Objective: Learn about polymorphism and casting.

What is Polymorphism?
Polymorphism is the ability of an object to take on many forms.
It is when a parent class references a child class object.


Why is polymorphism userful?
When we build a generic framework and it takes different object with the same
parent class, we can still use the framework as long as the object follows
rules of its parent class.


What is downcasting?
Converting an object from its supertype to its subtype.


How are methods called when using polymorphism?

  public class Top {
        public int value = 1;
        public void printValue() { System.out.println(value); }
  }

  public class Mid extends Top {
        public void printValue() { System.out.println(value); }
  }

  public class Bottom extends Mid {
        public int value = 3;
  }

  Top myVar = new Bottom();
  myVar.printValue();

Creates an object Bottom and upcasts it to Top (upcast done automatically).
Checks if printValue() present in Bottom and since its not goes to Mid and print 1.
If printValue() existed in Bottom it would have printed 3.


Example of polymorphism and downcasting.
// animal can be many objects such as dog, cat etc. this is example of polymorphism
Animal dog = new Dog();

// Example of downcasting. Converting from animal to dog
Dog newDog = (Dog) dog;