Some of the essential elements of the object-oriented programming are mentioned below:
Inheritance
Polymorphism
- Abstraction
- Objects and Classes
- Three OOP Principle
Inheritance
Polymorphism
- Persistence
- Genericity
- Composition/Aggregation
1. Abstraction
The abstraction is one of the essential elements of any programming language including the procedural languages. The concept of built-in data type is also an abstraction. The feature of defining own data type using struct in C language further extends this abstraction.
The basic purpose of abstraction is to reduce complexity by hiding details. For example, people do not think of car as a set of hundreds of components. They think of it as a well-defined object having some unique properties and behavior.
Abstraction can be defined as an act for identifying the essential properties and behaviors of an object without going into details. The properties and behaviors of an object differentiate it from other objects of similar type and also help in classifying/grouping the objects.
The object-oriented programming languages model abstractions using classes and objects.
2. Objects and Classes
Object
An object is a physical or abstract entity or thing, which can be distinguished from other objects of similar types. An object has three characteristics:
(i) Identification
(ii) Properties (Attributes)
(iii) Behaviors (Methods/Operations/Functions)
The object can also be thought of as an instance of class, where class is like a built-in data type and object is a variable.
Class
A class represents a category of objects with similar properties and behaviors. A class can be thought of as a blueprint for creating objects. An object has the properties and behaviors defined by its class. The class can be thought of as a user-defined data type and object as a variable/instance of the data type represented by the class.
The properties/attributes of an object are defined by data members/fields in Java. A data member/field in a class is a variable that can hold data. The behaviors/operations of an object are defined using methods in Java. Both data members and methods are referred to as members of the class.
A class may also be thought of as a user-defined data type and an object as a variable of that data type. Once a class has been defined we can create any number of objects belonging to that class.
3. Three OOP Principles
3.1 Encapsulation
Encapsulation is the mechanism that binds code and data on which the code acts. Java classes support this as they allow us to define the code and data together. Encapsulation also helps in achieving data hiding by declaring some of the class members as private so that they cannot be accessed from the code that does not belong to the class.
3.2 Inheritance
In object-oriented programming, inheritance refers to the properties of a class being available to other classes called sub-classes or derived-classes. A sub-class or derived class is one that is derived from an existing class. It inherits all the features of the existing class which is also referred as the base-class or super-class. So inheritance can be defined as the process of deriving a class from a super-class or a base-class. Inheriting a class does not introduce any changes in the base-class/super-class. The derived-class/sub-class has a larger set of properties and behaviors as compared to the base-class.
The major advantage of the inheritance is code reusability. If the base class is in use for a long time and there is a need to add some extra attributes and methods, we can do so by deriving another class. The derived class will be able to use code of the base class without debugging as it is in use for a long time
Class Hierarchy
All the classes derived from a common base class belong to a family and form a class hierarchy. The class hierarchy can be compared with a tree structure where one base class is at the root and does not have a super-class. All other classes are either derived from the class at the root of the hierarchy or from some other class, which is derived from the root class directly or indirectly.
More features are added as we go down the tree. The classes that are represented by the leaf nodes do not have any sub-classes. The following examples show some class hierarchies.
Example 1: You can derive classes Saving Account and Current Account from the class Account. Here Account is the Base/Super Class and Saving Account and Current Account are Derived/Sub classes.
Example 2: The following diagram shows the classes derived from the Base class Vehicle.
3.3 Polymorphism
Polymorphism is a feature that allows same interface to be used for a general class of actions.
Most of the object-oriented languages use polymorphism in the following situations:
2. Objects and Classes
Object
An object is a physical or abstract entity or thing, which can be distinguished from other objects of similar types. An object has three characteristics:
(i) Identification
(ii) Properties (Attributes)
(iii) Behaviors (Methods/Operations/Functions)
The object can also be thought of as an instance of class, where class is like a built-in data type and object is a variable.
Class
A class represents a category of objects with similar properties and behaviors. A class can be thought of as a blueprint for creating objects. An object has the properties and behaviors defined by its class. The class can be thought of as a user-defined data type and object as a variable/instance of the data type represented by the class.
The properties/attributes of an object are defined by data members/fields in Java. A data member/field in a class is a variable that can hold data. The behaviors/operations of an object are defined using methods in Java. Both data members and methods are referred to as members of the class.
A class may also be thought of as a user-defined data type and an object as a variable of that data type. Once a class has been defined we can create any number of objects belonging to that class.
3. Three OOP Principles
3.1 Encapsulation
Encapsulation is the mechanism that binds code and data on which the code acts. Java classes support this as they allow us to define the code and data together. Encapsulation also helps in achieving data hiding by declaring some of the class members as private so that they cannot be accessed from the code that does not belong to the class.
