Question

Java Homework Question:

Explain how class (static) variables and methods differ from their instance counterparts. Give an example of a class that contains at least one class variable and at least one class method. Don't forget to provide the code. Also, explain why using a class variable and method rather than an instance variable and method would be the correct choice in the example you select.

Answer 1

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.
• Initilisation of Instance Variable is not Mandatory. Its default value is 0
• Instance Variable can be accessed only by creating objects.
• 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 the start of program execution and destroyed automatically when execution ends.
• Initilisation of Static Variable is not Mandatory. Its default value is 0
• If we access the static variable like Instance variable (through an object), the compiler will show the warning message and it won’t halt the program. The compiler will replace the object name to class name automatically.
• If we access the static variable without the class name, Compiler will automatically append the class name.
• 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,

•   

  Live 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();
     }
  

• }