Object oriented system and core concepts

An object-oriented system is a system built as a collection of interacting objects. Each object has data and behaviour, and objects cooperate by sending messages to each other, instead of one program calling functions on shared data.

Class

A class is a blueprint or template that defines the attributes and operations common to a set of objects. Example: Student with attributes name, roll and operations register(), getResult().

Object

An instance of a class with its own identity, state and behaviour. Example: s1 = Student("Ram", 101).

Encapsulation

Binding data and methods together and hiding the internal data using access control (private). Example: balance of an account is private and changed only by deposit()/withdraw().

Abstraction

Showing only essential features and hiding the details. Example: a Car user uses start() and accelerate() without knowing the engine internals.

Message

A request sent to an object to invoke one of its operations. Example: account.withdraw(500) sends the message withdraw with argument 500 to the object account.

Inheritance

A subclass acquires attributes and methods of a superclass, giving reuse and an "is-a" hierarchy. Example: Student and Teacher inherit name and address from Person.

Interface

A named set of operation signatures without implementation, which a class promises to provide. Example: interface Printable { print(); } is implemented by Report and Invoice.

Polymorphism

The same message gives different behaviour depending on the object receiving it. Example: shape.draw() draws a circle for Circle and a rectangle for Rectangle.

         +---------+
         | Person  |
         +---------+
           ^     ^
           |     |
    +---------+ +---------+
    | Student | | Teacher |
    +---------+ +---------+
  s1:Student --- message: getResult() ---> object

Together these concepts make the system modular, reusable, easy to change and close to the real-world problem.