Identifiers are names in java program. Identifiers can be class name, 
method name or variable name.

  Rules for defining identifiers in java:
1) Identifiers must start with letter, Underscore or dollar($) sign.
2) Identifiers can’t start with numbers .
3) There is no limit on number of characters in identifier but not 
recommended to have more than 15
characters
4) Java identifiers are case sensitive.
5) First letter can be alphabet ,or underscore and dollar sign. 
From second letter we can have numbers
6) We should’nt use reserve words for identifiers in java.