The object-oriented paradigm encourages encapsulation. Encapsulation is used to hide the mechanics of the object, allowing the actual implementation of the object to be hidden, so that we don't need to understand how the object works. All we need to understand is the interface that is provided for us.

You can think of this in the case of the Television class, where the functionality of the television is hidden from us, but we are provided with a remote control, or set of controls for interacting with the television, providing a high level of abstraction. So, as in Figure 1.4, “The Television interface example.” there is no requirement to understand how the signal is decoded from the aerial and converted into a picture to be displayed on the screen before you can use the television.

There is a sub-set of functionality that the user is allowed to call, termed the interface. In the case of the television, this would be the functionality that we could use through the remote control or buttons on the front of the television.

The full implemenation of a class is the sum of the public interface plus the private implementation.

Figure 1.4. The Television interface example.

Encapsulation is the term used to describe the way that the interface is separated from the implementation. You can think of encapsulation as "data-hiding", allowing certain parts of an object to be visible, while other parts remain hidden. This has advantages for both the user and the programmer.

For the user (who could be another programmer):

For the programmer:

So, providing the programmer does not change the interface in any way, the user will be unaware of any changes, except maybe a minor change in the actual functionality of the application.

We can identify a level of 'hiding' of particular methods or states within a class using the public, private and protected keywords:

Figure 1.5, “The Television class example showing encapsulation.” shows encapsulation as it relates to the Television class. According to UML notation private methods are denoted with a minus sign and public methods are denoted with a plus sign. The private methods would be methods written that are part of the inner workings of the television, but need not be understood by the user. For example, the user would need to call the powerOn() method but the private displayPicture() method would also be called, but internally as required, not directly by the user. This method is therefore not added to the interface, but hidden internally in the implementation by using the private keyword.

Figure 1.5. The Television class example showing encapsulation.

If we have several descriptions with some commonality between these descriptions, we can group the descriptions and their commonality using inheritance to provide a compact representation of these descriptions. The object-oriented programming approach allows us to group the commonalities and create classes that can describe their differences from other classes.

Humans use this concept in categorising objects and descriptions. For example you may have answered the question - "What is a duck?", with "a bird that swims", or even more accurately, "a bird that swims, with webbed feet, and a bill instead of a beak". So we could say that a Duck is a Bird that swims, so we could describe this as in Figure 1.6, “The Duck class showing inheritance.”. This figure illustrates the inheritance relationship between a Duck and a Bird. In effect we can say that a Duck is a special type of Bird.

Figure 1.6. The Duck class showing inheritance.

For example: if were to be given an unstructured group of descriptions such as Car, Saloon, Estate, Van, Vehicle, Motorbike and Scooter, and asked to organise these descriptions by their differences. You might say that a Saloon car is a Car but has a long boot, whereas an Estate car is a car with a very large boot. Figure 1.7, “The grouped set of classes.” shows an example of how we may organise these descriptions using inheritance.

Figure 1.7. The grouped set of classes.

So we can describe this relationship as a child/parent relationship, where Figure 1.8, “The Base class and Derived class.” illustrates the relationship between a base class and a derived class. A derived class inherits from a base class, so in Figure 1.7, “The grouped set of classes.” the Car class is a child of the Vehicle class, so Car inherits from Vehicle.

Figure 1.8. The Base class and Derived class.

One way to determine that you have organised your classes correctly is to check them using the "IS-A" and "IS-A-PART-OF" relationship checks. It is easy to confuse objects within a class and children of classes when you first begin programming with an OOP methodology. So, to check the previous relationship between Car and Vehicle, we can see this in Figure 1.9, “The IS-A/IS-A-PART-OF relationships and the Vehicle class.”.

Figure 1.9. The IS-A/IS-A-PART-OF relationships and the Vehicle class.

