Unicode provides a unique number for every character, no matter what the platform, no matter what the program, no matter what the language.You can use Unicode characters anywhere in a Java program, including comments and identifiers such as variable names.

Case-Sensitivity and Whitespace
Java is a case-sensitive language. Its keywords are written in lowercase and must always be used that way. Java white space is defined as the ASCII space, horizontal tab, and form feed characters.Java ignores spaces, tabs, newlines, and other whitespace, except when it appears within quoted characters and string literals.

Keywords
The following words are reserved in Java:

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

The keywords const and goto are reserved, even though they are not currently used.

Comments
 There have two types of comments in Java.The first type is a single-line comment, A end-of-line comment: all the text from the ASCII  characters // to the end of the line is ignored

int i = 0;   // Initialize the loop variable

The second type  is a multiline comment. It begins with the characters /* and continues, over any number of lines, until the characters */. Any text between the /* and the */ is ignored by the Java compiler.

/*
 * text
 */

 Comments do not nest.

/* and */ have no special meaning in comments that begin with //.
// has no special meaning in comments that begin with /* or /**.

Identifier
An identifier is an unlimited-length sequence of Java letters and Java digits, the first of which must be a Java letter. An identifier cannot have the same spelling (Unicode character sequence) as a keyword , boolean literal, or the null literal

Literals
A literal is the source code representation of a value of a primitive type, the String type , or the null type. For example, the following are all literals:

2.4    '6'    "two"    true    false    null

Punctuation
Java  uses a number of punctuation characters as tokens. The Java Language Specification divides these characters (somewhat arbitrarily) into two categories, separators and operators. Separators are:

(   )   {   }   [   ]     <   >   :   ;     ,   .   @Operators are:

+    -    *    /    %    &   |    ^    <<   >>   >>> +=   -=   *=   /=   %=   &=  |=   ^=   <<=  >>=  >>>= =    =  =   !=   <    <=   >    >= !    ~    &&  ||   ++   --   ?    :