We can see that our code is very redundant in this class. All further changes would have to take a place in both classes, it'd make the code very hard to maintain. Now, we'll declare the administrator class using inheritance. We'll let the Administrator class to be inherited from the User class. We don't have to declare fields and methods again, VB. NET will add them automatically:.
In the example above, the private fields won't be accessible to the descendant. Only the fields and methods with the Public modifier will. Private fields and methods are understood as a special logic that belongs to a certain class. Meaning that, they are hidden from descendants - even a descendant is actually using this logic, it's unable to modify it.
To achieve the desired result, we'll use a new access modifier - Protected , which works just like Private but allows the inheritance of these fields. Now, if we created user and administrator instances, they'd both have name fields and LogIn methods. NET will inherit the User class and redeclare all its fields automatically.
The advantages of using inheritance are clear. We don't have to copy the same fields to two classes. All we have to do is declare new ones. The rest will be inherited. The benefits are tremendous, we can extend existing components of new methods and reuse them.
We don't have to write lots of redundant code, and most importantly - when we change a single field in the parent class, this change is then inherited everywhere automatically. We don't need to change something manually in 20 classes, which could potentially cause errors.
We're humans and we'll always make mistakes, we have to choose the right programming techniques to make as less mistakes as possible. The parent class is sometimes called the ancestor, our User class, and the class that inherits from it, a descendant, our Administrator class.
Descendants may add new methods or override the existing methods of the parent class see example below. Sometimes, you may encounter the terms superclass and subclass which essentially mean the same thing. Another way to design the object model would be to create a User parent class whose sole purpose would be inheritance. The User would be then inherited by Attendant and Admin classes. This technique would be useful if we had lots of user types. In that case, we're talking about a class hierarchy, which we'll get into further along in this online course.
Our example was simple so two classes were enough. There are also design patterns that are established and approved object models used for common situations. All of that stuff is advanced and interesting, and I promise we'll get to them later as well. In object modeling, we draw inheritance as an empty arrow pointing to the parent class. In our case, the graphical notation would look like this:. The biggest advantage to using inheritance is that if we create a variable of the parent data type, we can store the descendants in it as well.
We are able to do this because a descendant always includes everything that the parent class does, therefore, it meets the "requirements" of the data type more accurately it provides the necessary interface. What all descendants are is the parent class with some extra stuff. In this instance, the receiver is only aware that it is getting a Shape that has a method named Draw, but it is ignorant of the specific kind of Shape.
In oops world the polymorphisam is to ability inherits methods and properties of baseclass into derived class. Poly means many and morph means form. Thus, polymorphism refers to being able to use many forms of a type without regard to the details. Polymorphism Overview : - When a derived class inherits from a base class, it gains all the methods, fields, properties and events of the base class. To change the data and behavior of a base class, you have two choices: you can replace the base member with a new derived member, or you can override a virtual base member.
Replacing a member of a base class with a new derived member requires the new keyword. If a base class defines a method, field, or property, the new keyword is used to create a new definition of that method, field, or property on a derived class. The new keyword is placed before the return type of a class member that is being replaced. When the new keyword is used, the new class members are called instead of the base class members that have been replaced.
Those base class members are called hidden members. Hidden class members can still be called if an instance of the derived class is cast to an instance of the base class. Generally, the ability to appear in many forms. In object-oriented programming, polymorphism refers to a programming language's ability to process objects differently depending on their data type or class.
More specifically, it is the ability to redefine methods for derived classes. For example, given a base class shape, polymorphism enables the programmer to define different area methods for any number of derived classes, such as circles, rectangles and triangles.
No matter what shape an object is, applying the area method to it will return the correct results. Polymorphism is considered to be a requirement of any true object-oriented programming language.
Constructor overloading i. If I remember correctly, it is something like when you have the same method names but different implementations. For example in the base class there is a method called PrintMessage.
In the derived calss there is the same method name. The way that this can be achived without errors is to have Virtual in the base class and override in the derived class for the method names. What is Polymorphism? How does VB. Apr, Jul, Feb, 6. See my NET Interview questions 0. Visual Basic 6 provides this feature of object-oriented languages by implementing interfaces. By the way, an interface can be instantiated with several different classes that implement the interface, and it can polymorphically access the members of the class instances.
The classes and interfaces in this case are upgraded as indicated in the sections above. The issue in this case is the creation process for instances.
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