Identifier:  A name in java program is called identifier. It can be class name, variable name, method name, label name. 



                      
  Ex:
                              class Ab   //Ab is identifier {
                              public static void main(String args[])  //main is identifier 
                            {  
                                 public void m1(){  //m1 is identifier
int a=12;//a is identifier label 1://identifier
for (int i=0; i<5; i++)
            {              if(i==2){ 

  break;

 }

            }
Rules:
            1:- The only allowed characters in java for identifiers are: 
               a>  a-z,                 b> A-Z,                 c> 0-9 and _,$.
NOTE: -   If we are using other characters then we’ll get compile time error.
Ex:   a> all_Inmember (YES)  b> _123ab (YES)  c> abc@(No)  d>123foo (No)
             2:- Java Identifiers can’t start with digits.(Like 123va is wrong).
 3:-Java Identifiers are case sensitive.(Like int Num=12 is different from int num=12).  4:-There is no limit for the java identifiers but recommended length for java identifier is less        than 15
            5:-Reserved words can’t be used as identifiers.
            6:- All predefined class name and interface name can be used as identifier even it is legal but it is not recommended.
                            class T {                                           int Spring=12;System.out.println(Spring);//not recommended.
                                         Spring Runnable=”java”;//not recommended.}