Why Do We Need an Interface in OOP?

Most of us feel confused about the usefulness of interfaces at a first glance. We have questions like “Why do we need interfaces?" and “What is the advantage of using interfaces?" In this blog, we will try to find the answers to these questions.

Let’s begin with a simple example. Assume that you are a student and need to prepare for an exam. You know that there will be distractions during your exam preparation. We will use mobile applications and friends as our example. Let’s describe each as a class:

class Friend {

    public void askHelp(){
        System.out.println("I will do my best to help you!");
    }
}

class MobileApplication {

    public void installApp(){
        System.out.println("The installation is complete.");
    }
}

I think most would agree with me that not every friend and mobile application can be a distraction. For example, we can have hardworking friends. Or, we can install useful mobile applications on our smartphones. So, adding the distract() method to these classes is not good. That is why we need to describe these distractions in greater detail:

class AdventureLoverFriend extends Friend{
   public void getAdviceAboutTravel(){
        System.out.println("I will help you travel better :)");
   }
}

class Facebook extends MobileApplication{
    public void connectPeople(){
        System.out.println("Stay in touch with friends from all over the world!");
    }
}

You know that your upcoming exam is going to be very difficult and you don’t have much time to prepare. This is why you decided to print a list of distractions with headers like “SAY NO!” and stick them on the wall in your room. How can you achieve this?

Well, you can think about inheritance. Because if we create a superclass named Distraction, and both the Facebook class and AdventureLoverFriend class extend it, we can collect all distractable things in one list. Because we can refer to the subclass object with the superclass reference variable, we can conduct the needed operation on this list. But Facebook and AdventureLoverFriend cannot extend the Distraction class. This is because, in Java, one class cannot extend more than one class.

At this time, we can see how the interface is useful. Let’s create an interface named Distractable to further demonstrate this:

interface Distractable{
    void distract();
}

And then, let's implement it as follows:

class AdventureLoverFriend extends Friend implements Distractable{

    public void getAdviceAboutTravel(){
        System.out.println("I will help you travel better :)");
    }

    @Override
    public void distract() {
        System.out.println("I’m having a party this weekend and would love for you to come ^_^");
    }
}

class Facebook extends MobileApplication implements Distractable{

    public void connectPeople(){
        System.out.println("Stay in touch with friends from all over the world!");
    }

    @Override
    public void distract() {
        System.out.println("Go through your entire Facebook news feeds again and again :/");
    }
}

As you can see, the interfaces allow us to define common behavior that can be implemented by any class, regardless of its inheritance. Although the AdventureLoverFriend class extends the Friend class and the Facebook class extends the MobileApplication class, we can add common distractable behavior to them by implementing the Distractable interface. This means that we can “cut across” the inheritance hierarchy to implement functionality as we see fit.

Since Java allows us to refer implementation class objects with interface reference variable, we can write the following in theExamPreparation class:

class ExamPreparation {
    public static void main(String[] args) {
        List<Distractable> distractableList = getListOfDistractableThings();

        System.out.println("\t\t\t\t\t\t  SAY NO! ");
        printList(distractableList);
    }

    public static void printList(List<Distractable> distractableList){
        for(Distractable distractableThing: distractableList){
            distractableThing.distract();
        }
    }

    public static List<Distractable> getListOfDistractableThings(){
        List<Distractable> distractables = new ArrayList<>();

        Distractable facebook = new Facebook();
        distractables.add(facebook);
        Distractable adventureLoverFriend = new AdventureLoverFriend();
        distractables.add(adventureLoverFriend);

        return distractables;
    }
}

So, we print the list as we want:

                         SAY NO!
Go through your entire Facebook news feeds again and again :/
I’m having a party this weekend and would love for you to come ^_^

Also, consider that we only focus on common distractable behavior in the printList method. We don’t care about other behaviors because we look at them as a Distractable object, unlike the Facebook or AdventureLoverFriend object in the printList method. We can show it like this in the code:

Distractable facebook = new Facebook();
facebook.connectPeople(); //doesn't compile

There are many other reasons why we need interfaces in Java. In this post, I tried to explain one of the more important concepts. Hope it was helpful!