[oop] What is the difference between an interface and abstract class?

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

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?.

An interface is so called because it provides an interface of methods to a caller (or a COM client for instance) that are implemented by some class. By polymorphically casting an object pointer to the type of an interface that the object's class implements, it restricts the access of the object to functions and members of the interface that it implements, separated from other COM interfaces the coclass might implement. The client does not need to know what class implements the interface or what other methods are present in the class; the object presents as an instance of the interface it knows (where the instance of the class has been polymorphically cast to the interface subinstance) and it just uses the interface by calling the methods of the interface on the interface instance. All details of the actual implementation and extraneous functionality / details implemented by different interfaces are separated from the interface the caller expects -- the caller just uses the interface it has with the object, and the underlying object implementation is called without the caller having to know the location or the details of the implementation. Accessing the object through the interface (a pointer of the type of the interface) is a form of encapsulation that syntactically prevents unauthorised access to the object as well as hiding implementation details and other functionality that does not pertain to the interface and its defined personality.

An interface is where all methods are virtual and abstract (abstract is known as pure virtual in C++; all abstract methods contain the virtual specifier and therefore are virtual). An abstract class is where at least one of the methods is virtual and specified as abstract (or pure virtual in C++). Other details differ across languages. All interface attributes are implicitly public static final in java but they aren't in C++. Java allows non-static attributes in abstract classes but C++ allows them in both. Attributes cannot be virtual / abstract in either language.

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.

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”.

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, ...

usually Abstract class used for core of something but interface used for appending peripheral.

when you want to create base type for vehicle you should use abstract class but if you want to add some functionality or property that is not part of base concept of vehicle you should use interface,for example you want to add "ToJSON()" function.

interface has wide range of abstraction rather than abstract class. you can see this in passing arguments.look this example:

if you use vehicle as argument you just can use one of its derived type (bus or car-same category-just vehicle category). but when you use IMoveable interface as argument you have more choices.

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.

The general idea of abstract classes and interfaces is to be extended/implemented by other classes (cannot be constructed alone) that use these general "settings" (some kind of a template), making it simple to set a specific-general behaviour for all the objects that later extend it.

An abstract class has regular methods set AND abstract methods. Extended classes can include unset methods after being extended by an abstract class. When setting abstract methods - they are defined by the classes that are extending it later.

Interfaces have the same properties as an abstract class, but includes only abstract methods, which could be implemented in an other class/es (and can be more than one interface to implement), this creates a more permanent-solid definishion of methods/static variables. Unlike the abstract class, you cannot add custom "regular" methods.

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.

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");
        }
    }

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.

In practicality terms(JAVA), the major difference between abstract class and interface is Abstract class can hold state. Other than holding state we can achieve rest operations with Interface also.

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

A simple yet effective explanation of abstract class and interface on php.net:

An Interface is like a protocol. It doesn't designate the behavior of the object; it designates how your code tells that object to act. An interface would be like the English Language: defining an interface defines how your code communicates with any object implementing that interface.

An interface is always an agreement or a promise. When a class says "I implement interface Y", it is saying "I promise to have the same public methods that any object with interface Y has".

On the other hand, an Abstract Class is like a partially built class. It is much like a document with blanks to fill in. It might be using English, but that isn't as important as the fact that some of the document is already written.

An abstract class is the foundation for another object. When a class says "I extend abstract class Y", it is saying "I use some methods or properties already defined in this other class named Y".

So, consider the following PHP:

<?php
class X implements Y { } // this is saying that "X" agrees to speak language "Y" with your code.

class X extends Y { } // this is saying that "X" is going to complete the partial class "Y".
?>

You would have your class implement a particular interface if you were distributing a class to be used by other people. The interface is an agreement to have a specific set of public methods for your class.

You would have your class extend an abstract class if you (or someone else) wrote a class that already had some methods written that you want to use in your new class.

These concepts, while easy to confuse, are specifically different and distinct. For all intents and purposes, if you're the only user of any of your classes, you don't need to implement interfaces.

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.

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.

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

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.

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

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.

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).

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.

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.

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/

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..

Here is a very basic understanding over interface vs abstract class.

Some important differences:

In the form of a table:

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.

We have various structural/syntactical difference between interface and abstract class. Some more differences are

[1] Scenario based difference:

Abstract classes are used in scenarios when we want to restrict the user to create object of parent class AND we believe there will be more abstract methods will be added in future.

Interface has to be used when we are sure there can be no more abstract method left to be provided. Then only an interface is published.

[2] Conceptual difference:

"Do we need to provide more abstract methods in future" if YES make it abstract class and if NO make it Interface.

(Most appropriate and valid till java 1.7)

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...

Interfaces are generally the classes without logic just a signature. Whereas abstract classes are those having logic. Both support contract as interface all method should be implemented in the child class but in abstract only the abstract method should be implemented. When to use interface and when to abstract? Why use Interface?

class Circle {

protected $radius;

public function __construct($radius)

{
    $this->radius = $radius
}

public function area()
{
    return 3.14159 * pow(2,$this->radius); // simply pie.r2 (square);
}

}

//Our area calculator class would look like

class Areacalculator {

$protected $circle;

public function __construct(Circle $circle)
{
    $this->circle = $circle;
}

public function areaCalculate()
{
    return $circle->area(); //returns the circle area now
}

}

We would simply do

$areacalculator = new Areacalculator(new Circle(7)); 

Few days later we would need the area of rectangle, Square, Quadrilateral and so on. If so do we have to change the code every time and check if the instance is of square or circle or rectangle? Now what OCP says is CODE TO AN INTERFACE NOT AN IMPLEMENTATION. Solution would be:

Interface Shape {

public function area(); //Defining contract for the classes

}

Class Square implements Shape {

$protected length;

public function __construct($length) {
    //settter for length like we did on circle class
}

public function area()
{
    //return l square for area of square
}

Class Rectangle implements Shape {

$protected length;
$protected breath;

public function __construct($length,$breath) {
    //settter for length, breath like we did on circle,square class
}

public function area()
{
    //return l*b for area of rectangle
}

}

Now for area calculator

class Areacalculator {

$protected $shape;

public function __construct(Shape $shape)
{
    $this->shape = $shape;
}

public function areaCalculate()
{
    return $shape->area(); //returns the circle area now
}

}

$areacalculator = new Areacalculator(new Square(1));
$areacalculator->areaCalculate();

$areacalculator = new Areacalculator(new Rectangle(1,2));
$areacalculator->;areaCalculate();

Isn't that more flexible? If we would code without interface we would check the instance for each shape redundant code.

Now when to use abstract?

Abstract Animal {

public function breathe(){

//all animals breathe inhaling o2 and exhaling co2

}

public function hungry() {

//every animals do feel hungry 

}

abstract function communicate(); 
// different communication style some bark, some meow, human talks etc

}

Now abstract should be used when one doesn't need instance of that class, having similar logic, having need for the contract.

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).

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.

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.

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.

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.

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 behaviour among multiple related objects

From the Oracle documentation

Consider using abstract classes if :

1. You want to share code among several closely related classes.
2. You expect that classes that extend your abstract class have many common methods or fields, or require access modifiers other than public (such as protected and private).
3. You want to declare non-static or non-final fields.

Consider using interfaces if :

1. You expect that unrelated classes would implement your interface. For example,many unrelated objects can implement Serializable interface.
2. You want to specify the behaviour of a particular data type, but not concerned about who implements its behaviour.
3. You want to take advantage of multiple inheritance of type.

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.

Refer to this documentation page for more details.

Have a look at this SE question for code examples to understand better.

How should I have explained the difference between an Interface and an Abstract class?

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.