Abstract Class vs. Interface

Question

I have searched around SO as well as the rest of the web for a good answer but I have't found one that I really understand. I am going to present this in a different way and hopefully the answers will help others as well.

As far as I understand, the two concepts have the same rules except an abstract class is more flexible due to the method implementation ability. Also, I am aware you can implement multiple interfaces and only extend a single class but I'm sure there are more differences than the two I mentioned.

Please look at the two snippets of code and give me an example what I can do with each of my examples that would make me want or not want to use the other.

Abstract Class

abstract class Foo {
    abstract public function getValue();
    abstract public function setValue($value); 
}


class myObj extends Foo {
    function getValue() {

    }
    function setValue($value) {

    }
}

Interface

interface Foo {
    public function getValue();
    public function setValue($value);
}

class myObj implements Foo {
    function getValue() {

    }
    function setValue($value) {

    }
}

Solution 1

To resume the idea (globally, not in detail):

inheritance

is the notion to extend from something, and optionally add some new feature or override some existing feature (to do differently). But using inheritance, you share a big part of code with the parent. You are a parent + some other things.

interface

is representing some abilities (we says a class is implementing an interface to says that it has these abilities). An interface can be implemented by 2 classes which are completely different and do not share their code (except for methods they implements). When A and B are implementing interface C, A is not a B and B is not a A.

And one of the reason for interface is indeed to allow programmer to do the same as they could do with multi-inheritance, but without multi-inheritance problems.

This notion is used in some programming languages like JAVA, PHP...

Solution 2

Abstract

Abstract Classes focus on a kind of things similarity.

People are considered of type mammal and as such would not be considered of type vehicle.

Interface

Interfaces focus on collation of similar function.

For example: You are a human being and are of type mammal. If you want to fly then you will need to implement a flying Interface. If you want to shoot while flying, then you also need to implement the gun Interface.

See the examples below:

abstract class Mammal {
      protected $age_;
      //below are functions I think all mammals will have,including people
      abstract public function setAge($age);
      abstract public function getAge();
      abstract public function eat($food);
}
class Person extends Mammal {
      protected $job_; //Person's feature
      public function setAge($age){
        $this->age_ = $age;
      }

      public function getAge(){
        return $this->age_;
      }

      public function eat($food){
        echo 'I eat ' ,$food ,'today';
      }

      //People only attribute
      public function setJob($job){
         $this->job_ = $job;
      }
      public function getJob(){
         echo 'My job is ' , $this->job_;
      }

}

//Now a person wants to fly, but they are typically not able to do so.
//So we implement an interface
interface Plane{
  public function Fly(); 
}

//I also want shoot enemy
interface Gun{
  public function shoot();
}

class Person2 extends Mammal implements Plane,Gun{

      protected $job_;//Person feature
      public function setAge($age){
        $this->age_ = $age;
      }
      public function getAge(){
        return $this->age_;
      }
      public function eat($food){
        echo '<br/>I eat ' ,$food ,' today<br/>';
      }
      //Only a person has this feature.
      public function setJob($job){
         $this->job_ = $job;
      }
      public function getJob(){
         echo 'My job is ' , $this->job_;
      }

      //-----------------------------------------
      //below implementations from interfaces function. (features that humans do not have).
      //Person implements from other class
      public function fly(){
        echo '<br/>I use plane,so I can fly<br/>';
      }
      public function shoot(){
        echo 'I use gun,so I can shoot<br/>';
      }
}

$People = new Person();
echo '<pre>';
print_r( get_class_methods('People'));
echo '</pre>';

echo '<pre>';
print_r( get_class_methods('People2'));
echo '</pre>';

$People2 = new Person2();
$People2->setAge(24);
echo $People2->getAge();
$People2->eat('egg');
$People2->setJob('PHP devepop');
echo $People2->getJob();

$People2->fly();
$People2->shoot();

Solution 3

I have thought about this before, and the best I could conclude was that interfaces are a logically convenient abstraction of a pure abstract class (c++).

As for why you would choose interfaces over abstract classes, I quote (a c++ source but the concepts are the same):

Note that there is a great temptation to add concrete member functions and data to pure abstract base classes. This must be resisted, in general it is a sign that the interface is not well factored. Data and concrete member functions tend to imply a particular implementation and as such can inherit from the interface but should not be that interface. Instead if there is some commonality between concrete classes, creation of abstract class which inherits its interface from the pure abstract class and defines the common data and member functions of the concrete classes works well.

The thing is, when using interfaces, the first thing that comes to mind is decoupling. When using an interface, the user and the implementing-class are totally decoupled. The same applies for when you're using a pure abstract class which is basically an interface.