Interfaces in Java

Question

I really don't understand interfaces and when to use one. Can you explain it to me more simple and clear. Thanks.

Answer 1 · 2016-09-07T22:07:56Z

September 7, 2016 10:07pm

Let's say you have a game with different types of Entities.

You have an Entity interface that is used as a sort of foundation for each entity (obviously).

public interface Entity {
    public String getName();

    public int getHealth();
}

Now lets say you want to have a Zombie class as well as a Skeleton class. You would implement the Entity class, and take in all the methods from it.

public class Zombie implements Entity {
    private String name = "Zombie";    
    private int health = 16;

    public String getName() {
        return this.name;
    }

    public int getHealth() {
        return this.health;
    }
}

public class Skeleton implements Entity {
    private String name = "Skeleton";    
    private int health = 12;

    public String getName() {
        return this.name;
    }

    public int getHealth() {
        return this.health;
    }
}

That's all well and good, but if we have two classes that are identical other than the variables within them, why do we need an interface? Good question -- you might not, it depends on how you plan on using these classes.

Let's say that you don't actually know whether or not the entity will be a skeleton or a zombie. You could use the Entity interface inside of methods to take in any kind of Entity.

// this method will take in any class that implemented the Entity interface
public void doSomething(Entity entity) {
    if (entity instanceof Zombie) {
        System.out.println("Entity is a zombie!");
        Zombie zombie = (Zombie) entity; // easy casting Zombie to the entity variable
        // casting essentially just changes the variable's type
    }

    if (entity instanceof Skeleton) {
        System.out.println("Entity is a skeleton!");
        Skeleton skeleton = (Skeleton) entity; // easy casting Skeleton to the entity variable
    }

    /*
     * Let's say that the entity is a zombie. Since it's actual class is of the Zombie type, you would be
     * able to access any of the Zombie classes methods using the entity interface
     */
    entity.getName(); // this would return "Zombie" since it was defined in the Zombie class.
}

// example of the doSomething() method && creating a new instance of each class
Entity entity = new Zombie();

doSomething(entity);
doSomething(new Skeleton());

What if the entity class had a method that was implemented differently in both the Skeleton and Zombie class?

// for example, the skeleton's getHealth() method is implemented like this
public void getHealth() {
    Random random = new Random();
    return random.nextInt(); // returns a random integer value
}

// and this is how the zombie's getHealth() method is implemented
public void getHealth() {
    return this.health;
}

/*
 * There are three variable a Zombie variable, Skeleton Variable, and Entity variable that we don't know
 * which type it is.
 */
skeleton.getHealth(); // would return a random integer value
zombie.getHealth(); // would return 16 since we defined it in the class
entity.getHealth(); // would either return 16 or a random integer value depending on how the "entity" variable was initialized

Answer 2 · 2016-09-06T08:42:48Z

September 6, 2016 8:42am

I've spent a lot of time looking into this. I'm not sure I can tell you precisely what they are. It's more like a framework that you can use, built on classes.

We're taught to think of interfaces as like a "contract" that you sign a a programmer and then you do things in your script that deliver on that contract. You do this by running on default methods that java provides.

For example if you "implement" a Comparable interface on a class you'd use a compareTo() method.

public class NameOfClass implements Comparable {

}

This should point you in the right directio of what's out there in terms of interfaces. http://docs.oracle.com/javase/8/docs/api/

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Erwin Paolo Yumul

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Interfaces in Java

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