Identifiers in Java with Examples

Introduction to Identifiers in Java

In general terms, an identifier is a name given to an entity for identification. Java identifiers are any attribute/property of an entity that is utilized for identification. For example, a person's name, an employee's employee number, or an individual's social security number.

Identifiers in Java are names that distinguish between different Java entities, such as classes, methods, variables, and packages. Identifiers include the names of classes, methods, variables, packages, constants, etc. These identifiers are each specified using a specific syntax and naming scheme.

The syntax would depend on the type of identifier. For example, an integer variable in Java can be declared as below,

int <variable name(identifier)>;

The variable name is the name given by the programmer to uniquely identify the int variable. Later the programmer can use that variable in his program. Hence, here the variable name is the identifier. These Identifiers follow a certain naming convention/rules when they are named, which we shall look at in the next section of this article.

Example:

Example Java Code Snippet

Identifiers: Below is the list of identifiers that are present in the above sample code.

• MainClass (Class name)
• main (Method name)
• String (Predefined Class name)
• args (String variable name)
• var1 (integer variable name)
• var2 (double variable name)
• System(Predefined Class name)
• out(Variable name)
• println (Method name)

Rules for Identifiers in Java

Below are the rules that need to be followed when defining an identifier in Java. A compile-time error will be produced if any of the following rules are broken.

Rule 1:

Identifiers can only contain alphanumeric characters [a-z] [A-Z] [0-9] , dollar sign ($) and underscore ( _ ). No other character is allowed. Valid Examples:

Invalid Examples:

Rule 2:

Identifiers cannot start with a numeric value [0-9]. The starting character should be alphabet [A-Z] [a-z], dollar ($) or underscore ( _ ).

Valid Example:

Invalid Example:

Rule 3:

Identifiers should not contain spaces in their name.

Valid Example:

Invalid Example:

Rule 4:

Java identifiers are case-sensitive.

Eg: ‘Vendetta’ and ‘vendetta’ are considered as two different identifiers.

Rule 5:

The standard convention sets the size of the Java identifiers between 4 – 15, and it is advised to follow that length when defining an identifier. However, there is no upper limit on the length of an identifier.

Rule 6:

Reserve Keywords cannot be used as Identifiers. Java defines a total of 53 reserved keywords that are predefined. In the following part, we'll examine what a reserved keyword is and a list of predefined terms.

Example: int, float, public, static, etc.

Java Reserved Keywords

Reserved keywords are keywords that are used by java syntax for a particular functionality. Since each of these reserved keywords' functionality is predefined, these keywords cannot be used for any other purpose.

Java predefines a set of 53 reserved keywords that cannot to used as Identifiers. So these keywords cannot be used as Class names, Method names, Package names, or Variable names. We have some of these keywords in our sample program in the introduction. int, double, public, static, etc., are all examples of reserved keywords.

Example 1: int Var1; → Here, int is a reserved keyword that is used to define an integer variable. The variable name is Var1, which is an identifier for an int variable.

Example 2: int break; → Here, int is a reserved keyword, and the variable name is break, which is invalid as break itself is a reserved keyword with its predefined functionality in java. Hence this statement will throw an error.

Below is the table of references for all the 53 keywords in Java:

Valid Identifiers in Java

Below is an example list of valid Identifiers that can be used in java

Invalid Identifiers in Java

Below is an example list of Identifiers that are invalid in java.

Examples

Let's discuss few examples in detail related to Java identifiers to get a better understandin:

Valid identifier example:

Output

Explanation: In this example, myVariable is a valid identifier for an integer variable. It is assigned the value $10$, and then its value is printed, resulting in the output of $10$.

Invalid identifier example

Output/Error

Explanation: In this example, the identifier $123abc$ violates the naming rule as it starts with a digit. Identifiers must start with a letter, underscore, or dollar sign, not a digit.

Using the reserved word as an identifier

Output/Error

Explanation: In this example, the identifier class is a reserved word in Java and cannot be used as an identifier. Attempting to use a reserved word as an identifier will cause a compilation error.

Valid constant identifier example

Output

Explanation: In this example, $PI$ is a valid constant identifier representing the mathematical constant pi. It is declared as final to indicate that its value cannot be changed. The value of $PI$ is then printed, resulting in the output of 3.14159.

Valid method identifier example

Output

Explanation: In this example, "printMessage" is a valid identifier for a method. It takes a String parameter "message" and prints it. The method is called with the argument "Hello, world!", resulting in the output of Hello, world!.

Read Also

• Data Structures in Java
• Array in Java
• Strings in Java