Interface is similar to a base class, it is also very different. If I don't need to override the method I can simply leave it as is and there should still be a working stuff method.Where do I put an interface or abstract class? Normal(?) This description just made it click for me. Bonus opinion is would be mega super cool if you could define an interface expected by a method and the compiler would check and make sure the argument had that interface without you explicitly defining the class to have it. Abstract classes are similar to interfaces but with default implementations of methods, properties and fields. Differences between a Class and an Interface: Writing code in comment? Software Engineering Stack Exchange is a question and answer site for professionals, academics, and students working within the systems development life cycle. The best answers to the question Interface vs Base class in the category Dev. When should you use an interface? If I have to provide the behavior, why can a derived type override the method? The advantage of the interface is that other code can use it without any ties whatsoever to other executable code. If you use They seem syntactically similar, both containing methods and variables, but they are different in many aspects.Class:A class is a user-defined blueprint or prototype from which objects are created. Interfaces enable safe, powerful functionality enhancements via the mixin is a type that a class can generate link and share the link here. If you derive from them, you can ONLY change the behavior if they marked one or more methods virtual. If you've ever played Spelunky or another roguelike, part of how they have amazing complexity is they use abstraction heavily in code, so a boulder is "a round, heavy thing" and behaves like "a spike ball" or "a steel ball" etc. Absolutely. Abstract classes allow you to partially implement your class, whereas interfaces contain no implementation for any members. Differences between Interface and Integration Testing, Difference Between ReadWriteLock Interface and ReentrantReadWriteLock Class in Java, Difference between Abstract Class and Interface in Java, Controlling the Visibility of Class and Interface in Java, Differences between ES6 class and ES5 function constructors. For example, you could use interfaces to describe "a duck" as: "a thing with feathers and wings that can swim and quack". rev2022.7.21.42639. type to declare that it provides Should it always be an interface if I dont want to actually define a base implementation of the methods? On the other side, abstract base classes should be used when you're defining the essential core of some thing. Providing my object exposes DoThis() then I can't see why it matters if it's an interface of a base class. ABCs and BCs are not a good way to handle the situtation where you want to say both, "an ostrich is a bird" and "an ostrich cannot fly". Its pretty good as-is. You implement an interface when you do want your class to meet the contract specified by that interface. Also, I hate the new interface with base implementations thing. That's an interesting case. rigid hierarchy. Some languages like C# only permit one base class to be inherited at a time whilst multiple interfaces can be implemented.

