What is the difference between an interface and abstract class?

Asked 2023-09-20 21:01:03 View 930,760

What exactly is the difference between an interface and an abstract class?

  • This is an extremely common interview question. It's surprising since an abstract class is rarely used in solutions compared to other things. Your question has helped me Safraz. - anyone
  • This question might also help to understand the concept of interfaces stackoverflow.com/q/8531292/1055241 - anyone
  • I've removed the PHP tag form this question, since almost none of the answer are language specific, and the question itself is not language specific. - anyone
  • back in the day in c++ an interface is a pure abstract base class with all the method implementations = 0. If a single method was not = 0, then it has an implementation and the abstract base is no longer pure, and no longer an interface. I think the VMT has less indirection when multiple inheritance uses only pure abstract bases, but I don't remember how they look anymore, been too long. - anyone
  • In kotlin: Interfaces cannot store a state Only abstract classes can store state - anyone

Answers

Interfaces

An interface is a contract: The person writing the interface says, "hey, I accept things looking that way", and the person using the interface says "OK, the class I write looks that way".

An interface is an empty shell. There are only the signatures of the methods, which implies that the methods do not have a body. The interface can't do anything. It's just a pattern.

For example (pseudo code):

// I say all motor vehicles should look like this:
interface MotorVehicle
{
    void run();

    int getFuel();
}

// My team mate complies and writes vehicle looking that way
class Car implements MotorVehicle
{

    int fuel;

    void run()
    {
        print("Wrroooooooom");
    }


    int getFuel()
    {
        return this.fuel;
    }
}

Implementing an interface consumes very little CPU, because it's not a class, just a bunch of names, and therefore there isn't any expensive look-up to do. It's great when it matters, such as in embedded devices.


Abstract classes

Abstract classes, unlike interfaces, are classes. They are more expensive to use, because there is a look-up to do when you inherit from them.

Abstract classes look a lot like interfaces, but they have something more: You can define a behavior for them. It's more about a person saying, "these classes should look like that, and they have that in common, so fill in the blanks!".

For example:

// I say all motor vehicles should look like this:
abstract class MotorVehicle
{

    int fuel;

    // They ALL have fuel, so lets implement this for everybody.
    int getFuel()
    {
         return this.fuel;
    }

    // That can be very different, force them to provide their
    // own implementation.
    abstract void run();
}

// My teammate complies and writes vehicle looking that way
class Car extends MotorVehicle
{
    void run()
    {
        print("Wrroooooooom");
    }
}

Implementation

While abstract classes and interfaces are supposed to be different concepts, the implementations make that statement sometimes untrue. Sometimes, they are not even what you think they are.

In Java, this rule is strongly enforced, while in PHP, interfaces are abstract classes with no method declared.

In Python, abstract classes are more a programming trick you can get from the ABC module and is actually using metaclasses, and therefore classes. And interfaces are more related to duck typing in this language and it's a mix between conventions and special methods that call descriptors (the __method__ methods).

As usual with programming, there is theory, practice, and practice in another language :-)

Answered   2023-09-20 21:01:03

  • The key point about interfaces is not so much that they say what a class does, but allow objects that can Wizzle to make themselves useful to code that needs a Wizzler. Note that in many cases neither the person who writes the thing that can Wizzle, nor the person who needs a Wizzler, will be the person who writes the interface. - anyone
  • I don't think that CPU consumption is the highlight-worthy point on interfaces. - anyone
  • @e-satis Can you please explain your point on CPU utilization? Why does abstract class being a class increase CPU utilization? What kind of lookup are you referring to here? - anyone
  • @e-satis With Java 8, you can define default methods in interfaces which is the equivalent of having non-abstract methods in abstract classes. With this addition, I can no longer see the real difference between abstract classes and interface besides the fact that I should use interfaces because classes can implement multiple interfaces but can only inherit one class - anyone
  • I think the comparison between interface and class from Head First Java is vivid that A class defines who you are, and an interface tells what roles you could play - anyone

The key technical differences between an abstract class and an interface are:

  • Abstract classes can have constants, members, method stubs (methods without a body) and defined methods, whereas interfaces can only have constants and methods stubs.

  • Methods and members of an abstract class can be defined with any visibility, whereas all methods of an interface must be defined as public (they are defined public by default).

  • When inheriting an abstract class, a concrete child class must define the abstract methods, whereas an abstract class can extend another abstract class and abstract methods from the parent class don't have to be defined.

  • Similarly, an interface extending another interface is not responsible for implementing methods from the parent interface. This is because interfaces cannot define any implementation.

  • A child class can only extend a single class (abstract or concrete), whereas an interface can extend or a class can implement multiple other interfaces.

  • A child class can define abstract methods with the same or less restrictive visibility, whereas a class implementing an interface must define the methods with the exact same visibility (public).

