Java Identifier

A name in java program is called Identifier. It may be class name, method name, variable name and label name.

Example :- 

        class Test{

                public static void main(String[] args){

                    int x= 10;

                }

             } 

In above example all the underlined words are called identifiers in java.

Rules to define java identifier :-

     Rule 1 : The only allowed characters in java identifier are :

            1. a to z        2. A to Z

            3. 0 to 9        4. _(Underscore)

            5. $

       Rule 2 : If we are using any other character we will get compile time error.

             1. total_number -------- valid

             2. Total# ----------------Invalid

       Rule 3 : Identifiers are not allowed to start with digit.

             1. ABC123    ----------valid

             2. 123ABC     ---------Invalid

       Rule 4 : java identifiers are case sensitive up course java language itself treated as                        case sensitive language.

                public class CaseSensitiveDemo {

                        public static void main(String[] args) {

                                int num=10;

                                int Num=20;

                                int nUm=30;

                                int nuM=40;

                                int NUM=50;

                                System.out.println(num);

                                System.out.println(Num);

                                System.out.println(nUm);

                                System.out.println(nuM);

                                System.out.println(NUM);

                }

}       

       Rule 5 : There is no length limit for java identifiers but it is not recommended to                          take more than 15 lengths. 

       Rule 6 :  We can't use reserved words as identifiers.

             Example : int if = 10 ;     --------- Invalid

       Rule 7 : All predefined java class names and interface names we can use as                                   identifiers .

            Example : 

              public class IdentifierRule7 {

                     public static void main(String[] args) {

                                int String = 60;

              System.out.println(String);

                }

}

Output :

        60

Note  :  Although it is legal to use class names and interface names as identifiers but it is not a good programming concept.