3.2 Inheritance
In object-oriented programming, inheritance refers to the properties of a class being available to other classes called sub-classes or derived-classes. A sub-class or derived class is one that is derived from an existing class. It inherits all the features of the existing class which is also referred as the base-class or super-class. So inheritance can be defined as the process of deriving a class from a super-class or a base-class. Inheriting a class does not introduce any changes in the base-class/super-class. The derived-class/sub-class has a larger set of properties and behaviors as compared to the base-class.
The major advantage of the inheritance is code reusability. If the base class is in use for a long time and there is a need to add some extra attributes and methods, we can do so by deriving another class. The derived class will be able to use code of the base class without debugging as it is in use for a long time
Class Hierarchy
All the classes derived from a common base class belong to a family and form a class hierarchy. The class hierarchy can be compared with a tree structure where one base class is at the root and does not have a super-class. All other classes are either derived from the class at the root of the hierarchy or from some other class, which is derived from the root class directly or indirectly.
More features are added as we go down the tree. The classes that are represented by the leaf nodes do not have any sub-classes. The following examples show some class hierarchies.
Example 1: You can derive classes Saving Account and Current Account from the class Account. Here Account is the Base/Super Class and Saving Account and Current Account are Derived/Sub classes.
Polymorphism is a feature that allows same interface to be used for a general class of actions.
Most of the object-oriented languages use polymorphism in the following situations:
- Operator Overloading
- Method Overloading
- Method Overriding
Operator Overloading
Most of the languages use this form of polymorphism for the built-in operations. For example, all the arithmetic operators in C/C++ or Java can be used with many types of operands (int, long, float, double etc.). So same addition operator can be used to add two integers as well as to add
two floating-point numbers.
The C++ allows the user to overload the built-in operators. For example, you can overload the
arithmetic operators to handle the complex numbers also. Although Java uses operator
overloading for built-in operators but does not allow the user to overload the operators.
Method Overloading
This feature allows us to write more than one methods with the same name. Both C++ and Java have this feature. The methods (functions in C++) with same name are differentiated based on the parameters. The overloaded methods must either have different number of parameters or the types of the parameters must differ if their number is same.
If the call to an overloaded method can be resolved at compile time i.e. if the compiler can decide which of the overloaded method will be called then this is called static binding, which is an example of compile time polymorphism.
If the call to an overloaded method cannot be resolved at compile time i.e. if the compiler cannot
decide which of the overloaded method will be called then this is called dynamic binding, which
is an example of run-time polymorphism.
In general Java resolves calls to overloaded methods at run-time but there are many situations
where the calls to overloaded methods are resolved at compile-time.
Method Overriding
The sub-class can define a method with the same name as in the super-class, and same number and type of parameters. This is called method overriding.
The compiler can not resolve call to an overridden method. Java normally uses dynamic binding to resolve calls to overridden methods i.e. the decision takes place at run-time.
4. Persistence
Some object-oriented languages allow you to store/retrieve the state of a program to/from a persistent storage media i.e. a permanent storage media like secondary storage. This is called persistence.
Java allows you to store any object on the secondary storage and to retrieve it later on. If you attempt to store an object at the top of an object graph, all of the other referenced objects are recursively located and saved. Similarly when the object is retrieved later on, all of the objects and their references i.e. the entire object graph is correctly created in the main memory.
For example, it is possible to save an entire tree structure by just saving the root of the tree. At a
later stage it is possible to recreate the entire tree structure in the memory by just retrieving the root.
The C++ does not support this feature.
5. Genericity
The concept of defining an algorithm once, independently of any specific type of data, and then
applying that algorithm to a wide variety of data types without any additional effort is called Genericity.
C++ supports this feature using templates. Java also supports this feature through Object class,
which is at the top of any class hierarchy in Java.
For example, using this feature, we can implement a generic data type stack so that it is possible
to store element of any type in the stack.
This feature increases the degree of reusability to a large extent.
6. Composition/Aggregation
An object might be made up of other objects. Such an object is called Composite or Aggregate
object. For example, it is appropriate if an object of class vehicle is defined as a composite object
made up of objects like Engine, Body, Axle, Seats etc.
Inheritance v/s Composition
There are two basic mechanisms for deriving new classes from the existing ones: Inheritance and
Composition.
The class Bus can be derived by inheriting properties of class vehicle. Here Bus is a kind of vehicle which has some additional properties / behaviors beside the properties and behaviors which are common for all the vehicles. In other words class Bus has is-a relationship with the class vehicle as we can say that Bus is a vehicle.
The class vehicle itself might be derived from many other classes using composition. For example, an object of class vehicle might be composed of objects belonging to classes like Engine, Gear Box, Seats, Driver’s Seat, Body, Steering Wheel etc. The derived class in this case has whole-part relationship with the classes representing parts of the composite object. We can not say that Vehicle is an Engine or Vehicle is a Gear Box as Vehicle is made up of a number of parts.
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