Answered   2023-09-20 21:01:03

  • i think this is the best answer because it highlights all of the key differences. an example's not really necessary. - anyone
  • And normally with classes you can instantiate an object from it unlike the abstract classes which CANNOT be instantiated. - anyone
  • @Jiazzyuser If an abstract class implements an interface, it does not have to actually define the interface's methods. That requirement can be deferred to inheriting/child concrete classes. However, a concrete class must implement all interface methods that are not implemented by its parent class. I'll add example to illustrate this point. - anyone
  • "When inheriting an abstract class, the child class must define the abstract methods, whereas an interface can extend another interface and methods don't have to be defined." - This is not true. Just as an interface can extend an interface without defining methods, an abstract class can inherit an abstract class without defining methods. - anyone
  • Yes. "Methods and members of an abstract class can be defined with any visibility," - anyone

An Interface contains only the definition / signature of functionality, and if we have some common functionality as well as common signatures, then we need to use an abstract class. By using an abstract class, we can provide behavior as well as functionality both in the same time. Another developer inheriting abstract class can use this functionality easily, as they would only need to fill in the blanks.

Interface Abstract class
Interface support multiple implementations. Abstract class does not support multiple inheritance.
Interface does not contain Data Member Abstract class contains Data Member
Interface does not contain Constructors Abstract class contains Constructors
An interface Contains only incomplete member (signature of member) An abstract class Contains both incomplete (abstract) and complete member
An interface cannot have access modifiers by default everything is assumed as public An abstract class can contain access modifiers for the subs, functions, properties
Member of interface can not be Static Only Complete Member of abstract class can be Static

Taken from:

http://www.dotnetbull.com/2011/11/difference-between-abstract-class-and.html

http://www.dotnetbull.com/2011/11/what-is-abstract-class-in-c-net.html http://www.dotnetbull.com/2011/11/what-is-interface-in-c-net.html

Answered   2023-09-20 21:01:03

  • You need to say what language this applies to ("Abstract class does not support multiple inheritance" is far from being universally true) - anyone
  • Last comparison is confusing as per table! Methods in interface can't be static but variables are static final Implemented methods in abstract class can be static - anyone
  • Member of the interface must be static final . Last statement is wrong. - anyone
  • I think "functionality" in this answer means "implementation." Not sure what "behavior" means -- maybe "signatures"? - anyone
  • What is the targetted programming language here? C#? - anyone

An explanation can be found here: http://www.developer.com/lang/php/article.php/3604111/PHP-5-OOP-Interfaces-Abstract-Classes-and-the-Adapter-Pattern.htm

An abstract class is a class that is only partially implemented by the programmer. It may contain one or more abstract methods. An abstract method is simply a function definition that serves to tell the programmer that the method must be implemented in a child class.

An interface is similar to an abstract class; indeed interfaces occupy the same namespace as classes and abstract classes. For that reason, you cannot define an interface with the same name as a class. An interface is a fully abstract class; none of its methods are implemented and instead of a class sub-classing from it, it is said to implement that interface.

Anyway I find this explanation of interfaces somewhat confusing. A more common definition is: An interface defines a contract that implementing classes must fulfill. An interface definition consists of signatures of public members, without any implementing code.

Answered   2023-09-20 21:01:03

  • This is the most correct answer, since PHP interfaces differ from other languages in that PHP interfaces ARE abstract classes under the hood, whereas other languages' interfaces are signatures that classes must match. They behave the same as long as there are no errors though. - anyone
  • True, for PHP it's the real best anwser. But it's harder to get from the text blob than from a simple snippet. - anyone
  • From the definitions you provided, they look the same except for one detail: and interface is 100% abstract, while an abstract class is partially abstract and can have some method implementations (perhaps all methods can have implementations?). - anyone
  • So, All Interfaces are Abstract Base Classes but all ABCs are not interfaces ? - anyone

I don't want to highlight the differences, which have been already said in many answers ( regarding public static final modifiers for variables in interface & support for protected, private methods in abstract classes)

In simple terms, I would like to say:

interface: To implement a contract by multiple unrelated objects

abstract class: To implement the same or different behavior among multiple related objects

oracle documentation page provides general guidelines for the same.

Abstract class pros :

  1. Code and members can be shared among related classes.
  2. Supports non-static / non-final fields.

Interface pros :

  1. Establish relation among unrelated classes. e.g. many unrelated objects can implement Serializable OR Externalizable interface.
  2. Define the contract
  3. Multiple inheritance

abstract class establishes "is a" relation with concrete classes. interface provides "has a" capability for classes.

If you are looking for Java as programming language, here are a few more updates:

Java 8 has reduced the gap between interface and abstract classes to some extent by providing a default method feature. An interface does not have an implementation for a method is no longer valid now.

Interface use cases:

  1. Define a contract and it has to be implemented by its implementer.

    Take an example of custom serialization of object. My making your class as implementing Externalizable interface, you have to implement the contract for below methods

     void writeExternal(ObjectOutput out)
     void readExternal(ObjectInput in)
    
  2. Implementation of FactoryMethod ( creational pattern) can be done through interface and its implementation by implementer

    example : Factory Pattern. When to use factory methods?

  3. Implementation of Strategy design pattern ( behavioral pattern) : You have a family of strategies and you want to switch to different strategies at run time

    example : Real World Example of the Strategy Pattern

  4. You can find use of interface in Command design pattern (behavioral )

    example: Using Command Design pattern

abstract class use cases:

  1. You don't have implementation in base class (with shared data) and only sub-classes have to define their own implementation. You need abstract class instead of interface since you want to share state with sub-classes.

  2. Abstract classes are used in Abstract Factory design pattern implementation ( creational pattern )

    example: Factory Pattern. When to use factory methods?

  3. You can find many use cases of abstract classes in design patterns.

    example: Bridge pattern ( Structural pattern )

    Does the Bridge Pattern decouples an abstraction from implementation?

