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Interfaces and polymorphism
In my quest to understand the interface, the diamond problem explanation made some sense to me, but it didn't really satisfy me. Sure, the interface represented Java's way of dealing with the diamond problem, but was that the key insight into the interface? And how did this explanation help me understand how to use interfaces in my programs and designs?

As time went by I began to believe that the key insight into the interface was not so much about multiple inheritance as it was about polymorphism (see the explanation of this term below). The interface lets you take greater advantage of polymorphism in your designs, which in turn helps you make your software more flexible.

Ultimately, I decided that the "point" of the interface was:

Java's interface gives you more polymorphism than you can get with singly inherited families of classes, without the "burden" of multiple inheritance of implementation.

A refresher on polymorphism
This section will present a quick refresher on the meaning of polymorphism. If you are already comfortable with this fancy word, feel free to skip to the next section, "Getting more polymorphism."

Polymorphism means using a superclass variable to refer to a subclass object. For example, consider this simple inheritance hierarchy and code:

abstract class Animal {

    abstract void talk();
}

class Dog extends Animal {

    void talk() {
        System.out.println("Woof!");
    }
}

class Cat extends Animal {

    void talk() {
        System.out.println("Meow.");
    }
}

Given this inheritance hierarchy, polymorphism allows you to hold a reference to a Dog object in a variable of type Animal, as in:

Animal animal = new Dog();

The word polymorphism is based on Greek roots that mean "many shapes." Here, a class has many forms: that of the class and any of its subclasses. An Animal, for example, can look like a Dog or a Cat or any other subclass of Animal.

Polymorphism in Java is made possible by dynamic binding, the mechanism by which the Java virtual machine (JVM) selects a method implementation to invoke based on the method descriptor (the method's name and the number and types of its arguments) and the class of the object upon which the method was invoked. For example, the makeItTalk() method shown below accepts an Animal reference as a parameter and invokes talk() on that reference:

class Interrogator {

    static void makeItTalk(Animal subject) {
        subject.talk();
    }
}

At compile time, the compiler doesn't know exactly which class of object will be passed to makeItTalk() at runtime. It only knows that the object will be some subclass of Animal. Furthermore, the compiler doesn't know exactly which implementation of talk() should be invoked at runtime.

As mentioned above, dynamic binding means the JVM will decide at runtime which method to invoke based on the class of the object. If the object is a Dog, the JVM will invoke Dog's implementation of the method, which says, "Woof!". If the object is a Cat, the JVM will invoke Cat's implementation of the method, which says, "Meow!". Dynamic binding is the mechanism that makes polymorphism, the "subsitutability" of a subclass for a superclass, possible.

Polymorphism helps make programs more flexible, because at some future time, you can add another subclass to the Animal family, and the makeItTalk() method will still work. If, for example, you later add a Bird class:

class Bird extends Animal {

    void talk() {

        System.out.println("Tweet, tweet!");
    }
}

you can pass a Bird object to the unchanged makeItTalk() method, and it will say, "Tweet, tweet!".

Getting more polymorphism
Interfaces give you more polymorphism than singly inherited families of classes, because with interfaces you don't have to make everything fit into one family of classes. For example:

interface Talkative {

    void talk();
}

abstract class Animal implements Talkative {

    abstract public void talk();
}

class Dog extends Animal {

    public void talk() {
        System.out.println("Woof!");
    }
}

class Cat extends Animal {

     public void talk() {
        System.out.println("Meow.");
    }
}

class Interrogator {

    static void makeItTalk(Talkative subject) {
        subject.talk();
    }
}

Given this set of classes and interfaces, later you can add a new class to a completely different family of classes and still pass instances of the new class to makeItTalk(). For example, imagine you add a new CuckooClock class to an already existing Clock family:

class Clock {
}

class CuckooClock implements Talkative {

    public void talk() {
        System.out.println("Cuckoo, cuckoo!");
    }
}

Because CuckooClock implements the Talkative interface, you can pass a CuckooClock object to the makeItTalk() method:

class Example4 {

    public static void main(String[] args) {
        CuckooClock cc = new CuckooClock();
        Interrogator.makeItTalk(cc);
    }
}

With single inheritance only, you'd either have to somehow fit CuckooClock into the Animal family, or not use polymorphism. With interfaces, any class in any family can implement Talkative and be passed to makeItTalk(). This is why I say interfaces give you more polymorphism than you can get with singly inherited families of classes.