Instance variables in Java

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

• When space is allocated for an object in the heap, a slot for each instance variable value is created.

• 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 at 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 (when instance variables are given accessibility), they should be called using the fully qualified name. ObjectReference.VariableName.

Example

Online Demo

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();
   }
}

Output

This will produce the following result −

name  : Ransika
salary :1000.0