Java Variable Naming Rules and Conventions

Identifiers are basically the names of variables, methods, classes, packages and interfaces. The name of variable identifiers must be written according to the rules and conventions given by the programming language. Rules are mandatory to follow while conventions are optional, but it's good to follow them. Like every programming language, java has it's own set of rules and conventions that we should follow while writing the identifiers name.

Rules and conventions for naming variables in java

The rules and conventions for naming your variables in java can be summarized as follows :

• Every variable name should start with either alphabets, underscore ( _ ) or dollar ( $ ) symbol. It should not start with number or any other special symbols. However the convention is to always begin the variable names with a letter, not "$" or "_". Conventionally the dollar sign should never be used at all. So variable names like age, _age, $age are valid names in java.
• Variable names are case-sensitive. Names like age, Age, aGe, agE are different variables. These names can be used together in same program, but doing so is not a good practice.
• Spaces are not allowed in the variable names. For eg. int bike speed = 100; is not a valid declaration. It should be like int bikespeed = 100; or int bike_speed = 100; etc.
• Other characters apart from first character can be alphabets, numbers, $, or _ characters. For eg. user_age, a12$_ge, _$1age2 etc are valid names of variables.
• When choosing a name for your variables, the convention is to use full words instead of short forms, doing so will make your code easier to read and understand. Names like age, height and speed are more easy to understand than using short forms like a, h and s.
• The variable name you choose must not be a keyword(reserved word) of java. Java has defined some keywords like int, double, float, char, if, else etc. These keywords can not be used as your variable names. Refer java keyword table given below to see the list of all keywords in java.
• Variable name should always exist in the left hand side of assignment operator. int height = 5 is correct while 5 = int height is incorrect.
• One more convention is, if the name you choose consists of only one word, use that word in lower case letters. If it consists of more than one word, use first letter of each subsequent word as capital letter. The variable names like userName, minHeight and maxHeight are good examples of this convention.

List of java keywords

Table below shows the list of all java keywords that programmers can not use for naming their variables, methods, classes etc.

The keywords const and goto are reserved, but they are not currently used. The words true, false, and null might seem like keywords, but they are actually literals, you cannot use them as identifiers in your programs.

Java identifier rules program

 class NamingConvention {
   public static void main(String [] args) {
      byte $num1=20, _num2 = 10, num3$_ = 25;
      System.out.println("$num1 = "+ $num1 +", _num2 = "+ _num2 +", num3$_ = "+ num3$_);
      byte age=20, Age = 30, agE = 40;  
      System.out.println("age = "+ age +", Age = "+ Age +", agE = "+ agE);
      int For = 15, If = 30, Byte = 50;
      System.out.println("For = "+ For +", If = "+ If +", Byte = "+ Byte);                 
      int bike_speed = 50;
      System.out.println("bike_speed = "+ bike_speed);         
      int minSpeed = 10, maxSpeed = 80;           
      System.out.println("minSpeed = "+ minSpeed +", maxSpeed = "+ maxSpeed);         
    }
 }   	

Output:

Following are some incorrect variable naming which will result in compile time error.

 int 2num1 = 40, 12num = 10; // digits not allowed as first character
 int @num = 40, &num = 10; // only $ and _ are allowed as first character
 int bike speed = 30; // Space can not be used in variable name
 int byte = 20, float = 30, for = 40; // keywords can not be used as variable names