The IS-A relationship describes the inheritance in the figure, where we can say, "A Car IS-A Vehicle" and "A SaloonCar IS-A Car", so all relationships are correct. The IS-A-PART-OF relationship describes the composition (or aggregation) of a class. So in the same figure (Figure 1.9, “The IS-A/IS-A-PART-OF relationships and the Vehicle class.”) we can say "An Engine IS-A-PART-OF a Vehicle", or "An Engine, Colour and Wheels IS-A-PART-OF a Vehicle". This is the case even though an Engine is also a class! where there could be many different descriptions of an Engine - petrol, diesel, 1.4, 2.0, 16 valve etc.

So, using inheritance the programmer can:

When a class inherits from another class it inherits both the states and methods of that class, so in the case of the Car class inheriting from the Vehicle class the Car class inherits the methods of the Vehicle class, such as engineStart(), gearChange(), lightsOn() etc. The Car class will also inherit the states of the Vehicle class, such as isEngineOn, isLightsOn, numberWheels etc.

Polymorphism means "multiple forms". In OOP these multiple forms refer to multiple forms of the same method, where the exact same method name can be used in different classes, or the same method name can be used in the same class with slightly different paramaters. There are two forms of polymorphism, over-riding and over-loading.

Over-Riding

As discussed, a derived class inherits its methods from the base class. It may be necessary to redefine an inherited method to provide specific behaviour for a derived class - and so alter the implementation. So, over-riding is the term used to describe the situation where the same method name is called on two different objects and each object responds differently.

Over-riding allows different kinds of objects that share a common behaviour to be used in code that only requires that common behaviour.

Figure 1.10. The over-ridden draw() method.

Consider the previous example of the Vehicle class diagram in Figure 1.7, “The grouped set of classes.”. In this case Car inherits from Vehicle and from this class Car there are further derived classes SaloonCar and EstateCar. If a draw() method is added to the Car class, that is required to draw a picture of a generic vehicle. This method will not adequately draw an estate car, or other child classes. Over-Riding allows us to write a specialised draw() method for the EstateCar class - There is no need to write a new draw() method for the SaloonCar class as the Car class provides a suitable enough draw() method. All we have to do is write a new draw() method in the EstateCar class with the exact same method name. So, Over-Riding allows:

• A more straightforward API where we can call methods the same name, even thought these methods have slightly different functionality.

• A better level of abstraction, in that the implementation mechanics remain hidden.

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Abstract Classes

An abstract class is a class that is incomplete, in that it describes a set of operations, but is missing the actual implementation of these operations. Abstract classes:

• Cannot be instantiated.

• So, can only be used through inheritance.

For example: In the Vehicle class example previously the draw() method may be defined as abstract as it is not really possible to draw a generic vehicle. By doing this we are forcing all derived classes to write a draw() method if they are to be instantiated.

As discussed previously, a class is like a set of plans from which you can create objects. In relation to this analogy, an abstract class is like a set of plans with some part of the plans missing. E.g. it could be a car with no engine - you would not be able to make complete car objects without the missing parts of the plan.

Figure 1.11. The abstract draw() method in the Vehicle class.

Figure 1.11, “The abstract draw() method in the Vehicle class.” illustrates this example. The draw() has been written in all of the classes and has some functionality. The draw() in the Vehicle has been tagged as abstract and so this class cannot be instantiated - i.e. we cannot create an object of the Vehicle class, as it is incomplete. In Figure 1.11, “The abstract draw() method in the Vehicle class.” the SaloonCar has no draw() method, but it does inherit a draw() method from the parent Car class. Therefore, it is possible to create objects of SaloonCar.

If we required we could also tag the draw() method as abstract in a derived class, for example we could also have tagged the draw() as abstract in the Car class. This would mean that you could not create an object of the Car class and would pass on responsibility for implementing the draw() method to its children - see Figure 1.12, “The abstract draw() method in the Vehicle and Car classes.”

Figure 1.12. The abstract draw() method in the Vehicle and Car classes.