-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
-You can't use a Java keyword as an identifier.
-Identifiers in Java are case-sensitive; foo and FOO are two different identifiers.