Introduction to Java

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Introduction to Java

• Programming Paradigms

2
Computing in the 1990s

• The Internet and the WWW
• Graphical User Interfaces (GUIs)
• Object-oriented Programming (OOP)

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The Internet

• A global network of computers
• The World Wide Web (WWW)
• users retrieve documents
• remote programs may be invoked
• Need language platforms that are
• network-aware
• portable
• secure

4
Graphical User Interfaces

• Users interact with an application through visual
  objects
• Windows, Buttons, Text fields, etc.
• Mouse and pointer facilitate certain actions
• click, drag, drop, etc.
• Visual Programming
• focus is on handling UI objects and events

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Object-Oriented Programming

• Program (revised definition)
• collection of interacting objects
• Object
• a thing that has identity, state, and behavior
• instance of a class
• OOP
• focus is on developing classes that specify state
  (variables) and behavior (functions)

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Enter Java

• 1991
• Sun Microsystems develops a language (based on C)
  for consumer electronic devices
• 1993
• WWW explodes in popularity
• increased need for dynamic Web pages
• 1995
• Sun formally announces Java for web use

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What is Java?

• Java is a general purpose programming language
  that is
• object-oriented
• interpreted, architecture-neutral, portable
• distributed (network-aware), secure
• simple, robust
• multi-threaded
• high-performance (?)

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Two Types of Java Programs

• Applications
• general-purpose programs
• standalone
• executed through the operating system
• Applets
• programs meant for the WWW
• embedded in a Web page
• normally executed through a browser

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Simple Java Application

• File Hello.java
• // Hello World application
• public class Hello
• public static void main(String args)
• System.out.println(Hello world)

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Compilation and Execution

• Compile using javac (compiler)
• Cgt javac Hello.java
• Hello.class file is produced
• Execute using java (interpreter)
• Cgtjava Hello
• requires a Hello.class file

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Simple Java Applet

• File HelloAgain.java
• import java.awt.
• import java.applet.Applet
• public class HelloAgain extends Applet
• public void paint(Graphics g)
• g.drawString(Hello,50,50)

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Executing Applets

• After compiling the java program
• Embed an applet tag in an .html document that
  references the .class file
• Open the .html document using a browser or the
  appletviewer

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Sample .html Document

• File HA.html
• lth1gt My First Applet lt/h1gt
• ltapplet codeHelloAgain.class height200
  width100gt
• lt/appletgt

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What is HTML?

• Hypertext Markup Language
• Underlying language of Web pages
• A means of providing formatting instructions for
  presenting content
• Text-based
• .html documents
• collection of content and controls (tags)

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Java Program Structure

• Java Program
• (optional) import declarations
• class declaration
• Class
• class name should match its file name
• may extend an existing class (such as Applet)
• contains method/function declarations