Using Interfaces to reduce coupling

Consider the following three scenarios:

1. class Dog {
   ...
   public function Bark($str) {
      echo $str;
   }
   ... }
   
   

class AnimalCommunication { … public function DogBark(Dog $dog, $str) { $dog->Bark($str); } … }

2. class Dog {
   ...
   public function Bark($str) {
      echo $str;
   }
   ... }
   
   
   

class AnimalCommunication { … public function AnimalCommunicate($str) { echo $str; } … }

3. Interface IAnimal {
   public function Speak($str); }
   
   

Class Dog implements IAnimal { … public function Speak($str) { $this->Bark($str); } …. } class AnimalCommunication { … public function AnimalCommunicate(IAnimal $animal, $str) { $animal->speak($str); } … }

Note the following about these examples:

1. The first is tightly coupled to the dog class, and hence is not very extensible.
2. The second is coupled to echo. It does not allow different “animals” to output their text differently. What if we later added a TelepathicGecko class, which didn’t echo out to speak, but rather published to some ESP API somewhere? Clearly the coupling here is not ideal either.
3. The third is best. By programming to an interface, we reduce coupling to the minimum necessary amount.