Difference Betwixt Abstract Flat Vs Interface Inwards Java

When to utilization interface as well as abstract class is 1 of the most pop object oriented pattern questions as well as almost ever asked inwards Java, C#, as well as C++ interviews. In this article, nosotros volition mostly utter inwards the context of Java programming language, but it as applies to other languages as well. The query unremarkably starts alongside a departure betwixt abstract class as well as interface inwards Java, which is rather slowly to answer, especially if yous are familiar alongside the syntax of Java interface as well as abstract class. Things start getting hard when interviewer inquire well-nigh when to utilization abstract class as well as interface inwards Java, which is mostly based upon a company agreement of pop OOPS concepts similar Polymorphism, Encapsulation, Abstraction, Inheritance, as well as Composition. Many programmers fumbles here, which is natural because most of them haven't gone through existent organisation pattern procedure as well as haven’t seen the affect of choosing 1 over other. 

The repercussion of pattern decisions are best known during the maintenance phase, a skillful pattern allows seamless development acre maintaining a frail pattern is a nightmare.

 As I convey said previously, sometimes object oriented pattern interview questions also helps to sympathize a topic better, but solely if yous are willing to do some enquiry as well as non only mugging the answer. Questions similar when to utilization abstract class as well as interface autumn nether the same category. 

In guild to best sympathize this topic, yous demand to piece of employment out some scenarios, examples etc. It's best to acquire this variety of cognition as constituent of your piece of employment but fifty-fifty if yous don't acquire there, yous tin supplement them past times reading some skillful books like Head First pattern pattern and doing some object-oriented software pattern exercises. 

In this article, nosotros volition larn the difference betwixt abstract class as well as interface inwards Java programming linguistic communication as well as based on our agreement of those differences, nosotros volition attempt to discovery out some tips as well as guidelines to create upward one's remove heed when it's improve to utilization abstract class over the interface or vice-versa.

Difference betwixt abstract class as well as interface inwards Java

While deciding when to utilization interface as well as abstract class, it’s of import to know departure betwixt abstract class as well as interface inwards Java. In my opinion, next 2 differences betwixt them drives determination well-nigh when to utilization abstract class or interface inwards Java.

1) Interface inwards Java tin solely contains declaration. You tin non declare whatever concrete methods within interface. On the other manus abstract class may comprise both abstract as well as concrete methods, which makes abstract class an ideal house to supply mutual or default functionality. I advise reading my postal service 10 things to know well-nigh interface inwards Java to know to a greater extent than well-nigh interfaces, especially inwards Java programming language.

2) Java interface tin extend multiple interface as good Java class tin implement multiple interfaces, Which agency interface tin supply to a greater extent than Polymorphism back upward than abstract class . By extending abstract class, a class tin solely participate inwards 1 Type hierarchy but past times using interface it tin endure constituent of multiple type hierarchies. E.g. a class tin endure Runnable as well as Displayable at same time. One instance I tin scream back of this is writing GUI application inwards J2ME, where  class extends Canvas as well as implements CommandListener to supply both graphic as well as event-handling functionality..

3) In guild to implement interface inwards Java, until your class is abstract, yous demand to supply implementation of all methods, which is real painful. On the other manus abstract class may assist yous inwards this instance past times providing default implementation. Because of this reason, I prefer to convey minimum methods inwards interface, starting from only one, I don't similar thought of marker interface, 1 time annotation is introduced inwards Java 5. If yous await JDK or whatever framework similar Spring, which I does to sympathize OOPS as well as pattern patter better, yous volition discovery that most of interface contains solely 1 or 2 methods e.g. Runnable, Callable, ActionListener etc.

I haven't included all syntactical departure betwixt abstract class as well as interface inwards Java here, because focus hither to larn when to utilization abstract class as well as interface as well as choosing 1 over other. Nevertheless yous tin see difference betwixt interface as well as abstract class to find  all those syntactical differences.

When to utilization interface as well as abstract class inwards Java

As I said earlier, it's slowly to respond questions similar departure betwixt abstract class as well as interface inwards Java, but hard to respond follow-ups. Though most of  Java Interview starts alongside quondam one, after it goes to run into if yous convey actually used abstract class as well as interface or not. In guild to respond this question, yous demand to convey skillful agreement of OOPS concepts like Polymorphism, Encapsulation, Abstraction as well as Inheritance. Also familiarity alongside coupling as well as cohesion is important. You at to the lowest degree should know that attempt of designing should atomic number 82 to cut coupling as well as increased cohesion, ease of maintenance etc. In this part, nosotros volition run into some scenarios, guidelines, rules which tin assist yous to create upward one's remove heed when to utilization abstract class as well as interface inwards Java.

