JAVA APPLET BASICS
- 1. Java Applets
- 2. Introduction to Java Applet Programs • Applications are stand alone programs – executed with Java interpreter • Applet is a small program – can be placed on a web page – will be executed by the web browser – give web pages “dynamic content” 2
- 3. Java Applets • Built using one of general definitions of applets – Applet class – JAapplet class • Java applets are usually graphical – Draw graphics in a defined screen area – Enable user interaction with GUI elements 3
- 4. Java Applet Classes • Abstract Windowing Toolkit AWT – Earlier versions of Java – Applet class is one of the AWT components • Java Foundation Classes JFC – Extension to Java in 1997 – Has a collection of Swing components for enhanced GUIs – Swing component classes begin with J 4
- 5. Java Applets • Applets are Java programs that can be embedded in HTML documents – To run an applet you must create a .html file which references the applet – Ready to Program also will run an applet • When browser loads Web page containing applet – Applet downloads into Web browser – begins execution • Can be tested using appletviewer program 5
- 6. Contrast Application with Applet Application Applet •Object class extended •Class not declared public •Has a main() •static keyword used •Uses System.exit(1) •JApplet class extended •class declared to be public •init() instead of main() •init() not declared with static keyword 6
- 7. Applet Declaration • Syntax (note difference from application declaration) public class ClassName extends JAapplet 7 ClassName is an object that will be a subclass of JApplet
- 8. Body of an Applet • Note there is no main() method in an applet – JApplet class provides other methods instead of a main method • First method executed is the init() method 8
- 9. Applets • Applet – Program that runs in • appletviewer (test utility for applets) • Web browser (IE, Communicator) – Executes when HTML (Hypertext Markup Language) document containing applet is opened – Applications run in command windows • Notes – Focus on fundamental programming concepts first • Explanations will come later 9
- 10. Applets and Web Pages – HTML • Applets embedded in a web page – Executed when web page loaded by browser • Web pages structured with HTML codes –HyperText Mark-up Language • Syntax <command> . . . </command> 10 Turns format onTurns format on Turns the format offTurns the format off
- 11. Applets and Web Pages – HTML • Embedding Java applets – Insert applet tags <APPLET> </APPLET> • Call the specific applet by its file name <APPLET CODE = "Whatever.class" WIDTH = nnn HEIGHT = mmmm> <APPLET> Where nnn and mmm are specific pixel sizes 11
- 12. Applets and Web Pages – HTML • Create the web page code using a text editor • Save it with an .html suffix • Open this file with appletviewer or with a web browser that supports Java • Java Plug-in must be installed (part of J2SDK 1.4.1 from Sun) 12 <HTML> <HEAD> </HEAD> <BODY> <APPLET CODE = . . . > </APPLET> </BODY> </HTML>
- 13. Applets and Web Pages – HTML • Client Web browser anywhere can access this web page from its host server • Embedded Java applet runs on client browser (of any type platform) • This means a client anywhere on any type of platform can run a piece of software developed on any other type of platform 13 Platform Independence
- 14. Thinking About Objects • Java an object-oriented language – However, Java has constructs from structured programming • Object orientation – Natural way to think about world and writing computer programs • Object-oriented programming models the real world – Attributes - properties of objects • Size, shape, color, weight, etc. – Behaviors - actions that objects can perform • A ball rolls, bounces, inflates and deflates 14
- 15. Thinking About Objects • Object orientation (continued) – Inheritance • New classes of objects absorb characteristics of existing classes – Information hiding • Objects usually do not know how other objects are implemented • We can drive cars without knowing how every part works internally 15
- 16. Thinking About Objects Class - unit of Java programming • Java focuses on nouns (classes) – C focuses on verbs and is action oriented • Contain methods – Implement behaviors • Contain data – Implement attributes • Classes are reusable – Standardized, interchangeable parts 16
- 17. A Simple Java Applet: Drawing a String • Figure 3.6 – a welcome message applet • The .html code to run the applet in a browser • The program output shown in the Applet Viewer 17 <html> <applet code = "WelcomeApplet.class" width = "300" height = "45"> </applet> </html> <html> <applet code = "WelcomeApplet.class" width = "300" height = "45"> </applet> </html>
- 18. Running the Applet • Compile – Use Ready to Program – If no errors, bytecodes stored in WelcomeApplet.class • We must create an HTML file – Loads the applet into appletviewer or a browser – Ends in .htm or .html • To execute an applet – Create an HTML file indicating which applet the browser (or appletviewer) should load and execute 18
- 19. Running the Applet - Alternatively • Run from within Ready to Program • Prompt for applet window size appears • Applet window runs 19
- 20. 20 import allows us to use predefined classes (allowing us to use applets and graphics, in this case). extends allows us to inherit the capabilities of class JApplet. Method paint is guaranteed to be called in all applets. Its first line must be defined as above.
- 21. Running An Applet import java.applet.Applet; import java.awt.Graphics; public class HelloApplet extends Applet { public void paint (Graphics g) { g.drawString ("Hello. Welcome to",25,25); g.drawString ("Java Programming",25,40); } } import java.applet.Applet; import java.awt.Graphics; public class HelloApplet extends Applet { public void paint (Graphics g) { g.drawString ("Hello. Welcome to",25,25); g.drawString ("Java Programming",25,40); } } 21 • Enter this text into your Ready to Program editor • Compile the Java code • Enter this text into your Ready to Program editor • Compile the Java code
- 22. Running An Applet • Now create an .html file to run the applet • Save it as HelloApplet.html • Make sure you save it in the same directory as the .java file 22 <html> <applet code = "HelloApplet.class" width=275, height = 100> </applet> </html> <html> <applet code = "HelloApplet.class" width=275, height = 100> </applet> </html>