What is the main difference between Inheritance an

Question

I was presented with this question in an end of module open book exam today and found myself lost. I was reading Head first Javaand both definitions seemed to be exactly the same. I was just wondering what the MAIN difference was for my own piece of mind. I know there are a number of similar questions to this but, none I have seen which provide a definitive answer.

Answer 1

Oracle documentation quoted the difference precisely.

inheritance: A class inherits fields and methods from all its superclasses, whether direct or indirect. A subclass can override methods that it inherits, or it can hide fields or methods that it inherits. (Note that hiding fields is generally bad programming practice.)

polymorphism: polymorphism refers to a principle in biology in which an organism or species can have many different forms or stages. This principle can also be applied to object-oriented programming and languages like the Java language. Subclasses of a class can define their own unique behaviors and yet share some of the same functionality of the parent class.

polymorphism is not applicable for fields.

Related post:

Polymorphism vs Overriding vs Overloading

Answer 2

The main difference is polymorphism is a specific result of inheritance. Polymorphism is where the method to be invoked is determined at runtime based on the type of the object. This is a situation that results when you have one class inheriting from another and overriding a particular method. However, in a normal inheritance tree, you don't have to override any methods and therefore not all method calls have to be polymorphic. Does that make sense? It's a similar problem to all Ford vehicles are automobiles, but not all automobiles are Fords (although not quite....).

Additionally, polymorphism deals with method invocation whereas inheritance also describes data members, etc.

Answer 3

In Java, the two are closely related. This is because Java uses a technique for method invocation called "dynamic dispatch". If I have

public class A {
  public void draw() { ... }
  public void spin() { ... }
}

public class B extends A {
  public void draw() { ... }
  public void bad() { ... }
}

...

A testObject = new B();

testObject.draw(); // calls B's draw, polymorphic
testObject.spin(); // calls A's spin, inherited by B
testObject.bad(); // compiler error, you are manipulating this as an A

Then we see that B inherits spin from A. However, when we try to manipulate the object as if it were a type A, we still get B's behavior for draw. The draw behavior is polymorphic.

In some languages, polymorphism and inheritance aren't quite as closely related. In C++, for example, functions not declared virtual are inherited, but won't be dispatched dynamically, so you won't get that polymorphic behavior even when you use inheritance.

In javascript, every function call is dynamically dispatched and you have weak typing. This means you could have a bunch of unrelated objects, each with their own draw, have a function iterate over them and call the function, and each would behave just fine. You'd have your own polymorphic draw without needing inheritance.

Answer 4

Polymorphism is an approach to expressing common behavior between types of objects that have similar traits. It also allows for variations of those traits to be created through overriding. Inheritance is a way to achieve polymorphism through an object hierarchy where objects express relationships and abstract behaviors. It isn't the only way to achieve polymorphism though. Prototype is another way to express polymorphism that is different from inheritance. JavaScript is an example of a language that uses prototype. I'd imagine there are other ways too.

Answer 5

Inheritance is a concept related to code reuse. For example if I have a parent class say Animal and it contains certain attributes and methods (for this example say makeNoise() and sleep()) and I create two child classes called Dog and Cat. Since both dogs and cats go to sleep in the same fashion( I would assume) there is no need to add more functionality to the sleep() method in the Dog and Cat subclasses provided by the parent class Animal. However, a Dog barks and a Cat meows so although the Animal class might have a method for making a noise, a dog and a cat make different noises relative to each other and other animals. Thus, there is a need to redefine that behavior for their specific types. Thus the definition of polymorphism. Hope this helps.

Answer 6

Polymorphism: Suppose you work for a company that sells pens. So you make a very nice class called "Pen" that handles everything that you need to know about a pen. You write all sorts of classes for billing, shipping, creating invoices, all using the Pen class. A day boss comes and says, "Great news! The company is growing and we are selling Books & CD's now!" Not great news because now you have to change every class that uses Pen to also use Book & CD. But what if you had originally created an interface called "SellableProduct", and Pen implemented this interface. Then you could have written all your shipping, invoicing, etc classes to use that interface instead of Pen. Now all you would have to do is create a new class called Book & CompactDisc which implements the SellableProduct interface. Because of polymorphism, all of the other classes could continue to work without change! Make Sense?

So, it means using Inheritance which is one of the way to achieve polymorphism.

Polymorhism can be possible in a class / interface but Inheritance always between 2 OR more classes / interfaces. Inheritance always conform "is-a" relationship whereas it is not always with Polymorphism (which can conform both "is-a" / "has-a" relationship.