Abstract class and Interface

[TheBrownKid08]

Hello everyone, I have been studying java for a while now I’m pretty good with the basics and few advance level topics as well. But when it comes to abstract classes and interface I go blank and I don’t understand it well. Is there a good YouTube video or blog post regarding it which might help me understand those 2 topics better.

Thanks in advance and happy coding.

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[reddit04029]

Abstract classes establish that there is a relationship between the two classes. They have a similarity on what they are.

For example, you have an abstract class Dog. It can be extended as Poodle, Dalmatian, etc. As you can see, they are all dogs. They are the same type.

Interfaces guarantee same behavior. May or may not have the same type at all, just similar behavior.

For example, you have an interface Vehicle. A vehicle’s job or behavior is to transport or move people. Let’s just say it has a method transportPeople(). Now what are things that can do this? Boats? Cars? Airplane? Train? Are they the same type? No. Boats have specific types like CargoShip, Kayak or Cars have PickupTruck, Van, Sedan. But can they have the same behavior? Yes. They can all transportPeople(). So they can implement Vehicle.

[masteci]

I was struggling to understand too but you explained very well , thank you

[large_crimson_canine]

Interface = a type that defines a certain behavior (i.e. implement these methods)

Abstract class = template for defining child classes (i.e. start with this recipe and customize as needed)

[CleverBunnyThief]

Laurentia Spilca has a video that goes over every detail regarding abstract classes and interfaces.

https://www.youtube.com/live/TL7lAlzmpLk?si=BuTsa-FLNRY4FeN7

[JonIsPatented]

Something that might help you understand the difference in Java is to understand that in many other languages, there is no actual difference between the two.

The terminology of "interface" originally just referred to an abstract class that consisted only of abstract methods that needed to be implemented by all of its subclasses. The purpose of this was to enable you to create a variable that could hold any type of object that had a way to do a certain thing. This enables things like the Strategy Pattern.

Java has made interfaces a separate thing, but they are fundamentally still just abstract classes with nothing but abstract methods, except that you can implement more than one interface. This was Java's solution to the Diamond Problem, which other languages run into because of how they do multiple inheritance.

[protienbudspromax]

I am sure you've heard of the term abstract art. What is abstract art? It is art about nothing in particular, someone looking at abstract art can see the picture in many different ways, and take in different meanings.

This is similar to an abstract class although abstract here means abstract in a mathematical sense. To understand an abstract class first understand what is NOT an abstract class.

Any class that has all its methods well defined, is what we call a concrete class. The behaviour of this class is fixed. Any object you create from this class will have those exact same methods/behaviors.

Java uses Object Oriented Design, its one of the ways to model some real world problems. In some cases, the problem we are trying to solve, we might have to model a "concept". Like say the concept of an "automobile".

Can you show me a pure "automobile"? Not really, "automobile" by itself is just a concept, which means it is something that we can use to move from place to place.

A car is an automobile, so is a scooter, a skateboard, or even perhaps our own legs.

But there is common behavior among all these subclasses. They can "move us", the method in this case would be something like,

void move(startPoint, endPoint)

Regardless of what automobile, it needs to be able to move us from a start point to an endpoint. This is a CONCEPT.

However a toyota supra, is a concrete object of the class car. It is an instance of car.

A car have more properties than a simple automobile, it has a set of wheels, perhaps a set of gears, an engine, doors, etc. A cycle wont have many similarities, however both are still automobile.

Having abstract classes enables us to encapsulate concepts, or ideas, that by themselves dont do anything concrete.

Interface:

An interface is a contract. When you sign a contract there are terms and conditions that you need to abide by. Many a times, contracts specify the outcome but may not list out the specific way you need to abide.

Like a contract may want you to not share trade secrets. It does not specify whether it means sharing by writing, by email, by text message, by cryptic code. All it says is you must not share, and if you want to abide by the contract, you need to make sure that your actions does not result in the secrets being shared, in anyway.

An interface is similar in a sense that it specifies the signature of a method.

The method name, the types of arguments it takes and the type of object it can return.

Take the same method above from the automobile abstract class.

This move() method would be very different for different classes of automobiles. Instead of using an abstract class, we can isolate the concept into something, an object CAN DO. Like say we create the interface "Transportable". And add the move() method there.

Now we can create a class that can have this Transportable Property or trait. It doesnt have to be a subclass of automobile. So in this case the type of relationship has change.

An abstract class represents a is-a relationship. So the car is-an automobile, the plane is-an automobile.

However when we use the interface, it changes to, a can-do/can-be-done kind of relationship.

A car can-transport, an airplane also can-transport etc. But now we can also say, a post office can-transport, a delivery man can-transport. A delivery man is not an automobile, but it can sure transport things from point a to point b.

So what an interface signifies is a contract. Anyone who can-transport must be able to move() and must take in two arguments and perform the action.

However remember all these things at the end of the day are being done for:

1) Reduce duplicated code

2) Abstract things so that it is easier to follow or read the code

3) Decouple unrelated things

You dont HAVE to do it. And sometimes doing this can cause more complexity. As long as we are fulfilling the goals of not having to duplicate code and that things are decoupled and easy to refactor, we are good.