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
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.
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):
The user need only understand the interface.
The user need not understand how the implementation works or was created.
For the programmer:
The programmer can change the implementation, but need not notify the user.
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 methods - describe the interface.
private methods - describe the implementation.
Figure 1.5, “The
Television class example showing encapsulation.” shows encapsulation as it relates to the
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
powerOn() method but the private
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
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
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.
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
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
Vehicle, we can see this in Figure 1.9, “The IS-A/IS-A-PART-OF relationships and the
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:
Inherit a behaviour and add further specialised behaviour - for example a
Car IS A
Vehicle with the addition of four
Wheel objects, Seats etc.
Inherit a behaviour and replace it - for example the
class will inherit from
Car and provide a new "boot" implementation.
Cut down on the amount of code that needs to be written and debugged - for example
in this case only the differences are detailed, a
SaloonCar is essentially
identical to the
Car, with only the differences requiring description.
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
Car class inherits the methods of the
Vehicle class, such
lightsOn() etc. The
Car class will also inherit the
states of the
Vehicle class, such as
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.
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.
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
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
method for the
SaloonCar class as the
Car class provides
a suitable enough
draw() method. All we have to do is write a
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.
Over-Loading is the second form of polymorphism. The same method name can be used, but the number of parameters or the types of parameters can differ, allowing the correct method to be chosen by the compiler. For example:
add (int x, int y) add (String x, String y)
are two different methods that have the same name and the same number of parameters. However, when we pass
String objects instead of two int variables then we expect different
functionality. When we add two int values we expect an int result - for example
6 + 7 = 13. However, if we passed two
String objects we would expect a result of
"6" + "7" = "67". In other words the strings should be concatenated.
The number of arguments can also determine which method should be run. For example:
channel() channel(int x)
will provide different functionality where the first method may simply display the current channel number, but the second method will set the channel number to the number passed.
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
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.” 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
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
If we required we could also tag the
draw() method as abstract in a derived class, for example we could also have
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
© 2006 Dr. Derek Molloy (DCU).