't perfect, but they should help If you follow these rules and the screen still doesn’t change, I probably won’t be able to find the problem, either : -( 17

Other useful Applet methods n System. out. println(String s) n n n Works from appletviewer, not from browsers Automatically opens an output window. show. Status(String) displays the String in the applet’s status line. n n Each call overwrites the previous call. You have to allow time to read the line! 18

Applets are not magic! n n Anything you can do in an applet, you can do in an application. You can do some things in an application that you can’t do in an applet. If you want to access files from an applet, it must be a “trusted” applet. Trusted applets are beyond the scope of this course. 19

Structure of an HTML page n HTML n HEAD BODY TITLE (content) Most HTML tags are containers. A container is <tag> to </tag> 20

HTML <html> <head> <title> Hi World Applet </title> </head> <body> <applet code="Hi. World. class” width=300 height=200> <param name="arraysize" value="10"> </applet> </body> </html> 21

<param name="arraysize" value="10"> n public String get. Parameter(String name) n String s = get. Parameter("arraysize"); n try { size = Integer. parse. Int (s) } catch (Number. Format. Exception e) {…} 22

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