Session 1 Lecture Notes for First Course in Java
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Introduction of instructors and explanation of the syllabus

Quiz (easy) - distribute print outs

Demonstration of the “Hello World” Java application

Demonstration of the Java Glossary applet

LECTURE

Four Major Releases of Java

1.0, 1.1, 1.2 (Java 2), 1.3 (the current version of Java 2).

Free download of the current SDK, Software Development Kit at http://www.sun.com

Hello World Code

public class HelloWorld 
{
   public static void main (String[] args)
   {
     System.out.println("Hello World");
   }
}

What is Programming?

Programming is the process of creating applications. Although there are visual programming environments, such as Visual Basic, this course focuses on text-based programming.

Applications (and applets)

An application is software that runs on top of the operating system, has a specific purpose, and usually allows the user to interact. An example of an application is Microsoft Word. On the other hand, the software that runs your printer (a "driver") is something for the operating system or an application, rather than for the human user.

Applets run in a browser

Java introduced the concept of applets, which are applications that can run inside of a web browser on the client computer. A web browser is an application. 

An applet is a "safe" application that can only run inside of another, very specific application. Some of the key restrictions on an applet 

• confine the applet inside a "sand box" within the client's browser
• prevent the applet from accessing the client's file system or running native code
• give web surfers confidence  to download applets from the web, even if the user does not know the company that created the applet

Note: If an applet provider registers a specific applet with a registration company, the "signed applet", if you accept it, can have access to your file system.

Note: A Java application, like any application, does have access to your file system.

Java compared to C, C++

A computer platform is the combination of the operating system and the central processing unit (CPU). 

Examples of operating systems include 

• Mac OS
• UNIX (Solaris, HP/UX)
• Linux
• Windows 

Examples of CPUs include 

• Intel Pentium (x86 family)
• PowerPC (Apple/IBM/Motorola)
• UltraSparc (Sun)
• Alpha (Compaq)

A standard, compiled application runs machine code that is specific for a platform. Machine code is the raw zeros and ones that a computer "chip" understands. If you want your UNIX application to run on Windows, for example, you must "port" the code and recompile it using a compiler that produces machine code that is "native" to the target CPU.

Platform independence

Java code is different from standard, compiled code. You compile your Java source code into a special language, "bytecode", that a special application, the Java virtual machine (JVM), interprets. 

The JVM is an application that emulates a computer. It translates Java bytecode into native machine code.

There is a JVM for Mac OS, UNIX, Linux, and Windows. This means that the same source code should be valid for multiple platforms. You do not need to "port" the application to another platform. It should be "write once, run anywhere", assuming there is a JVM for that platform.

There are even JVMs for smaller platforms, such as the PalmOS that runs on the PalmPilot. 

Syntax and Semantics

When you use any language, whether it is a computer language or a human language, you have to follow the rules. The term "syntax" refers to the rules of word order and word combination. Some languages have a flexible syntax that lets you say "I like ice cream" as well as "Ice cream I like." Other language are more rigid in their rules. Java syntax is more rigid than, say Perl syntax. 

 When you use any language, whether it is a computer language or a human language, you make statements, declarations, expressions, and ask questions (test conditions). The words that you use to make statements, declarations, and expressions have meanings. The term "semantics" refers to the meanings of words. Human languages often allow one word to have multiple meanings, depending on the context. For example, the meaning of the word "love" in the sentence, "I love ice cream" is somewhat different from its meaning in "I love my spouse." The slang expression, "He kicked the bucket", for example, could have a literal meaning as well as a metaphorical meaning. A computer language might allow the same word to have multiple meanings, depending on the context, or it might restrict the word to one kind of use. The compiler, unlike a human being, does not tolerate ambiguity for any given context. 

For example, Java has certain "reserved words" that you use according to Java's syntax and semantics. The Hello World application consists almost exclusively of reserved words or predefined words:

public class HelloWorld 
{
   public static void main (String[] args)
   {
     System.out.println("Hello World");
   }
}

The only words that are not reserved words or predefined words are the three variables: 

• The class name HelloWorld 
• the string that the JVM prints to the screen, "Hello World"
• the variable for command line arguments, args

Just as human languages have punctuation in addition to words, so Java has special symbols, such as the curly braces { }, parentheses ( ), the brackets [], the comma separator , the wildcard asterisk *, quotation marks " ", the forward slash /, the backslash \, the dot . and the semicolon ; that completes a statement.

********

Overview of your assignment for Thursday:  

1. Downloading the Java SDK.
2. Writing, compiling, and running the HelloWorld application.
3. Using Sun's Java Tutorial on Getting Started with the HelloWorld application and applet.

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