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Fundamentals of
Web Programming
Lecture 15: Java Basics
20-753: Fundamentals of
Web Programming
Lecture 15: Java Basics
Copyright © 1999, Carnegie Mellon. All Rights Reserved.
1
Today’s Topics
• What is Java?
• Why Java?
• Basic Language Constructs
• HelloWorld.java
20-753: Fundamentals of
Web Programming
Lecture 15: Java Basics
Copyright © 1999, Carnegie Mellon. All Rights Reserved.
2
What is Java?
• “Java is a simple, robust, objectoriented, platform-independent,
multi-threaded, dynamic, generalpurpose programming
environment.”
http://java.sun.com/aboutJava/
20-753: Fundamentals of
Web Programming
Lecture 15: Java Basics
Copyright © 1999, Carnegie Mellon. All Rights Reserved.
3
Java Virtual Machine (VM)
C++
Java
Source
Program
Source
Program
Native
Compiler
Byte
Compiler
Universal
Binary
Native
Binary
• Generated on a specific
platform
• Runs only on that
platform
“The portability of scripting
with the performance of
compilation” (mostly)
“Write once, run anywhere”
Developer’s Computer
Java
VM
User’s Computer
• Generated on a specific
platform
• Runs on any platform with
a Java VM
20-753: Fundamentals of
Web Programming
Lecture 15: Java Basics
Copyright © 1999, Carnegie Mellon. All Rights Reserved.
4
Two Flavors of Java
• Java Applications
– Run as separate, standalone
programs on a computer or other
device
– Do not require a web browser
• Java Applets
– Run as embedded programs inside a
web browser
– Require web browser with Java VM
20-753: Fundamentals of
Web Programming
Lecture 15: Java Basics
Copyright © 1999, Carnegie Mellon. All Rights Reserved.
5
Why Java?
• Provide dynamic content
• Great support for network
programming
• Leverage client-side resources
• Platform-independence simplifies
distribution
• Small byte-code size shortens
download time
20-753: Fundamentals of
Web Programming
Lecture 15: Java Basics
Copyright © 1999, Carnegie Mellon. All Rights Reserved.
6
Why Java?
• Faster than other platform-neutral
languages (e.g., Perl)
• Developed with an intense focus
on distributed computing for the
Internet
• Comprehensive support libraries
for networking, GUI development,
client-server apps, etc.
20-753: Fundamentals of
Web Programming
Lecture 15: Java Basics
Copyright © 1999, Carnegie Mellon. All Rights Reserved.
7
Basic Language Constructs
• Basic Types (Table 38.1)
• Operators (Table 38.2)
• Control Flow (Table 38.3)
• Comments (Table 38.4)
• Documentation from Sun:
http://java.sun.com/docs/
20-753: Fundamentals of
Web Programming
Lecture 15: Java Basics
Copyright © 1999, Carnegie Mellon. All Rights Reserved.
8
Java Is Not JavaScript
• Strongly Typed
– variables must be declared as a
certain type
– variables can only hold values of that
type
• Compiled
– Java programs must be compiled
before they are run
20-753: Fundamentals of
Web Programming
Lecture 15: Java Basics
Copyright © 1999, Carnegie Mellon. All Rights Reserved.
9
Java Is Not C++
• In C++, but not in Java:
– Overloading
– Multiple Inheritance
– Destructors
– Templates
20-753: Fundamentals of
Web Programming
Lecture 15: Java Basics
Copyright © 1999, Carnegie Mellon. All Rights Reserved.
10
Java Is Not C++
• Java is “simpler and safer”
• In Java, but not in C++:
– Packages
– Interfaces
• Java requires object-oriented
analysis and design (optional in
other languages like C++)
• Result: Better Code Reusability
20-753: Fundamentals of
Web Programming
Lecture 15: Java Basics
Copyright © 1999, Carnegie Mellon. All Rights Reserved.
11
Getting Started: Tools
• javac
– The Java byte compiler
– e.g. javac HelloWorld.java
• java
– The Java virtual machine
– e.g. java HelloWorld
20-753: Fundamentals of
Web Programming
Lecture 15: Java Basics
Copyright © 1999, Carnegie Mellon. All Rights Reserved.
12
Getting Started: Tools
• appletviewer
– To view applets outside the browser
– e.g. appletviewer HelloWorld.html
• javadoc
– Generates HTML doc files for all your
Java classes
– e.g. javadoc *.java
20-753: Fundamentals of
Web Programming
Lecture 15: Java Basics
Copyright © 1999, Carnegie Mellon. All Rights Reserved.
13
Getting Started: Tools
• Linux
– kaffe implementation of Java 1.1
– pre-installed on your laptops
• Win32
– Sun’s Java Development Kit (JDK)
– Version 1.2.1 available from Sun
(or download from a local mirror)
20-753: Fundamentals of
Web Programming
Lecture 15: Java Basics
Copyright © 1999, Carnegie Mellon. All Rights Reserved.
14
First Example
• Application: HelloWorld.java
• Applet: HelloWorldApplet.java,
HelloWorldApplet.html
• http://omaha.mt.cs.cmu.edu/20-753/HelloWorld/
20-753: Fundamentals of
Web Programming
Lecture 15: Java Basics
Copyright © 1999, Carnegie Mellon. All Rights Reserved.
15