What are main rules regarding Legal Identifiers?
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 (for example: enum)
Identifiers in Java are case-sensitive (foo and FOO are two different identifiers)