What does "common interface" mean in OOP?

Question

I have seen the term "common interface" used a lot while reading books about OOP.

For example, the book The Essence of Object-Oriented Programming with Java and UML says the following:

Abstract classes usually define a common interface for subclasses by specifying methods that all subclasses must override and define

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My understanding of the term "common interface" is the following:

Assume that we have a superclass (or an interface or an abstract class) called Animal and two subclasses called Dog and Cat, and Animal have two virtual methods called makeSound() and move().

Now the common interface would be composed of two methods which are Animal.makeSound() and Animal.move().

Assume that we have the following code:

Animal animal1 = new Dog();
animal1.makeSound();
animal1.move();

animal1 = new Cat();
animal1.makeSound();
animal1.move();

The explanation of the above code is the following:

Animal animal1 = new Dog() creates an Animal common interface and associate a Dog object with it:

animal1.makeSound() sends an Animal.makeSound() message to the common interface, and then the common interface sends a Dog.makeSound() message to the Dog object:

Same thing happens in the case of animal1.move() (which is the Animal.move() message is sent to the common interface, etc.).

animal1 = new Cat() removes the Dog object from the common interface, and associate a Cat object with the common interface:

animal1.makeSound() sends an Animal.makeSound() message to the common interface, and then the common interface sends a Cat.makeSound() message to the Cat object:

Same thing happens in the case of animal1.move() (which is the Animal.move() message is sent to the common interface, etc.).

Am I correct in my understanding?

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Edit:

The Animal common interface in my diagram is not an object, but rather it is the virtual methods (or I guess I can call them the "message handlers") that receive the Animal.makeSound() and Animal.move() messages and then send the appropriate messages to the associated subclass object.

Answer 1

An interface is not a separate entity or object - an interface is just a characteristic (or set of characteristics) of an object. Specifically characteristics which describe how to use/interact with an object.

The term "common interface" is a way of describing the fact that Dog and Cat happen to share some of the same characteristics, and that these shared characteristics can allow these to be used interchangeably in code. (provided that there are no Dog-specific or Cat-specific characteristics involved).

To shift to another very simple example, most programming languages have data types called int and double - these nearly always share a common interface because they can both be used with the 'BODMAS' arithmetic operators despite the fact that integer representation and floating-point representation work very differently to each other.

In the given example of animal1.move(), animal1 is just a plain variable (i.e. a name) - it has no behaviour of its own. It's a simple programming language construct which acts as a name/alias for an object, and which enables a programmer to write code that uses/interacts with an object

• if the animal1 variable is an alias to a Dog object, then animal1.move() sends a move() message directly to that Dog object.
• if the animal1 variable is an alias to a Cat object, then animal1.move() sends a move() message directly to that Cat object.

A better analogy for the term interface could be when thinking about how a human interacts with a car while they are sitting in the driver's seat. From a driver's point of view, all cars have a common interface for someone driving the car:

• A steering wheel
• Accelerator pedal
• Brake pedal
• Hazard Lights
• Indicators

These are all characteristics of any car, however, any individual car will always have their own Steering wheel, pedals, lights, etc.

The term "common interface" is conceptual rather than physical (the interface isn't separate from the object, it is part of the object) - the common interface of a car means that any qualified driver of one car is able to seamlessly take control over any other car without needing to adapt their understanding of how to drive. The rules and actions involved in driving one car do not change when switching to a different car.

Answer 2

Conceptually Correct - at a very High Level.

Technically incorrect though. The message isn't sent to an Interface and then on forwarded to the instance.

What actually happens is that Animal establishes a protocol of behaviours. This protocol is filled in by implementations such as Dog, and this protocol is then used by callers to find the correct behaviour for the given implementation.

As such Dog implements its version of makeSound to make a barking noise. It publishes this behaviour as appropriate to the Animal protocol so that users of Animal can find this barking version of makeSound.

When the caller holding Animal a then makes a call a.makeSound(). The caller knowing about Animals follows the protocol and gets the behaviour to call. In this case its Dog.makeSound(). It then calls this Dog implementation directly.