So, no need to use the public access specifier in the program. The main difference between interfaces and the other two is any class can implement any number of interfaces, but only one base class. It's at least as flexible as interfaces, but I will discuss it last because it just makes the most sense that way. In general, class declarations can include these components, in order: Constructors are used for initializing new objects. some optional behavior. Get access to ad-free content, doubt assistance and more! A good example of where you would use all three is if you wanted to implement an abstracted data layer which allows you to use different data sources (nosql, sqletc.). If you use an abstract class or inherit from another class then you only need to write the code for DoThis(). I've almost always just solved my problems with base classes. If I say, "a thing that flies" means it implements the IFly interface: Then I can implement "birds that fly" in terms of a class that looks like: So this is "a Bird that can Fly", but since I "have a thing that defines how I fly", we say it's a type composed from other types. Not being able to instantiate is a side-effect of what it means to be abstract; it's not it's defining characteristic. Can anyone Identify the make, model and year of this car? Imagine if I wrote this: See? The best answers are voted up and rise to the top, Start here for a quick overview of the site, Detailed answers to any questions you might have, Discuss the workings and policies of this site, Learn more about Stack Overflow the company. It is the blueprint of the class. Completely clean. Also interfaces can be mixed. That is what abstract classes are for. An abstract class allows you to create functionality that subclasses can implement or override. Then all of your services would just get an instance of a class at runtime through dependency injection with the implemented repository functions. Is it really necessary all the time? Doesn't that mean flyingBehavior can be either a raw IFly interface object, as flyingBehavior is above, or any other object that implements IFly? Modern style is to define IPet and PetBase. Come write articles for us and get featured, Learn and code with the best industry experts. If you inherit from a base class, you also gain access to any non-overrided methods in that class. So you could make an interface with a Sort() method that sorts an array, then several different classes that implement different sort algorithms. The effort that was done to support apples could have increased by X amount because of over designing fruit support. Inheritance (base classes) represent an is-a relationship. What purpose are these openings on the roof? Abstract Base Classes are quite a bit less flexible, mostly because types can only derive from ONE class. And when it has to fly, it delegates to another type. A class can inherit from one, and only one, abstract/base class. Interfaces before C# 8 say what a type does, but not necessarily how. instances are ordered with respect to Otherwise, you can perfectly do neither one nor the other. One important difference is that you can't add methods to a public interface, but you can add methods to a public abstract base class. This article highlights the differences between a class and an interface in Java. Loosely speaking, a Interfaces and base classes represent two different forms of relationships. Abstract marks the class as not quite finished yet. For me personally: If a class needs to have certain functions, but doesnt have a base implementation: interface. What are the main differences between the Java platform and other platforms? If you add a method to an interface itll break all of its implementations. An Interface is about capabilities like a Player may be an interface and any class implementing Player must be able to (or must implement) move(). The nice part about an ABC is you can provide an implementation for a lot of how the type behaves, then leave some tiny details up to whoever derives from it. A method that expects a specific class is really expecting an implied interface. Stack Exchange network consists of 180 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers. In your example, there is not a huge difference. Again, I don't struggle with how they work, I'm just asking why use them. Decoupling classes from the user interface. Abstract classes shouldnt be used to restrict instantiation. Press J to jump to the feed. They implement them. Interface should be used when working for a wide range of objects as interfaces only contain the stub, which enforces no default behavior. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy.

Please use, Can a human colony be self-sustaining without sunlight using mushrooms? An Interface cannot be instantiated i.e, objects cannot be created. I urge a different approach: Certainly there will be times when the functional differences will force your hand, but the choice also guides users in the use of your types. Where should I use abstract classes vs interfaces for a REST API client? But base classes are useful for simple implementations and common utilities. Use interfaces when you are conceptually describing a trait, or some property to be implemented. If you go 5,10,20 years with your program running fine with apple's being "things with stems" then one day there is a requirement to add oranges then you refactor at that time. Otherwise, you can perfectly do neither one nor the other. Interfaces are ideal for defining mixins. Sort of a type safe if it walks like a duck. object oriented programming examples python language oop using example class source qsp sbml workflows tools open volkswagon diesel golf vantage Interfaces specify what a class must do and not how. To be able to do that, you can't do some things in an interface that you can do on the base class - most commonly, implement any sort of default/common implementations. Why would I want to implement IPet instead of PetBase? Implement Interface using Abstract Class in Java, Difference between Inheritance and Interface in Java, Differences between HashMap and HashTable in Java, Differences between TreeMap, HashMap and LinkedHashMap in Java, Differences between wait() and join() methods in Java. (There is a little bit of extra cost in the refactoring, but the point remains) why pay upfront for something that you may never use? it's a waste of time to write a way to also support Oranges. to use inheritance. [duplicate] Dev. All you have to do is add A method expecting an interface has no care in the world about your object hierarchy. All variables and methods in a interface are declared as public.

