Fundemantals

Source code of the java program is first written in plain text files ending with the .java extension. When you compile the java program Those source files are then converted into .class files by the javac compiler. A .class file does not contain code that is native to your processor; it instead contains bytecodes


Java VM is available on many different operating systems, the same .class files are capable of running on Microsoft Windows, the Solaris ^TM Operating System (Solaris OS), Linux, or Mac OS.

Identifiers and Keywords

The rules the compiler uses to determine whether a name is legal. 

• Identifiers must start with a letter, a currency character ($), or a connecting character such as the underscore ( _ ). Identifiers cannot start with a number!
• After the first character, identifiers can contain any combination of letters, currency characters, connecting characters, or numbers.
• In practice, there is no limit to the number of characters an identifier can contain.
• You can't use a Java keyword as an identifier.
• Identifiers in Java are case-sensitive; foo and FOO are two different identifiers

Examples:
The following identifiers are legal

int _a;  //starting with underscore

int $c;  //starting with $ sign

int ______2_w;  //starting with underscore

int _$;       //starting with underscore

int this_is_a_very_detailed_name_for_an_identifier; // starting with a letter

The following identifiers are illegal

int :b; // starting letter is not valid

int -d; // starting letter is not valid

int e#; // special charactor # is not legal

int .f; // starting (.) is not valid

int 7g; //starting with number not allowed.

Java Keywords

You cannot use any of the following as identifiers in your programs

abstract
   continue
   for 
   new 
switch 
   assert*** 
   default
   goto* 
package 
   synchronized 
   boolean 
   do 
if
   private 
   this 
   break 
double 
   implements 
   protected 
   throw 
byte 
   else  
   import 
   public 
case 
   enum**** 
   instanceof 
   return 
transient 
   catch 
   extends 
   int 
short 
   try 
   char 
   final 
interface 
   static 
   void 
   throws 
class 
   finally 
   long 
   strictfp** 
volatile 
   const* 
   float 
   native 
super 
   while 

* not used 

** added in 1.2 *** added in 1.4 **** added in 5.0

Through the Java VM, the same application is capable of running on multiple platforms.

Legal Identifiers

Technically, legal identifiers must be composed of only Unicode characters, numbers, currency symbols, and connecting characters (like underscores).

All the Java components —classes, variables, and methods—need names. In Java these names are called

there are rules for what constitutes a legal Java identifier.