Rules For Defining Java Identifiers

There are certain rules for defining a valid Java identifier. These rules must be followed, otherwise, we get a compile-time error. These rules are also valid for other languages like C, and 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 contains a ‘@’ a special character.
• Identifiers should not start with digits([0-9]). For example “123geeks” is 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 a while is a reserved word. There are 53 reserved words in Java.

Examples of valid identifiers : 

MyVariable
MYVARIABLE
myvariable
x
i
x1
i1
_myvariable
$myvariable
sum_of_array
geeks123

Examples of invalid identifiers : 

My Variable  // contains a space
123geeks   // Begins with a digit
a+c // plus sign is not an alphanumeric character
variable-2 // hyphen is not an alphanumeric character
sum_&_difference // ampersand is not an alphanumeric character

In Java, identifiers are used for identification purposes. Java Identifiers can be a class name, method name, variable name, or label. 

Example of Java Identifiers

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.