1) In Java particularly, determination betwixt choosing Abstract class as well as interface may influence past times the fact that multiple inheritance is non supported inwards Java. One class tin solely extend some other class inwards Java. If yous lead abstract class over interface than yous lost your peril to extend some other class, acre at the same fourth dimension yous tin implement multiple interfaces to demo that yous convey multiple capability. One of the mutual example, inwards favor of interface over abstract class is Thread vs Runnable case. If yous desire to execute a chore as well as demand run() method it's improve to implement Runnable interface than extending Thread class.

2) Let's run into some other instance where an abstract class suits improve than interface. Since abstract class tin include concrete methods, it’s slap-up for maintenance indicate of view, especially when your base of operations class is evolving as well as pop off along changing. If yous demand a functionality across all your implementation e.g. a mutual method, than, yous demand to alter every unmarried implementation to include that alter if  you convey chosen interface to depict your base of operations class. Abstract class comes handy inwards this instance because yous tin only define novel functionality inwards abstract super class as well as every sub class volition automatically gets it. In short, abstract class are slap-up inwards price of evolving functionality. If yous are using interface, yous demand to do extra aid acre defining contracts because its non slowly to alter them 1 time published.

3) Interface inwards Java is slap-up for defining Types. Programming for interfaces than implementation is as good 1 of the useful Object oriented pattern principle which suggests do goodness of using interface as statement to function, render type etc.

4) One to a greater extent than full general dominion of when to utilization abstract class as well as interface is to discovery out whether a surely class volition shape a IS-A hierarchy or CAN-DO-THIS hierarchy. If yous know that yous volition endure creating classes e.g. Circle, Square than it's improve to create an abstract class Shape which tin convey area() as well as perimeter() as abstract method, rather than defining Shape as interface inwards Java. On the other manus if yous are going to create classes which tin do thinks like, tin fly, yous tin utilization interface Flyable instead of abstract class.

5) Interface to a greater extent than oft than non define capability e.g. Runnable tin run(), Callable tin call(), Displayable tin display(). So if yous demand to define capability, consider using interface. Since a class tin convey multiple capabilities i.e. a class tin endure Runnable as good as Displayable at same time. As discussed inwards starting fourth dimension point, Since java does non allow multiple inheritance at class level, only way to supply multiple capability is via interfaces.

6) Let's run into some other instance of where to utilization Abstract class as well as Interface inwards Java, which is related to before point. Suppose yous convey lot of classes to model which are birds, which tin fly, than creating a base of operations abstract class as Bird would endure appropriate  but if yous convey to model other things along alongside Birds, which tin wing e.g. Airplanes, Balloons or Kites than it's improve to create interface Flyable to stand upward for flight functionality. In conclusion, if yous demand to supply a functionality which is used past times same type of class than utilization Abstract class as well as if functionality tin endure used past times completely unrelated classes than utilization interface.

7) Another interesting utilization of Abstract class as well as interface is defining contract using interface as well as providing skeletal using abstract class. java.util.List from Java collection framework is a skillful instance of this pattern. List is declared as interface as well as extends Collection as well as Iterable interface as well as AbstractList is an abstract class which implements List. AbstractList provides skeletal implementation of List interface. Benefit of using this approach is that it minimize the attempt to implement this interface past times concrete class e.g. ArrayList or LinkedList. If you don't utilization skeletal implementation e.g. abstract class as well as instead create upward one's remove heed to implement List interface than non solely yous demand to implement all List methods but as good yous powerfulness endure duplicating mutual code. Abstract class inwards this instance cut attempt to implement interface.

8) Interface as good supply to a greater extent than decoupling than abstract class because interface doesn't comprise whatever implementation detail, acre abstract class may comprise default implementation which may couplet them alongside other class or resource.

9) Using interface as good assist acre implementing Dependency Injection pattern pattern and makes testing easy. Many mock testing framework utilize this behavior.

That's all on When to utilization Abstract class as well as interface inwards Java. Though give-and-take hither is centered only about Java but given concept of abstract class as well as interface goes beyond Java as well as as good applicable to other Object oriented language, some of the tips are as good applicable to other OOPS languages. Key matter to scream back hither is at that topographic point Definition of abstract class as well as interface e.g. inwards C++ as well as C# it varies a lot similar inwards C++ as well as Java. Single most departure is multiple inheritance. We convey as good discussed some substitution differences betwixt abstract class as well as interface inwards Java, which influence determination of choosing abstract class over interface or vice-versa. Last matter to scream back is that interface is extremely hard to evolve, then set extra aid acre designing interfaces.

PS: Effective Java, which is 1 of the best mass on Java programming as good has couplet of items on interface as well as abstract class. Joshua Bloch has advised to prefer interface over abstract class inwards some scenario, which is worth reading.

Further Learning
Design Pattern Library
From 0 to 1: Design Patterns - 24 That Matter - In Java
Java Design Patterns - The Complete Masterclass

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