OOPS Features

what is OOPS

* The object oriented programming (OOP) is a programming model 

where Programs are organized around object and data rather than 

action and logic. 


* OOP allow decomposition of a problem into a number of entities called

Object and then builds data and function around these objects.

• The Program is divided into number of small units called Object. The data and function are build around these objects.
• The data of the objects can be accessed only by the functions associated with that object.
• The functions of one object can access the functions of other object.

OOP has the following important features.

 Class:

A class is the core of any modern Object Oriented Programming language such as C#.

In OOP languages it is must to create a class for representing data. 

Class is a blueprint of an object that contains variables for storing data and functions to performing operations on these data. 

Class will not occupy any memory space and hence it is only logical 

representation of data.

To create a class, you simply use the keyword "class" followed by the class name:

class Employee

{

}

Object: 

Objects are the basic run-time entities in an object oriented system.They may represent a person,a place or any item that the program has to handle. 

"Object is a Software bundle of related variable and methods. "

“Object is an instance of a class”

Class will not occupy any memory space. Hence to work with the data represented by the class you must create a variable for the class, which is called as an object. 

When an object is created by using the keyword new, then memory will be allocated for the class in heap memory area, which is called as an instance and its starting address will be stored in the object in stack memory area.

 When an object is created without the keyword new, then memory will not be allocated in heap I.e. instance will not be created and object in the stack contains the value null.

When an object contains null, then it is not possible to access the members of the class using that object.

class Employee

{

}

Syntax to create an object of class Employee:-

Employee objEmp = new Employee();

All the programming languages supporting object oriented Programming will be supporting these three main concepts:

1. Encapsulation
2. Inheritance
3. Polymorphism

Inheritance and Polymorphism—Specialization and Generalization

Specialization and Generalization

Classes and their instances (objects) do not exist in a vacuum, but rather in a network of interdependencies and relationships, just as we, as social animals, live in a world of relationships and categories.

One of the most important relationships among objects in the real world is specialization, which can be described as the is-a relationship. When we say that a dog is a mammal, we mean that the dog is a specialized kind of mammal. It has all the characteristics of any mammal (it bears live young, nurses with milk, has hair), but it specializes these characteristics to the familiar characteristics of canis domesticus. A cat is also a mammal. As such, we expect it to share certain characteristics with the dog that are generalized in Mammal, but to differ in those characteristics that are specialized in cats.

The specialization and generalization relationships are both reciprocal and hierarchical. Specialization is just the other side of the generalization coin: Mammal generalizes what is common between dogs and cats, and dogs and cats specialize mammals to their own specific subtypes.

These relationships are hierarchical because they create a relationship tree, with specialized types branching off from more generalized types. As you move "up" the hierarchy, you achieve greater generalization. You move up toward Mammal to generalize that dogs, cats, and horses all bear live young. As you move "down" the hierarchy you specialize. Thus, the cat specializes Mammal in having claws (a characteristic) and purring (a behavior).

Similarly, when you say that ListBox and Button are Windows, you indicate that there are characteristics and behaviors of Windows that you expect to find in both of these types. In other words, Window generalizes the shared characteristics of both ListBox and Button, while each specializes its own particular characteristics and behaviors.

The Unified Modeling Language (UML) is a standardized language for describing an object-oriented system. The UML has many different visual representations, but in this case, all you need to know is that classes are represented as boxes. The name of the class appears at the top of the box, and (optionally) methods and members can be listed in the sections within the box.

In the UML, you model specialization relationships, as shown in Figure 11-1. Note that the arrow points from the more specialized class up to the more general class. In the figure, the more specialized Button and ListBoxclasses point up to the more general Window class.

Figure 11-1. An is-a relationship

It is not uncommon for two classes to share functionality. When this occurs, you can factor out these commonalities into a shared base class, which is more general than the specialized classes. This provides you with greater reuse of common code and gives you code that is easier to maintain, because the changes are located in a single class rather than scattered among numerous classes.

For example, suppose you started out creating a series of objects, as illustrated in Figure 11-2. After working with RadioButtons, CheckBoxes, and Command buttons for a while, you realize that they share certain characteristics and behaviors that are more specialized than Window, but more general than any of the three. You might factor these common traits and behaviors into a common base class, Button, and rearrange your inheritance hierarchy, as shown in Figure 11-3. This is an example of how generalization is used in object-oriented development.

Figure 11-2. Objects deriving from Window

Figure 11-3. Factoring a Button class

The UML diagram in Figure 11-3 depicts the relationship among the factored classes and shows that bothListBox and Button derive from Window, and that Button is specialized into CheckBoxand Command. Finally, RadioButton derives from CheckBox. You can thus say that RadioButton is a CheckBox, which in turn is aButton, and that Buttons are Windows. This is not the only, or even necessarily the best, organization for these objects, but it is a reasonable starting point for understanding how these types (classes) relate to one another.