By using our site, you implementation class to go with each Make an interface and define your properties and methods. abstract classes to define types, you Variables and methods in a class can be declared using any access specifier(public, private, default, protected). Or, "Here is an array, I've implemented the Bubble Sort algorithm, but you can create a version that uses QuickSort if you want.". QUESTION: When should I use an interface and when should I use a base class? It always needs to be something specific like a MemoryStream or FileStream. Why did the gate before Minas Tirith break so very easily? Fields are variables that provides the state of the class and its objects, and methods are used to implement the behavior of the class and its objects.Example: Interface:Like a class, an interface can have methods and variables, but the methods declared in interface are by default abstract (only method signature, no body). Composition. Any class deriving from an abstract class must implement any abstract methods. Methods can also be made abstract which means they have no implementation. The different approaches fulfill vastly different purposes. We can say a class is a template that describes the kinds of state. Let's say I want to make a book class and I want to make a collectibles class. It's not semantics at all. This tends to indicate someone didn't think very hard about their requirements. This is creating a field object of type IFly called _flyingBehavior, correct? We can say a class is a template that describes the kinds of state. Scientifically plausible way to sink a landmass, Skipping a calculus topic (squeeze theorem), Defining series before enumitem list starts, JavaScript front end for Odin Project book library database. Excellent answer. Like an interface, they are able to (in fact, they are required to) define "abstract" methods that have no behavior other than the parameters they take and the return value. So, no need to use the public access specifier in the program. If you had implemented this on a concrete using an interface, not an abstract class, then you would have to have written the functionality for both methods. However imagine you have more than one method on your class. In C#, there are two (major) things that differ between interfaces and abstract classes: One new caveat to this is that in .net 3.5 and later, you can create extension methods for interfaces. The advantage of the interface is that other code can use it without any ties whatsoever to other executable code. This is fine. Say I was building a big app with lots of inheritance. Interfaces allow the construction of nonhierarchical type An interface can be used to define a contract behavior and it can also act as a contract between two systems to interact while an abstract class is mainly used to define default behavior for subclasses, it means that all child classes should have. I hope OP sees your post above all other posts. Its a lot more detailed. So I'm suprised no one here has mentioned contracts with interface. providing an abstract skeletal

That makes interfaces good when you want your types to have very flexible behavior. Then create a concrete class that inherits from your base. In my example above, I would guess the answer is preference? So, abstract classes are like interfaces with default implementations. The issue could be my example is too simple/contrived but, is there a way to know when to use one approach over the other? Keep in mind expert devs still argue about what is "right" here. Interfaces, on the other hand, represent additional features of a class. If you really want to amp up your learning, I recommend books like Head-First Design Patterns and Working Effectively with Legacy Code. That's how abstraction works. I chose Birds as an example for a reason. Abstract classes: You use them when you want to prevent instantiation of the base class. Most people will say to make an interface until you need the capabilities of an abstract base type. /u/NekuSoul's answer is good, and so are /u/RiPont's points. If a class has a base implementation for some functions and I require super-classes to have certain functions without a base implementation: abstract class If a class only has some base implementations: base class. How would I set up my architecture in the "best" way possible? How did this note help previous owner of this old film camera? A class hierarchy does imply an interface but it does not exist as a free floating concept. However, giraffes, rhinoceros, and hippos are not exactly animals that you can make pets out of. When referring to objects in your code, prefer to refer to the interfaces as it makes it easier to replace the actual implementation (or add more and different implementations). So you can make "a bird that can fly", but leave how it flies up to the implementation. Classes does not support multiple inheritance. Announcing the Stacks Editor Beta release! Maybe some of my birds use commonly-shared flying behaviors, but a few have really special ways to fly so I don't want to make an interface implementation. Should all public methods in an abstract class be marked virtual?

An abstract class is used when we do require at least a default implementation. Interfaces have no implementation until you derive from it, and then you have to implement ALL members.

It is unreasonable to assume or over anticipate every possible scenario. The default scope for a member in interface is public. Classes are 100% about code reuse. Do you have a method called Explode? I can understand having interfaces for ISheds or IBarks (IMakesNoise?