  4. Abstract classes are used in Decorator design pattern

    example : When to Use the Decorator Pattern?

  5. Abstract class is best fit for template method pattern where you have to create skeleton.

    example: Template design pattern in JDK, could not find a method defining set of methods to be executed in order

Answered   2023-09-20 21:01:03

Some important differences:

In the form of a table:

Difference

As stated by Joe from javapapers:

1.Main difference is methods of a Java interface are implicitly abstract and cannot have implementations. A Java abstract class can have instance methods that implements a default behavior.

2.Variables declared in a Java interface is by default final. An abstract class may contain non-final variables.

3.Members of a Java interface are public by default. A Java abstract class can have the usual flavors of class members like private, protected, etc..

4.Java interface should be implemented using keyword “implements”; A Java abstract class should be extended using keyword “extends”.

5.An interface can extend another Java interface only, an abstract class can extend another Java class and implement multiple Java interfaces.

6.A Java class can implement multiple interfaces but it can extend only one abstract class.

7.Interface is absolutely abstract and cannot be instantiated; A Java abstract class also cannot be instantiated, but can be invoked if a main() exists.

8.In comparison with java abstract classes, java interfaces are slow as it requires extra indirection.

Answered   2023-09-20 21:01:03

  • I've edited your answer to provide correct attribution. You can't just drop a link at the bottom of your answer. You need to quote all of the language that was copied from another source, as well. Also, if that table was drawn from somewhere, you should clearly indicate where that is from. - anyone
  • Please mention for C++ as well.. though there is no keyword "interface" in C++ as such, but its a commonly asked Qn regd C++ as well. - anyone
  • @cbinder : There is no keyword 'interface' in c++. For difference in c++, please refer 1. tutorialspoint.com/cplusplus/cpp_interfaces.htm 2. tutorialspoint.com/cplusplus/cpp_interfaces.htm - anyone
  • @MageshBabu Perhaps defining a function in a class containing pure virtual function makes it an abstract class rather than the interface - anyone
  • With Java 8, the differences are less now. Check updated differences here: journaldev.com/1607/… - anyone

The main point is that:

  • Abstract is object oriented. It offers the basic data an 'object' should have and/or functions it should be able to do. It is concerned with the object's basic characteristics: what it has and what it can do. Hence objects which inherit from the same abstract class share the basic characteristics (generalization).
  • Interface is functionality oriented. It defines functionalities an object should have. Regardless what object it is, as long as it can do these functionalities, which are defined in the interface, it's fine. It ignores everything else. An object/class can contain several (groups of) functionalities; hence it is possible for a class to implement multiple interfaces.

Answered   2023-09-20 21:01:03

  • Thank you now we are getting somewhere with a good high level response. Funny how far deep into the comments you have to go to find a more understanding-based response. - anyone
  • The other answers are too technical. This is getting towards what I would feel is a 'right' answer. The whole point of OOP is semantics, and whether or not a private nested classes public getters are invoked through CPU expensive lookups is pretty much irrelevant here - anyone

When you want to provide polymorphic behaviour in an inheritance hierarchy, use abstract classes.

When you want polymorphic behaviour for classes which are completely unrelated, use an interface.

Answered   2023-09-20 21:01:03

I am constructing a building of 300 floors

The building's blueprint interface

  • For example, Servlet(I)

Building constructed up to 200 floors - partially completed---abstract

  • Partial implementation, for example, generic and HTTP servlet

Building construction completed-concrete

  • Full implementation, for example, own servlet

Interface

  • We don't know anything about implementation, just requirements. We can go for an interface.
  • Every method is public and abstract by default
  • It is a 100% pure abstract class
  • If we declare public we cannot declare private and protected
  • If we declare abstract we cannot declare final, static, synchronized, strictfp and native
  • Every interface has public, static and final
  • Serialization and transient is not applicable, because we can't create an instance for in interface
  • Non-volatile because it is final
  • Every variable is static
  • When we declare a variable inside an interface we need to initialize variables while declaring
  • Instance and static block not allowed

Abstract

  • Partial implementation
  • It has an abstract method. An addition, it uses concrete
  • No restriction for abstract class method modifiers
  • No restriction for abstract class variable modifiers
  • We cannot declare other modifiers except abstract
  • No restriction to initialize variables

Taken from DurgaJobs Website

Answered   2023-09-20 21:01:03

  • An abstract class can have a constructor - anyone
  • I completely disagree with this view. The blueprint is a completely different concept to 'interface.' Blueprint is more analogous to a static model or design specification for a specific implementation. It is closer to 'class,' as the blueprint can be instantiated multiple times through its constructor, but even this is not close enough as the 'class' also contains the specification for how to construct (the ctor), and the means to do so. Interface as a concept is intended to represent some behavior, such as Heat Up/ Cool Down, that can be applied to arange of things, eg: buildings, ovens, etc - anyone

Let's work on this question again:

The first thing to let you know is that 1/1 and 1*1 results in the same, but it does not mean that multiplication and division are same. Obviously, they hold some good relationship, but mind you both are different.

I will point out main differences, and the rest have already been explained:

Abstract classes are useful for modeling a class hierarchy. At first glance of any requirement, we are partially clear on what exactly is to be built, but we know what to build. And so your abstract classes are your base classes.

