In programming languages, identifiers are used for identification purposes. In Java, an identifier can be a class name, method name, variable name, or label. For example :
public class Test
{
public static void main(String[] args)
{
int a = 20;
}
}
In the above java code, we have 5 identifiers namely :
Test : class name.
main : method name.
String : predefined class name.
args : variable name.
a : variable name.
Rules for defining Java Identifiers
There are certain rules for defining a valid java identifier. These rules must be followed, otherwise we get compile-time error. These rules are also valid for other languages like C,C++.
The only allowed characters for identifiers are all alphanumeric characters([A-Z],[a-z],[0-9]), ‘$‘(dollar sign) and ‘_‘ (underscore).For example “geek@” is not a valid java identifier as it contain ‘@’ special character.
Identifiers should not start with digits([0-9]). For example “123geeks” is a not a valid java identifier.
Java identifiers are case-sensitive.
There is no limit on the length of the identifier but it is advisable to use an optimum length of 4 – 15 letters only.
Reserved Words can’t be used as an identifier. For example “int while = 20;” is an invalid statement as while is a reserved word. There are 53 reserved words in Java.