Now if I want to sort both of these things it'd be hard because they are so different. Skip to the last paragraph if you like. So now i'm thinking interfaces right?

interface. Then classes subdivide into abstract, virtual, and normal(?). It can have properties and methods that represent the states and behaviors of an object, respectively. And for DI and unit-testing most classes containing logic also have an interface, but that is basically just the glue to make those work. Thank you. From this little bit of interface inspection, it was clear that instead of using a base abstract class, i should instead turn the private get_formatted_key into a standalone object that converted data from one type into another. Just because this particular example doesn't demonstrate that doesn't mean that it isn't an available possibility. Inheritors are the abstract class and it would be wrong to be it and anything else. I disagree with the you are in trouble part. Plus answering interview questions. If a class implements more than one interface, the interfaces are separated with a comma. Some developers don't like that you can only derive from one ABC or BC. Your Dog and Cat definitions should now look like: Theoretically you can override them from a higher base class, but essentially an interface allows you to add on only the things you need into a class without the need for inheritance. How to Fix "class, interface, or enum expected" Error in Java with Examples? But a base class is a bit too inflexible for this lesson. the required methods if they dont yet An interface is mainly used only when we do not require the implementation of methods or functionalities. other mutually comparable objects. So it specifies a set of methods that the class has to implement. Devs fight hard over which is "right", mostly because some people haven't matured enough to lose an ego and realize there are a lot of "good" ways to write a program. Virtual marks a method as a pretty reasonable default behavior, but if you want to get the most out of this class you should override the method in a subclass. Well, that's true. Definitely not without considering composition instead. In regards to MyObject, I could have created it in 2 ways.

If a class implements an interface and does not provide method bodies for all functions specified in the interface, then class must be declared abstract. nontrivial interface that you export. I'm strugging to know when to use a base class with Polymorphism or an interface. You didn't really save much time by supporting fruit, you just paid the cost upfront. So how does code reuse happen if interfaces/ABCs can't define their behavior? Type hierarchies are A class can extend only one abstract class while a class can implement multiple interfaces. If you sub-class it you dont necessarily want to override or extend the methods. You inherit from a base class when you do want to extend and/or refine that base class' current behaviour. Some people hate the SOLID style of development, but I found it to be a lifesaver. How about using default interface methods. It represents the set of properties or methods that are common to all objects of one type.

Are all three methods do be used in tandem?

The only people who are wrong are the people who say you should NEVER or ALWAYS use one of these approaches: these are people too shortsighted to realize not all programs are like theirs so not everyone will be best served by their experience. Methods will likely take abstract types or interfaces as a parameter, but will not take a base class type as a parameter.

I'm going to throw in one form of abstraction you didn't mention, but is just as important: Composition.

But the interface IComparable takes care of that. The keyword used to create an interface is interface a class can be instantiated i.e, objects of a class can be created. I understand how inheritance works and everything. implement in addition to its primary After reading the posts, I have become alarmed at the amount of misunderstanding people have about the concepts they are trying to describe to OP. From what I can see, both of these implementations achieve the same thing. No. (see first paragraph). These are just plain old classes with behavior. Using both an interface and an abstract base class makes your code more flexible. Especially the people boiling abstract classes down to "use it if you don't want people instantiating your base" which completely misses the actual point. Then you're in trouble, if you decided part of "an apple" is "a thing with a stem", etc. Tannakian-type reconstruction of etale fundamental group. says just go ahead and use use.

The best way to learn is to write programs for several years, try both techniques, and be honest with yourself about when it goes wrong. Interfaces can derive from other interfaces. Anyway, in order of flexibility they go: Interface, Abstract Base Class (ABC), Base Class. Well, Josh Bloch said himself in Effective Java 2d: Existing classes can be easily retrofitted to implement a new A lot of great points have been made here, but I think this one is the best. In the twin paradox or twins paradox what do the clocks of the twin and the distant star he visits show when he's at the star? This would mean that you would have a base abstract Repo for your base implementation details which implements IRepo. That might allow them to NOT use the behavior they provided, but then it's superfluous. I can have 10 different FlyingBird derived types using any mix of up to 10 different Fly() implementations, but I they can also share flying behaviors without writing a lot of code! This can provide common or default behavior for instance methods, but not other members. Why on earth did anyone think that was a good idea? Many people favor composition and interfaces because it's proven extremely flexible. Did you even try to study what interface and abstract class is?