Interfaces are useful for letting other hierarchy or classes to know that what I am capable of doing. And when you say I am capable of something, you must have that capacity. Interfaces will mark it as compulsory for a class to implement the same functionalities.

Answered   2023-09-20 21:01:03

  • Good answer, but the math metaphor is useless and made me waste roughly an equivalent amount of time reading it as writing this comment. Now multiply that by all the other people that have read this question. - anyone
  • " math metaphor is useless" , why do you think so ? - anyone

If you have some common methods that can be used by multiple classes go for abstract classes. Else if you want the classes to follow some definite blueprint go for interfaces.

Following examples demonstrate this.

Abstract class in Java:

abstract class Animals
{
    // They all love to eat. So let's implement them for everybody
    void eat()
    {
        System.out.println("Eating...");
    }
    // The make different sounds. They will provide their own implementation.
    abstract void sound();
}
 
class Dog extends Animals
{
    void sound()
    {
        System.out.println("Woof Woof");
    }
}
 
class Cat extends Animals
{
    void sound()
    {
        System.out.println("Meoww");
    }
}

Following is an implementation of interface in Java:

interface Shape
{
    void display();
    double area();
}
 
class Rectangle implements Shape 
{
    int length, width;
    Rectangle(int length, int width)
    {
        this.length = length;
        this.width = width;
    }
    @Override
    public void display() 
    {
        System.out.println("****\n* *\n* *\n****"); 
    }
    @Override
    public double area() 
    {
        return (double)(length*width);
    }
} 
 
class Circle implements Shape 
{
    double pi = 3.14;
    int radius;
    Circle(int radius)
    {
        this.radius = radius;
    }
    @Override
    public void display() 
    {
        System.out.println("O"); // :P
    }
    @Override
    public double area() 
    { 
        return (double)((pi*radius*radius)/2);
    }
}

Some Important Key points in a nutshell:

  1. The variables declared in Java interface are by default final. Abstract classes can have non-final variables.

  2. The variables declared in Java interface are by default static. Abstract classes can have non-static variables.

  3. Members of a Java interface are public by default. A Java abstract class can have the usual flavors of class members like private, protected, etc..

Answered   2023-09-20 21:01:03

It's pretty simple actually.

You can think of an interface as a class which is only allowed to have abstract methods and nothing else.

So an interface can only "declare" and not define the behavior you want the class to have.

An abstract class allows you to do both declare (using abstract methods) as well as define (using full method implementations) the behavior you want the class to have.

And a regular class only allows you to define, not declare, the behavior/actions you want the class to have.

One last thing,

In Java, you can implement multiple interfaces, but you can only extend one (Abstract Class or Class)...

This means inheritance of defined behavior is restricted to only allow one per class... ie if you wanted a class that encapsulated behavior from Classes A,B&C you would need to do the following: Class A extends B, Class C extends A .. its a bit of a round about way to have multiple inheritance...

Interfaces on the other hand, you could simply do: interface C implements A, B

So in effect Java supports multiple inheritance only in "declared behavior" ie interfaces, and only single inheritance with defined behavior.. unless you do the round about way I described...

Hopefully that makes sense.

Answered   2023-09-20 21:01:03

The comparison of interface vs. abstract class is wrong. There should be two other comparisons instead: 1) interface vs. class and 2) abstract vs. final class.

Interface vs Class

Interface is a contract between two objects. E.g., I'm a Postman and you're a Package to deliver. I expect you to know your delivery address. When someone gives me a Package, it has to know its delivery address:

interface Package {
  String address();
}

Class is a group of objects that obey the contract. E.g., I'm a box from "Box" group and I obey the contract required by the Postman. At the same time I obey other contracts:

class Box implements Package, Property {
  @Override
  String address() {
    return "5th Street, New York, NY";
  }
  @Override
  Human owner() {
    // this method is part of another contract
  }
}

Abstract vs Final

Abstract class is a group of incomplete objects. They can't be used, because they miss some parts. E.g., I'm an abstract GPS-aware box - I know how to check my position on the map:

abstract class GpsBox implements Package {
  @Override
  public abstract String address();
  protected Coordinates whereAmI() {
    // connect to GPS and return my current position
  }
}

This class, if inherited/extended by another class, can be very useful. But by itself - it is useless, since it can't have objects. Abstract classes can be building elements of final classes.

Final class is a group of complete objects, which can be used, but can't be modified. They know exactly how to work and what to do. E.g., I'm a Box that always goes to the address specified during its construction:

final class DirectBox implements Package {
  private final String to;
  public DirectBox(String addr) {
    this.to = addr;
  }
  @Override
  public String address() {
    return this.to;
  }
}

In most languages, like Java or C++, it is possible to have just a class, neither abstract nor final. Such a class can be inherited and can be instantiated. I don't think this is strictly in line with object-oriented paradigm, though.

Again, comparing interfaces with abstract classes is not correct.

Answered   2023-09-20 21:01:03

The only difference is that one can participate in multiple inheritance and other cannot.

The definition of an interface has changed over time. Do you think an interface just has method declarations only and are just contracts? What about static final variables and what about default definitions after Java 8?

Interfaces were introduced to Java because of the diamond problem with multiple inheritance and that's what they actually intend to do.

Interfaces are the constructs that were created to get away with the multiple inheritance problem and can have abstract methods, default definitions and static final variables.

