introduction to java programming cs 21a: introduction to computing i first semester, 2013-2014
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Introduction to Java
ProgrammingCS 21a: Introduction to
Computing IFirst Semester, 2013-2014
Programming
►Recall that a program is defined as:a sequence of instructions for a computer
►A large part (but not all) of CS 21a is about how to write programs in a programming language (Java)
The Java Programming Language
► Java: an object-oriented programming language that is► simple► safe► platform independent► designed for the internet
►Many universities use Java as the introductory programming language for beginning programmers► Ateneo adopted Java for CS 21a in 1997
Java (a brief history)
►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
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
Simple Java Application
File: Hello.java
// Hello World application
public class Hello
{
public static void main( String args[] )
{
System.out.println( "Hello world" );
}
}
The Programming Process
Create/EditProgram
CompileProgram
ExecuteProgram
Compile Errors? Run-Time Errors?
SourceProgram
ObjectProgram
Creation, Compilation, and Execution
►Create Java programC:\> edit Hello.java► Hello.java file is created
►Compile using javac (compiler)C:\> javac Hello.java► Hello.class file is produced
►Execute using java (interpreter)C:\>java Hello► requires a Hello.class file
Simple Java Applet
File: HelloAgain.java
import javax.swing.*;
import java.awt.*;
public class HelloAgain extends JApplet
{
public void paint( Graphics g )
{
g.drawString( "Hello", 50, 50 );
}
}
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
Sample .html Document
File: HA.html
<h1> My Sample Applet </h1>
<applet code="HelloAgain.class" height=200 width=100>
</applet>
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)
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 JApplet)►contains method/function
declarations
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