Java Variables

Variables >
Siva Nookala - 16 Feb 2019

Variables can be used to store information/data. Once a variable is declared, it can be initialized with some value and later can be changed to some other value. A variable can be used further for performing any calculations, printing it etc., Every variable should have a datatype like int, boolean etc., and that type should be specified at the time of declaring the variable. Every variable should also have a unique (or non duplicate) name to identify it.

For example, in a student program we can have one variable to store 'the section of the student', one for 'roll number' etc.

char section;
int roll_number;

Here char tells the data type of the variable and section gives the variable name. Similarly int is the data type and the variable name is roll_number. You can assign a value to a variable at the same time that it is declared. This process is known as initialization.

char section = 'C';
int roll_number = 76; //assign 76 to roll number

Here int defines the data type of the variable and roll_number gives the variable name, where 76 is the current value of the variable. As shown above, one variable (roll_number) in your program can store numeric data while another variable (section) can store char data. Java has special keywords to signify what type of data each variable stores. We can declare variables using the keywords/types explained in Data Types In Java and initialze the value of the variable.

Rules for naming variables:

No spaces in variable names

No special symbols in variable names such as !@#%^&*

Variable names can only contain alphabets, numbers, and the underscore ( _ ) symbol

Variable names can not start with numbers, but variable name can contain numbers

Variables are case-sensitive, numberOfStudents is different from Numberofstudents, NUMBEROFSTUDENTS, numberofstudents etc.,

Valid variable names	`section roll_number MIB2 arya2 with_lot_of_under_scores__ _starting_with_under_score_ numberOfStudents runsCount i j ABC345 PI`
Invalid variable names	`3idiots 7/GBrundavanColony name with spaces name_with_special_chars@@# #oo#`

Lets see a simple program using the variables.

Print variable value

CODE

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class PrintVariableValue
{
    public static void main(String arg[])
    {
        int numberOfStudents = 34; // LINE A
        int highestMarks = 98;
        
        System.out.println("The class has " + numberOfStudents + " students and the highest marks are " + highestMarks + ".");    
    }
}

OUTPUT

The class has 34 students and the highest marks are 98.

DESCRIPTION

Here we declared variables numberOfStudents and highestMarks. Both of them are of type int. We have concatenated varialables and strings to print the formatted output.

THINGS TO TRY

Change the numberOfStudents to 56 and highestMarks to 25 and observe the output.

Add one more double variable averageMarks and set it to 53.2. Print the average marks in the format "Average marks are 53.2".

In LINE A, change the variable name from numberOfStudents to numberofstudents (change O and S from capitals to small letters) but do not change it in the print statement. Observe the compilation error you get. Note that the variable names are case-sensitive.

Try declaring the variables with names like 3idiots, v@r## and number of students and see what compilation errors you get.

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