Rules for creating an identifier in Java

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