See Why does Java allow static final variables in interfaces when they are only intended to be contracts?.

Answered   2023-09-20 21:01:03

  • While that is an important difference, that is not the only difference. - anyone

Interface: Turn ( Turn Left, Turn Right.)

Abstract Class: Wheel.

Class: Steering Wheel, derives from Wheel, exposes Interface Turn

One is for categorizing behavior that can be offered across a diverse range of things, the other is for modelling an ontology of things.

Answered   2023-09-20 21:01:03

In short the differences are the following:

Syntactical Differences Between Interface and Abstract Class:

  1. Methods and members of an abstract class can have any visibility. All methods of an interface must be public. //Does not hold true from Java 9 anymore
  2. A concrete child class of an Abstract Class must define all the abstract methods. An Abstract child class can have abstract methods. An interface extending another interface need not provide default implementation for methods inherited from the parent interface.
  3. A child class can only extend a single class. An interface can extend multiple interfaces. A class can implement multiple interfaces.
  4. A child class can define abstract methods with the same or less restrictive visibility, whereas class implementing an interface must define all interface methods as public.
  5. Abstract Classes can have constructors but not interfaces.
  6. Interfaces from Java 9 have private static methods.

In Interfaces now:

public static - supported
public abstract - supported
public default - supported
private static - supported
private abstract - compile error
private default - compile error
private - supported

Answered   2023-09-20 21:01:03

Many junior developers make the mistake of thinking of interfaces, abstract and concrete classes as slight variations of the same thing, and choose one of them purely on technical grounds: Do I need multiple inheritance? Do I need some place to put common methods? Do I need to bother with something other than just a concrete class? This is wrong, and hidden in these questions is the main problem: "I". When you write code for yourself, by yourself, you rarely think of other present or future developers working on or with your code.

Interfaces and abstract classes, although apparently similar from a technical point of view, have completely different meanings and purposes.

Summary

  1. An interface defines a contract that some implementation will fulfill for you.

  2. An abstract class provides a default behavior that your implementation can reuse.

Alternative summary

  1. An interface is for defining public APIs
  2. An abstract class is for internal use, and for defining SPIs

On the importance of hiding implementation details

A concrete class does the actual work, in a very specific way. For example, an ArrayList uses a contiguous area of memory to store a list of objects in a compact manner which offers fast random access, iteration, and in-place changes, but is terrible at insertions, deletions, and occasionally even additions; meanwhile, a LinkedList uses double-linked nodes to store a list of objects, which instead offers fast iteration, in-place changes, and insertion/deletion/addition, but is terrible at random access. These two types of lists are optimized for different use cases, and it matters a lot how you're going to use them. When you're trying to squeeze performance out of a list that you're heavily interacting with, and when picking the type of list is up to you, you should carefully pick which one you're instantiating.

On the other hand, high level users of a list don't really care how it is actually implemented, and they should be insulated from these details. Let's imagine that Java didn't expose the List interface, but only had a concrete List class that's actually what LinkedList is right now. All Java developers would have tailored their code to fit the implementation details: avoid random access, add a cache to speed up access, or just reimplement ArrayList on their own, although it would be incompatible with all the other code that actually works with List only. That would be terrible... But now imagine that the Java masters actually realize that a linked list is terrible for most actual use cases, and decided to switch over to an array list for their only List class available. This would affect the performance of every Java program in the world, and people wouldn't be happy about it. And the main culprit is that implementation details were available, and the developers assumed that those details are a permanent contract that they can rely on. This is why it's important to hide implementation details, and only define an abstract contract. This is the purpose of an interface: define what kind of input a method accepts, and what kind of output is expected, without exposing all the guts that would tempt programmers to tweak their code to fit the internal details that might change with any future update.

An abstract class is in the middle between interfaces and concrete classes. It is supposed to help implementations share common or boring code. For example, AbstractCollection provides basic implementations for isEmpty based on size is 0, contains as iterate and compare, addAll as repeated add, and so on. This lets implementations focus on the crucial parts that differentiate between them: how to actually store and retrieve data.

APIs versus SPIs

Interfaces are low-cohesion gateways between different parts of code. They allow libraries to exist and evolve without breaking every library user when something changes internally. It's called Application Programming Interface, not Application Programming Classes. On a smaller scale, they also allow multiple developers to collaborate successfully on large scale projects, by separating different modules through well documented interfaces.

Abstract classes are high-cohesion helpers to be used when implementing an interface, assuming some level of implementation details. Alternatively, abstract classes are used for defining SPIs, Service Provider Interfaces.

The difference between an API and an SPI is subtle, but important: for an API, the focus is on who uses it, and for an SPI the focus is on who implements it.

Adding methods to an API is easy, all existing users of the API will still compile. Adding methods to an SPI is hard, since every service provider (concrete implementation) will have to implement the new methods. If interfaces are used to define an SPI, a provider will have to release a new version whenever the SPI contract changes. If abstract classes are used instead, new methods could either be defined in terms of existing abstract methods, or as empty throw not implemented exception stubs, which will at least allow an older version of a service implementation to still compile and run.

A note on Java 8 and default methods

Although Java 8 introduced default methods for interfaces, which makes the line between interfaces and abstract classes even blurrier, this wasn't so that implementations can reuse code, but to make it easier to change interfaces that serve both as an API and as an SPI (or are wrongly used for defining SPIs instead of abstract classes).

