Identifiers are designed by the programmer. They are used for naming classes, methods, variables, objects, labels, packages and interfaces in a program. Java identifiers follow the following rules:
1. They can have alphabets, digits, underscore and dollar sign characters.
2. They cannot begin with a digit.
3. Uppercase and lowercase letters are distinct.
4. They can be of any length.
Note: Identifier must be meaningful, short enough to be quickly and easily typed and long enough to be descriptive and easily read. For example: sum, average, totalMarks, PRINCIPAL_AMOUNT.