The Essence of OOP using Java, Runtime Polymorphis

What is polymorphism?

The meaning of the word polymorphism is something like one name, many forms.

How does Java implement polymorphism?

Polymorphism manifests itself in Java in the form of multiple methods having the same name.

In some cases, multiple methods have the same name, but different formal argument lists (overloaded methods, which were discussed in a previous lesson).

In other cases, multiple methods have the same name, same return type, and same formal argument list (overridden methods).

Three distinct forms of polymorphism

From a practical programming viewpoint, polymorphism manifests itself in three distinct forms in Java:

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

I covered method overloading as one form of polymorphism (compile-time polymorphism) in a previous lesson.  I also explained automatic type conversion and the use of the cast operator for type conversion in a previous lesson.

In this lesson ...

I will begin the discussion of runtime polymorphism through method overriding and inheritance in this lesson.  I will cover interfaces in a subsequent lesson.

The essence of runtime polymorphic behavior

With runtime polymorphism based on method overriding, the decision as to which version of a method will be executed is based on the actual type of object whose reference is stored in the reference variable, and not on the type of the reference variable on which the method is invoked.

Late binding

The decision as to which version of the method to invoke cannot be made at compile time.  That decision must be deferred and made at runtime.  This is sometimes referred to as late binding.

http://www.developer.com/tech/article.php/983081/The-Essence-of-OOP-using-Java-Runtime-Polymorphism-through-Inheritance.htm