Polymorphism

lPolymorphism in a Java program

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• The ability of a reference variable to change behavior according to what object instance it is holding.
• This allows multiple objects of different subclasses to be treated as objects of a single super class, while automatically selecting the proper methods to apply to a particular object based on the subclass it belongs to.
  

There are 3 forms of Polymorphism

• Method overloading
  
• Method overriding through inheritance
  
• Method overriding through the Java interface

Example:

Given the parent class Person and the subclass Student of the previous examples, we add another subclass of Person which is Employee

lIn Java, we can create a reference that is of type super class to an object of its subclass. For example,

public static main( String[] args ) {

Student studentObject = new Student();

Employee employeeObject = new Employee();

Person ref = studentObject; // Person reference points              // to a Student object

   String name = ref.getName();

}

• Now suppose we have a getName method in our super class Person, and we override this method in both Student and Employee subclass's
  

public class Student {

public String getName(){

System.out.println(“Student Name:” + name);

return name;

}

}

public class Employee {

public String getName(){

System.out.println(“Employee Name:” + name);

return name;

}

}

l

• Going back to our main method, when we try to call the getName method of the reference Person ref, the getName method of the Student object will be called. 
  

l

• Now, if we assign ref to an Employee object, the getName method of Employee will be called.
  

l

• Another example that illustrates polymorphism is when we try to pass a reference to methods as a parameter
  

l

• Suppose we have a static method printInformation that takes in a Person reference as parameter
  

public static printInformation( Person p ){

       // It will call getName() method of the

       // actual object instance that is passed

       p.getName();

}

• We can actually pass a reference of type Employee and type Student to the printInformation method as long as it is a subclass of the Person class.
  

public static main( String[] args )‏

{

Student studentObject = new Student();

Employee employeeObject = new Employee();

printInformation( studentObject );

printInformation( employeeObject );

}