What is the Instance variables in Java?

• Instance variables are declared in a class, but outside a method, constructor or any block.

• Instance variables are created when an object is created with the use of the keyword 'new' and destroyed when the object is destroyed.

• Instance variables hold values that must be referenced by more than one method, constructor or block, or essential parts of an object.s state that must be present throughout the class.

• Instance variables can be declared in the class level before or after use.

• Access modifiers can be given for instance variables.

• The instance variables are visible for all methods, constructors and block in the class.

• Normally it is recommended to make these variables private (access level).

• However, visibility for subclasses can be given for these variables with the use of access modifiers.

• Instance variables have default values.

• For numbers the default value is 0, for Booleans, it is false and for object references, it is null.

• Values can be assigned during the declaration or within the constructor.

• Instance variables can be accessed directly by calling the variable name inside the class.

• However within static methods and different class ( when instance variables are given accessibility) the should be called using the fully qualified name.

ObjectReference.VariableName.

Example of Instance variables in Java:-

import java.io.*;

public class Employee
{
 //this instance variable is visible for any child class. 
 public String name;

 //salary variable is visible in Employee class only. 
 private double salary;
 
 //The name variable is assigned in the constructor. 
 public Employee (String empName)
 {
  name = empName;
 }

 //The salary variable is assigned a value.
 public void setSalary(double empSal)
 { 
  salary = empSal;
 }

 // This method prints the employee details. 
 public void printEmp()
 {
  System.out.println("name : " + name ); 
  System.out.println("salary :" + salary);
 }
 
 public static void main(String args[])
 {
  Employee empOne = new Employee("Ransika"); 
  empOne.setSalary(1000);
  empOne.printEmp(); 
 }
}


 

This would produce the following result:

name : Ransika
salary :1000.0