Variables in Java


A variable is the name given to a memory location. It is the basic unit of storage in a program.

  • The value stored in a variable can be changed during program execution.
  • A variable is only a name given to a memory location, all the operations done on the variable effects that memory location.
  • In Java, all the variables must be declared before they can be used.

How to declare variables?
We can declare variables in java as follows:
Variables in Java

datatype: Type of data that can be stored in this variable.
variable_name: Name given to the variable.
value: It is the initial value stored in the variable.

Examples:

float simpleInterest; //Declaring float variable
int time = 10, speed = 20; //Declaring and Initializing integer variable
char var = 'h'; // Declaring and Initializing character variable

Types of variables

There are three types of variables in Java:

  • Local Variables
  • Instance Variables
  • Static Variables

Let us now learn about each one of these variables in detail.

  1. Local Variables: A variable defined within a block or method or constructor is called local variable.

    • These variable are created when the block in entered or the function is called and destroyed after exiting from the block or when the call returns from the function.
    • The scope of these variables exists only within the block in which the variable is declared. i.e. we can access these variable only within that block.

    Sample Program 1:

    public class StudentDetails
    {
        public void StudentAge() 
        {   //local variable age
        	int age = 0;
        	age = age + 5;
        	System.out.println("Student age is : " + age);
        }
    
        public static void main(String args[])
        {
        	StudentDetails obj = new StudentDetails();
        	obj.StudentAge();
        }
    }
    

    Output:

    Student age is : 5
    

    In the above program the variable age is local variable to the function StudentAge(). If we use the variable age outside StudentAge() function, the compiler will produce an error as shown in below program.

    Sample Program 2:

    public class StudentDetails
    {
        public void StudentAge() 
        {   //local variable age
        	int age = 0;
        	age = age + 5;
        }
    
        public static void main(String args[]) 
        {   
            //using local variable age outside it's scope
        	System.out.println("Student age is : " + age);
        }
    }
    
    

    Output:

    error: cannot find symbol
     " + age);
    
  2. Instance Variables: Instance variables are non-static variables and are declared in a class outside any method, constructor or block.

    • As instance variables are declared in a class, these variables are created when an object of the class is created and destroyed when the object is destroyed.
    • Unlike local variables, we may use access specifiers for instance variables. If we do not specify any access specifier then the default access specifier will be used.

    Sample Program:

    import java.io.*;
    class Marks
    {   
        //These variables are instance variables.
        //These variables are in a class and are not inside any function
        int engMarks;
        int mathsMarks;
        int phyMarks;
    }
    
    class MarksDemo
    {
        public static void main(String args[])
        {   //first object
            Marks obj1 = new Marks();
            obj1.engMarks = 50;
            obj1.mathsMarks = 80;
            obj1.phyMarks = 90;
    
            //second object
            Marks obj2 = new Marks();
            obj2.engMarks = 80;
            obj2.mathsMarks = 60;
            obj2.phyMarks = 85;
    
            //displaying marks for first object
            System.out.println("Marks for first object:");
            System.out.println(obj1.engMarks);
            System.out.println(obj1.mathsMarks);
            System.out.println(obj1.phyMarks);
        
            //displaying marks for second object
            System.out.println("Marks for second object:");
            System.out.println(obj2.engMarks);
            System.out.println(obj2.mathsMarks);
            System.out.println(obj2.phyMarks);
        }
    }
    

    Output:

    Marks for first object:
    50
    80
    90
    Marks for second object:
    80
    60
    85
    

    As you can see in the above program the variables, engMarks , mathsMarks , phyMarksare instance variables. In case we have multiple objects as in the above program, each object will have its own copies of instance variables. It is clear from the above output that each object will have its own copy of instance variable.

  3. Static Variables: Static variables are also known as Class variables.

    • These variables are declared similarly as instance variables, the difference is that static variables are declared using the static keyword within a class outside any method constructor or block.
    • Unlike instance variables, we can only have one copy of a static variable per class irrespective of how many objects we create.
    • Static variables are created at start of program execution and destroyed automatically when execution ends.

    To access static variables, we need not to create any object of that class, we can simply access the variable as:

    class_name.variable_name;

    Sample Program:

    import java.io.*;
    class Emp {
    
       // static variable salary
       public static double salary;
       public static String name = "Harsh";
    }
    
    public class EmpDemo
    {
         public static void main(String args[]) {
          
          //accessing static variable without object         
          Emp.salary = 1000;
          System.out.println(Emp.name + "'s average salary:" + Emp.salary);
       }
        
    }
    

    output:

    Harsh's average salary:1000.0
    

Instance variable Vs Static variable

  • Each object will have its own copy of instance variable whereas We can only have one copy of a static variable per class irrespective of how many objects we create.
  • Changes made in an instance variable using one object will not be reflected in other objects as each object has its own copy of instance variable. In case of static, changes will be reflected in other objects as static variables are common to all object of a class.
  • We can access instance variables through object references and Static Variables can be accessed directly using class name.
  • Syntax for static and instance variables:
    class Example
        {
            static int a; //static variable
            int b;        //instance variable
        }
    

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