Which one to use?

  1. Is the thing supposed to be publicly used by other parts of the code, or by other external code? Add an interface to it to hide the implementation details from the public abstract contract, which is the general behavior of the thing.
  2. Is the thing something that's supposed to have multiple implementations with a lot of code in common? Make both an interface and an abstract, incomplete implementation.
  3. Is there ever going to be only one implementation, and nobody else will use it? Just make it a concrete class.
    1. "ever" is long time, you could play it safe and still add an interface on top of it.

A corollary: the other way around is often wrongly done: when using a thing, always try to use the most generic class/interface that you actually need. In other words, don't declare your variables as ArrayList theList = new ArrayList(), unless you actually have a very strong dependency on it being an array list, and no other type of list would cut it for you. Use List theList = new ArrayList instead, or even Collection theCollection = new ArrayList if the fact that it's a list, and not any other type of collection doesn't actually matter.

Answered   2023-09-20 21:01:03

Not really the answer to the original question, but once you have the answer to the difference between them, you will enter the when-to-use-each dilemma: When to use interfaces or abstract classes? When to use both?

I've limited knowledge of OOP, but seeing interfaces as an equivalent of an adjective in grammar has worked for me until now (correct me if this method is bogus!). For example, interface names are like attributes or capabilities you can give to a class, and a class can have many of them: ISerializable, ICountable, IList, ICacheable, IHappy, ...

Answered   2023-09-20 21:01:03

You can find clear difference between interface and abstract class.

Interface

  • Interface only contains abstract methods.
  • Force users to implement all methods when implements the interface.
  • Contains only final and static variables.
  • Declare using interface keyword.
  • All methods of an interface must be defined as public.
  • An interface can extend or a class can implement multiple other interfaces.

Abstract class

  • Abstract class contains abstract and non-abstract methods.

  • Does not force users to implement all methods when inherited the abstract class.

  • Contains all kinds of variables including primitive and non-primitive

  • Declare using abstract keyword.

  • Methods and members of an abstract class can be defined with any visibility.

  • A child class can only extend a single class (abstract or concrete).

Answered   2023-09-20 21:01:03

I am 10 yrs late to the party but would like to attempt any way. Wrote a post about the same on medium few days back. Thought of posting it here.

tl;dr; When you see “Is A” relationship use inheritance/abstract class. when you see “has a” relationship create member variables. When you see “relies on external provider” implement (not inherit) an interface.

Interview Question: What is the difference between an interface and an abstract class? And how do you decide when to use what? I mostly get one or all of the below answers: Answer 1: You cannot create an object of abstract class and interfaces.

ZK (That’s my initials): You cannot create an object of either. So this is not a difference. This is a similarity between an interface and an abstract class. Counter Question: Why can’t you create an object of abstract class or interface?

Answer 2: Abstract classes can have a function body as partial/default implementation.

ZK: Counter Question: So if I change it to a pure abstract class, marking all the virtual functions as abstract and provide no default implementation for any virtual function. Would that make abstract classes and interfaces the same? And could they be used interchangeably after that?

Answer 3: Interfaces allow multi-inheritance and abstract classes don’t.

ZK: Counter Question: Do you really inherit from an interface? or do you just implement an interface and, inherit from an abstract class? What’s the difference between implementing and inheriting? These counter questions throw candidates off and make most scratch their heads or just pass to the next question. That makes me think people need help with these basic building blocks of Object-Oriented Programming. The answer to the original question and all the counter questions is found in the English language and the UML. You must know at least below to understand these two constructs better.

Common Noun: A common noun is a name given “in common” to things of the same class or kind. For e.g. fruits, animals, city, car etc.

Proper Noun: A proper noun is the name of an object, place or thing. Apple, Cat, New York, Honda Accord etc.

Car is a Common Noun. And Honda Accord is a Proper Noun, and probably a Composit Proper noun, a proper noun made using two nouns.

Coming to the UML Part. You should be familiar with below relationships:

  • Is A
  • Has A
  • Uses

Let’s consider the below two sentences. - HondaAccord Is A Car? - HondaAccord Has A Car?

Which one sounds correct? Plain English and comprehension. HondaAccord and Cars share an “Is A” relationship. Honda accord doesn’t have a car in it. It “is a” car. Honda Accord “has a” music player in it.

When two entities share the “Is A” relationship it’s a better candidate for inheritance. And Has a relationship is a better candidate for creating member variables. With this established our code looks like this:

abstract class Car
{
   string color;
   int speed;
}
class HondaAccord : Car
{
   MusicPlayer musicPlayer;
}

Now Honda doesn't manufacture music players. Or at least it’s not their main business.

So they reach out to other companies and sign a contract. If you receive power here and the output signal on these two wires it’ll play just fine on these speakers.

This makes Music Player a perfect candidate for an interface. You don’t care who provides support for it as long as the connections work just fine.

You can replace the MusicPlayer of LG with Sony or the other way. And it won’t change a thing in Honda Accord.

Why can’t you create an object of abstract classes?

Because you can’t walk into a showroom and say give me a car. You’ll have to provide a proper noun. What car? Probably a honda accord. And that’s when a sales agent could get you something.

