Identifiers are the names of variables, methods, classes, packages, and interfaces. Unlike liberals, they are not the things themselves, just ways of referring to them. In the HelloWorld program, HelloWorld, String, args, main and println are identifiers.
Rules:
1. Identifier created by below convention only
a. a to z
b. A to Z
c. 0 to 9
d. $
e. _ (underscore)
2. The identifier should not start with a digit. e.g. 123Hello is an invalid class whereas Hello123 is valid class.
3. Java Identifier is case sensitive.
4. There is no length limit for Java Identifier.
5. We can’t use reserved words as Identifier.
6. All predefined class and interface names we can use as Identifiers.