Dynamic Binding and Typecasting

• A Child class object can be assigned to Base class reference
• Assume class Customer  is base class of RegularCustomer class     

          Customer newObj = new RegularCustomer();

• The parent class reference can be used to invoke child class methods
• Only the methods which are overridden by child class and other method of parent class can not be accessed by Parent class reference.
• JVM bind the method to its linking based on the data type of Object referred by it, NOT by data type of reference.
• This decision is taken at Run time and hence it is called as Dynamic binding 
• The binding of method call to its definition happens at run-time 
• Dynamic binding is also called run-time polymorphism and late binding

Type Casting

• An instance of subclass can be casted to its parent class without explicit typecasting
• Explicit type casting can be done only between Sub class and super class

      Example:
      Customer cObject = new RegularCustomer
       RegulatCustoomer = ( RegularCustomer )cObject;

Note : Typecasting can not be done between subclasses vice versa.

2. Similarly if cRef (in the example given in the slide) refers to Customer class , then the following typecasting is not valid :

Customer cRef=new Customer();

RegularCustomer rRef=(RegularCustomer)cRef;

Method Overloading vs Method Overriding

      
Self study
What is the output of the following? 

class Base{

    private static int baseVar=0;

    public static int commonVar=0;

    public  Base(){

         baseVar++;

    }

    protected void printDetails(){

         System.out.println("BaseVariable:"+baseVar);

    }

}

class Derived extends Base{

    private static int derVar=0;

    public Derived(){

         commonVar++;

         derVar++;

    }

public void printDetails(){

         super.printDetails();

         System.out.println("Derived variable: "+derVar);

         System.out.println("CommonVar:"+commonVar);

    }

    public static void main(String args[]){

         Derived childOne=new Derived();

         Derived childTwo=new Derived();

         Base baseOne=new Base();

         childTwo.printDetails();

    }

}

output: ?

2.class Base{

   private int baseVar=1000;

   public Base(){

     baseVar++;

   }

   public void display(){

        System.out.println("Base Variable:"+baseVar);

   }

}

class Derived extends Base{

   private int derVar;

   public Derived(int derVar){

        this.derVar=derVar;

   }

   public void display(){

    System.out.println("Base Variable:"+baseVar+"Derived                  Variable:"+derVar);

   }

}

class Demo{

   public static void main(String args[]){

        Base baseOne=new Base();

        baseOne.display();

        Derived derOne=new Derived(10);

        derOne.display();

   }

}

Output: ??