Why can’t you create an object of an interface? Because you can’t walk into a showroom and say give me a contract of music player. It won’t help. Interfaces sit between consumers and providers just to facilitate an agreement. What will you do with a copy of the agreement? It won’t play music.

Why do interfaces allow multiple inheritance?

Interfaces are not inherited. Interfaces are implemented. The interface is a candidate for interaction with the external world. Honda Accord has an interface for refueling. It has interfaces for inflating tires. And the same hose that is used to inflate a football. So the new code will look like below:

abstract class Car
{
    string color;
    int speed;
}
class HondaAccord : Car, IInflateAir, IRefueling
{
    MusicPlayer musicPlayer;
}

And the English will read like this “Honda Accord is a Car that supports inflating tire and refueling”.

Answered   2023-09-20 21:01:03

  • although this answer has its own merit, but much of its conclusions are language dependent. For example, C++/hack allows multiple inheritance and distinct between interface and abstract classes only are by convention: one can replace all interfaces by put abstract classes - anyone

Key Points:

  • Abstract class can have property, Data fields ,Methods (complete / incomplete) both.
  • If method or Properties define in abstract keyword that must override in derived class.(its work as a tightly coupled functionality)
  • If define abstract keyword for method or properties in abstract class you can not define body of method and get/set value for properties and that must override in derived class.
  • Abstract class does not support multiple inheritance.
  • Abstract class contains Constructors.
  • An abstract class can contain access modifiers for the subs, functions, properties.
  • Only Complete Member of abstract class can be Static.
  • An interface can inherit from another interface only and cannot inherit from an abstract class, where as an abstract class can inherit from another abstract class or another interface.

Advantage:

  • It is a kind of contract that forces all the subclasses to carry on the same hierarchies or standards.
  • If various implementations are of the same kind and use common behavior or status then abstract class is better to use.
  • If we add a new method to an abstract class then we have the option of providing default implementation and therefore all the existing code might work properly.
  • Its allow fast execution than interface.(interface Requires more time to find the actual method in the corresponding classes.)
  • It can use for tight and loosely coupling.

find details here... http://pradeepatkari.wordpress.com/2014/11/20/interface-and-abstract-class-in-c-oops/

Answered   2023-09-20 21:01:03

The shortest way to sum it up is that an interface is:

  1. Fully abstract, apart from default and static methods; while it has definitions (method signatures + implementations) for default and static methods, it only has declarations (method signatures) for other methods.
  2. Subject to laxer rules than classes (a class can implement multiple interfaces, and an interface can inherit from multiple interfaces). All variables are implicitly constant, whether specified as public static final or not. All members are implicitly public, whether specified as such or not.
  3. Generally used as a guarantee that the implementing class will have the specified features and/or be compatible with any other class which implements the same interface.

Meanwhile, an abstract class is:

  1. Anywhere from fully abstract to fully implemented, with a tendency to have one or more abstract methods. Can contain both declarations and definitions, with declarations marked as abstract.
  2. A full-fledged class, and subject to the rules that govern other classes (can only inherit from one class), on the condition that it cannot be instantiated (because there's no guarantee that it's fully implemented). Can have non-constant member variables. Can implement member access control, restricting members as protected, private, or private package (unspecified).
  3. Generally used either to provide as much of the implementation as can be shared by multiple subclasses, or to provide as much of the implementation as the programmer is able to supply.

Or, if we want to boil it all down to a single sentence: An interface is what the implementing class has, but an abstract class is what the subclass is.

Answered   2023-09-20 21:01:03

Inheritance is used for two purposes:

  • To allow an object to regard parent-type data members and method implementations as its own.

  • To allow a reference to an objects of one type to be used by code which expects a reference to supertype object.

In languages/frameworks which support generalized multiple inheritance, there is often little need to classify a type as either being an "interface" or an "abstract class". Popular languages and frameworks, however, will allow a type to regard one other type's data members or method implementations as its own even though they allow a type to be substitutable for an arbitrary number of other types.

Abstract classes may have data members and method implementations, but can only be inherited by classes which don't inherit from any other classes. Interfaces put almost no restrictions on the types which implement them, but cannot include any data members or method implementations.

There are times when it's useful for types to be substitutable for many different things; there are other times when it's useful for objects to regard parent-type data members and method implementations as their own. Making a distinction between interfaces and abstract classes allows each of those abilities to be used in cases where it is most relevant.

Answered   2023-09-20 21:01:03

Differences between abstract class and interface on behalf of real implementation.

Interface: It is a keyword and it is used to define the template or blue print of an object and it forces all the sub classes would follow the same prototype,as for as implementation, all the sub classes are free to implement the functionality as per it's requirement.

Some of other use cases where we should use interface.

Communication between two external objects(Third party integration in our application) done through Interface here Interface works as Contract.

Abstract Class: Abstract,it is a keyword and when we use this keyword before any class then it becomes abstract class.It is mainly used when we need to define the template as well as some default functionality of an object that is followed by all the sub classes and this way it removes the redundant code and one more use cases where we can use abstract class, such as we want no other classes can directly instantiate an object of the class, only derived classes can use the functionality.

