Object-Oriented Paradigm is where we focus real life objects while programming any solution. By focusing real life objects we mean that over solutions revolves around different objects, which represent respective objects in real life situation.
The objective of this section is to provide a thorough understanding of the principles of object-oriented paradigm.
Data abstraction is the reduction of a particular body of data to a simplified representation of the whole. Abstraction, in general, is the process of taking away or removing characteristics from something in order to reduce it to a set of essential characteristics.
In object-oriented programming, objects interact with each other by messages. The only thing that an object knows about another object is the object's interface. Each object's data and logic is hidden from other objects. In other words, the interface encapsulates the object's code and data.
This allow the developer to separate an object's implementation from its behaviour. This separation creates a "black-box" affect where the user is isolated from implementation changes. As long as the interface remains the same, any changes to the internal implementation is transparent to the user. For example, if the name message is sent to the Student object, it does not matter to the user how the developer implemented the code to handle this message. All the sending object needs is the correct protocol for interacting with the Student object. The developer can change the implementation at any time, but the name message would still work because the interface is the same.
Another benefit of separating implementation from behaviour is polymorphism. Polymorphism allows two or more objects respond to the same message. A method called name could also be implemented for an object of the class Course. Even though the implementation of this name message may return a course number and a course title, its protocol is the same as the name message to the Student object. Polymorphism allows a sending object to communicate with different objects in a consistent manner without worrying about how many different implementations of a message.
An analogy of polymorphism to daily life is how students response to a school bell. Every student knows the significant of the bell. When the bell (message) rings, however, it has its own meaning to different students (objects). Some students go home, some go to the library, and some go to other classes. Every student responds to the bell, but how they response to it might be different.
Another example of polymorphism is the function of printing. Every printable object must know how to print itself. The message is the same to all the different objects: print, but the actual implementation of what they must do to print themselves varies.
The sending object does not have to know how the receiving object implement the message. Only the receiving objects worries about that. Assume that there is a print Page method in a Document object that has the responsibility of printing a page. To print the page, the print Page method sends the print message to each object on the page. The Document does not need to know what types of objects are on the page, only that each object supports the behaviour of printing.
New objects can be added to the page without affecting the print Page method. This method still sends the print message and the new object provides its own print method in response to that message.
Polymorphism allows the sending object to communicate with receiving objects without having to understand what type of object it is, as long as the receiving objects support the messages.
Another important concept of object-oriented programming is inheritance. Inheritance allows a class to have the same behaviour as another class and extend or tailor that behaviour to provide special action for specific needs.
Let's use the following application as an example. Both Graduate class and Undergraduate class have similar behaviour such as managing a name, an address, a major, and a GPA. Rather than put this behaviour in both of these classes, the behaviour is placed in a new class called Student. Both Graduate and Undergraduate become subclass of the Student class, and both inherit the Student behaviour.
Both Graduate and Undergraduate classes can then add additional behaviour that is unique to them. For example, Graduate can be either Master's program or phD program. On the other hand, Undergraduate class might want to keep track of either the student is Freshman, Sophomores, Junior or Senior.
Classes that inherit from a class are called subclasses. The class a subclass inherits from are called super class. In the example, Student is a super class for Graduate and Undergraduate. Graduate and Undergraduate are subclasses of Student.