Java Identifiers

In programming languages, identifiers are used for identification purpose. In Java, an identifier can be a class name, method name, variable name or a 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.

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

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