Java

1
Java

• Lecture 16
• Dolores Zage

2
WWW

• Was a method for distributing passive information
• added forms and image maps
• interaction was only a new way to get at the same
  information
• enter Java, and the capability for Web pages of
  containing Java applets

3
Java applet

• is a dynamic and interactive program that can run
  inside a Web page displayed by a Java-capable
  browser
• Small programs that
• create animations
• multimedia presentations
• real-time video games
• multi-user networked games
• ream interactivity
• most anything a small program can do

4
What is Java?

• OO programming language developed by Sun
  Microsystems
• modeled after C
• small, simple, portable across platforms and
  operating systems
• complete full-fledge programming language
• HotJava (Suns browser) was completely written in
  Java

5
Applets and Applications

• Applets- Java programs that are downloaded over
  the WWW and executed by a Web browser on the
  readers machine
• applications - more general programs, do not
  require a browser to run.
• A single Java program can be both, depending on
  how you write that program and the capabilities
  that program uses.

6
Platform-Independent

• Most significant advantages that Java has over
  other programming languages
• at both the source and the binary level
• Javas foundation class libraries make it easy to
  write code that can be moved from platform to
  platform without the need to rewrite it to work
  with that platform

7
Bytecodes

• Binary files are also platform-independent
• use a form called bytecodes
• bytecodes are a set of instructions that looks
  like some machine codes, but that is not specific
  to any one processor

8
Traditional versus Java compilation
Traditional Code
Java Code
Binary File (Pentium)
Compiler (Pentium)
------- ------- ------- ----- ------ ----
Java Compiler (Pentium)
Java Interpreter (Pentium)
----- ----- -- ----- --- ------
Binary File (Power PC)
Java Bytecode (Platform Independent)
Java Interpreter (PowerPC)
Compiler (Power PC)
Java Compiler (PowerPC)
Binary File (SPARC)
Java Interpreter (SPARC)
Java Compiler (SPARC)
Compiler (SPARC)
9
Bytecodes

• Programs can run on any platform and any
  operating or window system as long as the Java
  interpreter is available

• Disadvantage - execution speed.

10
Creating the Hello World Java Application
All of the program is enclosed in a class
definition!
Class HelloWorld Public static
void main (String args)
System.out.println(Hello World!)

Main is the first routine that is run when the
program is executed
11
Creating and Executing Hello World

• Use your favorite editor
• filename Helloworld.java
• compile the source file using the Java compiler
  (Suns -- javac Helloworld.java)
• if no errors - end up with a file called
  Helloworld.class - this is the bytecode file
• then use the Java interpreter to execute
• java Helloworld

12
To create an applet

• Creating applets may be more complex than
  creating an application because of the special
  rules for how they behave in a browser
• for example, Hello World as an applet
• need to make space for your message
• use graphics operations to paint the message to
  the screen

13
Creating the Hello World Java Applet
Enables this applet to interact with the JDK
classes for creating applets and drawing graphics
on the screen
import java.awt.Graphics Class HelloWorldApplet
extends java.applet.Applet Public
void paint (Graphics g)
g.drawString(Hello World!, 5, 25)

Applets use several standard methods to take the
place of main, which include init(), start() and
paint()
14
Creating and Executing Hello World

• Use your favorite editor
• filename HelloworldApplet.java
• compile the source file using the Java compiler
  (Suns -- javac HelloworldApplet.java)
• if no errors - end up with a file called
  HelloworldApplet.class - this is the bytecode
  file
• now use inside of a web page

15
Using the Hello World Java Applet
Hello to Everyone!
My Java applet
says WIDTH150 HEIGHT25