Example of Abstract Class:

 public abstract class DesireCar
  {

 //It is an abstract method that defines the prototype.
     public abstract void Color();

  // It is a default implementation of a Wheel method as all the desire cars have the same no. of wheels.   
 // and hence no need to define this in all the sub classes in this way it saves the code duplicasy     

  public void Wheel() {          

               Console.WriteLine("Car has four wheel");
                }
           }


    **Here is the sub classes:**

     public class DesireCar1 : DesireCar
        {
            public override void Color()
            {
                Console.WriteLine("This is a red color Desire car");
            }
        }

        public class DesireCar2 : DesireCar
        {
            public override void Color()
            {
                Console.WriteLine("This is a red white Desire car");
            }
        }

Example Of Interface:

  public interface IShape
        {
          // Defines the prototype(template) 
            void Draw();
        }


  // All the sub classes follow the same template but implementation can be different.

    public class Circle : IShape
    {
        public void Draw()
        {
            Console.WriteLine("This is a Circle");
        }
    }

    public class Rectangle : IShape
    {
        public void Draw()
        {
            Console.WriteLine("This is a Rectangle");
        }
    }

Answered   2023-09-20 21:01:03

The topic of abstract classes vs interfaces is mostly about semantics.

Abstract classes act in different programming languages often as a superset of interfaces, except one thing and that is, that you can implement multiple interfaces, but inherit only one class.

An interface defines what something must be able to do; like a contract, but does not provide an implementation of it.

An abstract class defines what something is and it commonly hosts shared code between the subclasses.

For example a Formatter should be able to format() something. The common semantics to describe something like that would be to create an interface IFormatter with a declaration of format() that acts like a contract. But IFormatter does not describe what something is, but just what it should be able to to. The common semantics to describe what something actually is, is to create a class. In this case we create an abstract class... So we create an abstract class Formatter which implements the interface. That is a very descriptive code, because we now know we have a Formatter and we now know what every Formatter must be able to do.

Also one very important topic is documentation (at least for some people...). In your documentation you probably want to explain within your subclasses what a Formatter actually is. It is very convenient to have an abstract class Formatter to which documentation you can link to within your subclasses. That is very convenient and generic. On the other hand if you do not have an abstract class Formatter and only an interface IFormatter you would have to explain in each of your subclasses what a Formatter actucally is, because an interface is a contract and you would not describe what a Formatter actually is within the documentation of an interface — at least it would be not something common to do and you would break the semantics that most developers consider to be correct.

Note: It is a very common pattern to make an abstract class implement an interface.

Answered   2023-09-20 21:01:03

I'd like to add one more difference which makes sense. For example, you have a framework with thousands of lines of code. Now if you want to add a new feature throughout the code using a method enhanceUI(), then it's better to add that method in abstract class rather in interface. Because, if you add this method in an interface then you should implement it in all the implemented class but it's not the case if you add the method in abstract class.

Answered   2023-09-20 21:01:03

To give a simple but clear answer, it helps to set the context : you use both when you do not want to provide full implementations.

The main difference then is an interface has no implementation at all (only methods without a body) while abstract classes can have members and methods with a body as well, i.e. can be partially implemented.

Answered   2023-09-20 21:01:03

  • Since you answered it just now, your answer does not take into account the default keyword in Java 8 with which you can define concrete methods in interfaces as well. - anyone
  • As I said this was meant to be a "simple but clear answer" for someone at the stage of learning what the difference is. For someone like that, it isn't of any benefit to know about that sort of exception, it would only be very confusing. - anyone

Similarities
Both forces classes extending or implementing them to override base methods.

Differences

  1. A class can implement multiple interfaces.
  2. A class can only extend from one abstract class.
  3. Fields declared in interfaces must be static and final because all objects that created from such implementation share same values.
  4. In Abstract classes, fields can be named and not assigned. subclasses can override them.

Usecases

  1. Abstract classes are used in subclasses that are closely related, or have almost same functionalities and behaviours.
  2. Interfaces are used for unrelated classes that you want to force a certain thing or behaviour, because its Just a contract without implementation.

Answered   2023-09-20 21:01:03

  • This is actually one of the most detailed and compact answers. Cheers! - anyone

An abstract class is a class whose object cannot be created or a class which cannot be instantiated. An abstract method makes a class abstract. An abstract class needs to be inherited in order to override the methods that are declared in the abstract class. No restriction on access specifiers. An abstract class can have constructor and other concrete(non abstarct methods ) methods in them but interface cannot have.

An interface is a blueprint/template of methods.(eg. A house on a paper is given(interface house) and different architects will use their ideas to build it(the classes of architects implementing the house interface) . It is a collection of abstract methods , default methods , static methods , final variables and nested classes. All members will be either final or public , protected and private access specifiers are not allowed.No object creation is allowed. A class has to be made in order to use the implementing interface and also to override the abstract method declared in the interface. An interface is a good example of loose coupling(dynamic polymorphism/dynamic binding) An interface implements polymorphism and abstraction.It tells what to do but how to do is defined by the implementing class. For Eg. There's a car company and it wants that some features to be same for all the car it is manufacturing so for that the company would be making an interface vehicle which will have those features and different classes of car(like Maruti Suzkhi , Maruti 800) will override those features(functions).

Why interface when we already have abstract class? Java supports only multilevel and hierarchal inheritance but with the help of interface we can implement multiple inheritance.

Answered   2023-09-20 21:01:03

In an interface all methods must be only definitions, not single one should be implemented.

But in an abstract class there must an abstract method with only definition, but other methods can be also in the abstract class with implementation...

Answered   2023-09-20 21:01:03