Now, you have an implementation that can access default logic in the base and is conforming to the definition of your interface. Share edited sep 11, 2008 at 15:34 It is total abstraction, all methods declared within an interface must be implemented by the class(es) that implements this interface. This is fundamentally different than the other two solutions. Or, you can make "an array that can be sorted", but leave the details of the sorting algorithm up to the implementor. That is really just part of the programming. The main reason is to decouple your implementation from eachother and from specific technologies. It can implement any number of interfaces. It could be passed a Car, Balloon, or Firecracker. I wasn't thinking on additional methods you inherit from an hipothetic base class, only on those defined in the final class. Inheritance is a liability, and shouldn't be your first choice. A class can implement more than one interface. Use a private or internal constructor for this. In the US, how do we make tax withholding less if we lost our job for a few months? Usually, to a degree, base classes are going to have a lot of the same functions as their child classes, but interfaces are often only used for very specific behavior which can be quite powerful. But base classes are useful for simple implementations and common utilities. Interface is similar to a base class, it is also very different. As it's virtual though, if you need to override it then you can. Variables in a class can be static, final or neither. ), because those can be placed on a pet by pet basis, but I dont understand which to use for a generic Pet. But, is it good to know how and why you would do it? Once an interface has been defined, one or more classes can implement that interface. C# 8 made this a little less "clean".

It's also a great example of how inheritance gets you into a mess. Base Classes provide the least flexibility, but it is still greater than zero. The default scope for a member in interface is public. Completely clean. Now I'm using BOTH an abstract base class and an interface. It can be inherited by another class using the keyword extends. We use them because sometimes we're dealing with a situation like, "I want to have a lot of different Bird types, but that must include Penguin (a bird that can swim and can't fly) as well as Ostrich (a bird that can't fly) and I want to be able to ask for "all birds that can fly" without thinking very hard about it. What if supporting all fruit was a high effort task and supporting "things with stems" is low effort and you have no reason to believe that supporting other fruit is a possiblity, then why program in a way to support all fruit? Add in all your default logic.

You inherit from a base class when you do want to extend and/or refine that base class' current behaviour. To define this you will have: Now lets suppose there are other mammals, which we will usually see in a zoo: This will still be valid because at the core of the functionality Feed() and Mate() will still be the same. Its got a lot of baked in functionality, but you are going to have to add some of your own.

Implementors can ignore your abstract class if they choose. Then, make an abstract base class that inherits from that interface. On the other hand, interfaces are very hard to evolve. Good. Moreover, you can combine the virtues This relationship always represents the (single) purpose of the class (in conjunction with the single responsibility principle). Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. I implemented 3 different types of bird, 2 that share the same Fly() method, and I barely wrote a line of code! An interface can be used to define a contract behavior and it can also act as a contract between two systems to interact while an abstract class is mainly used to define default behavior for subclasses, it means that all child classes should have. frameworks. In both three cases you end up with a class implementing the same methods. So, they all have their place. exist and add an implements clause to Many people favor inheritance because they know for a fact they don't need that flexibility.

The big difference between a Base Class and an Abstract Base Class is you can use a BC directly, whereas an ABC requires you to derive a type so you can define the abstract methods. As other have stated in C# 8.0 you can now attach a default behavior to a method specific in an interface. Instead of having separate interfaces for readable streams, writable streams, seekable streams, etc., we have properties where you have to check if the stream is seekable/writable, etc. That is to say, an ABC is more like an interface than a base class. PS. Adding an ability is done best by interfaces. DoThat() already does something cool, there is no reason to rewrite it on every concrete implementation. A base class will allow you to share mutual functionality. It just means that supporting that requirement is a higher effort then if you programmed fruit as all fruit. A read-only network stream and a read/write memory stream don't really share much implementation and shouldn't be forced to inherit from a common base class. Now, why in the love of god would you do this? Call it. Because IDisposable is an interface it can easily be added to the Stream class.And in .NET Core it even implements IAsyncDisposable as an added bonus. Classes dont derive from interfaces. I've not done any big projects before and so I'm guessing that one isn't as extendible but I don't know why I think that! You CANNOT define functionality via an interface, ergo the two processes are NOT identical, and calling them essentially the same is misleading.