Download CS 21a - Ateneo

Introduction to Java
Programming Paradigms
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Computing in the 1990s
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The Internet and the WWW
Graphical User Interfaces (GUIs)
Object-oriented Programming (OOP)
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The Internet
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A global network of computers
The World Wide Web (WWW)
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users retrieve documents
remote programs may be invoked
Need language platforms that are:
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network-aware
portable
secure
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Graphical User Interfaces
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Users interact with an application
through visual objects:
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Mouse and pointer facilitate certain
actions:
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Windows, Buttons, Text fields, etc.
click, drag, drop, etc.
“Visual Programming”
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focus is on handling UI objects and events
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Object-Oriented Programming
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Program (revised definition):
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Object:
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collection of interacting objects
a thing that has identity, state, and
behavior
instance of a class
OOP:
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focus is on developing classes that specify
state (variables) and behavior (functions)5
Enter Java
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1991
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1993
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Sun Microsystems develops a language
(based on C) for consumer electronic
devices
WWW explodes in popularity
increased need for “dynamic” Web pages
1995
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Sun formally announces Java for web use6
What is Java?
Java is a general purpose programming
language that is:
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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
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Applications
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general-purpose programs
standalone
executed through the operating system
Applets
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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
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Compile using javac (compiler)
C> javac Hello.java
 Hello.class file is produced
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Execute using java (interpreter)
C>java 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:
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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
<h1> My First Applet </h1>
<applet code=“HelloAgain.class” height=200
width=100>
</applet>
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What is HTML?
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Hypertext Markup Language
Underlying language of Web pages
A means of providing formatting
instructions for presenting content
Text-based
.html documents:
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collection of content and controls (tags)
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Java Program Structure
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Java Program
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(optional) import declarations
class declaration
Class
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class name should match its file name
may extend an existing class (such as
Applet)
contains method/function declarations
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