Java Identifiers | Rules for Java Identifiers

What is an identifier in Java? 

Java Identifiers are nothing but the name of class, interface, method, or variable. For example

int message = “Welcome”

Where int is the data type and message is the identifier/ variable name.

Let’s take a look at the simple program and identify its identifiers.

Class Welcome
{
         public static void main (String args[])
         {
                 int message ="Welcome to JavaInterviewPoint"
           }
}

In the above code, we have 5 identifiers

• Welcome – Class name
• main – Method name
• String – Class name
• args – Variable name
• message – Variable name

Rules for Java Identifiers

Below are the rules which needs to be followed while defining an identifier

1. Identifiers can only have characters (a-z, A-Z, 0-9), dollar sign ($), and underscore (_) characters.

Example: String Java –> Valid Identifier
int total# –> Invalid Identifier as # is not allowed in Identifier.

2. Identifiers cannot start with a number or any other character, other than character (a-z, A-Z), dollar sign ($), or underscore (_) character.

Example:     String msg –> Valid Identifier
int $cash –> Valid Identifier
int _total –> Valid Identifier
int 123total –> Invalid Identifier, as an identifier cannot start with a number
int #total –> Invalid Identifier, # cannot be used as the starting character

3. There is no length limitation for Java Identifiers

Example: String mmmmmmeeeeeeeeessssssssssaaaaaaaagggggeeeeeeeeeee, is still a valid identifier, but it is not recommended to use a longer identifier as it will spoil the readability of the code.

4. Java Identifiers are case sensitive

Example: String message = “Welcome”
String Message =” To”
String MESSAGE = “JavaInterviewPoint”
All the above identifiers are different

5. Java reserved keywords cannot be used as an Identifier. The below list should never be used as an identifier

abstract	continue	for	new	switch
assert	default	goto	package	synchronized
boolean	do	if	private	this
break	double	implements	protected	throw
byte	else	import	public	throws
case	enum	instanceof	return	transient
catch	extends	int	short	try
char	final	interface	static	void
class	finally	long	strictfp	volatile
const	float	native	super	while

6. All the predefined classes and interface names can be used as an identifier.

Example: int String
int Exception
String Runnable

The above identifiers are still a valid identifier but not recommended.

Below are some of the valid identifiers

String WelcomeMessage;
int $rate;
int total_number;
String _flag;
int $;
String _$_$;

Invalid Identifiers

int 99Value;
String #total;
int @123;
int 5G;