Also, implementation inheritance's ability to declare an operation abstract enables it to act similar to interface inheritance and force subclasses to implement the operation instead. NET class can only enter into an implementation inheritance relationship with one superclass also called base or ancestor class. However, VB. NET, like VB6, lets that same class enter into as many interface inheritance relationships as it chooses.
An example may help. Say you are developing an app in which the Store object wants to ask an instance of the Stereo class to calculate availability inventory.
However, the Stereo class wants to borrow the functionality already provided by its superclass, Product. The Store object might implement such functionality like this:. The real implementation of this behavior is in Product's calcInventory method. Because VB6 doesn't support implementation inheritance you need to put some code in the Stereo class:.
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VB6 allows the interface, the Product class, to implement the actual behavior. Stereo keeps an instance of Product containment then asks that reference to do some work for it delegation.
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This type of interface inheritance is not a true interface because it allows you to add code to Stereo to provide the actual behavior. With VB. NET you can remove the containment and delegation code by using implementation inheritance. For example, in the Stereo class:. The Store class makes a calculation request similar to the way the Stereo class makes this request, but Stereo carries out the work much differently in VB.
The Stereo instance, myStereo, doesn't contain a calcInventory method so VB looks up its superclass, Product, and executes its calcInventory procedure. Implementation inheritance also allows you to override superclass operations. An Items class might use this to provide its own implementation of calcInventory which would cancel out the behavior provided by its Product superclass.obdacounrecon.gq
Object Oriented Programming Language Features
For instance:. All of the above may sound a bit confusing if you have not used inheritance in the past. Take a second to reread the above and you will see that it is really not that difficult. Interface inheritance tends to be a little harder to grasp for some but, again, it is not that though of a concept. Implementation inheritance allows you to reduce your code base drastically since subclasses gain the methods of their superclass.
This is a mixed blessing. If you start with a poor design you will end up with code that is more difficult than ever to debug and maintain. Code and attributes once common in the superclass may not remain common as the app's business needs evolve over time. Eventually many subclasses may end up overriding the behavior of the superclass. Worse, the subclass may override the superclass, do its own work, then call the same operation again on the superclass. Believe me, I've seen it happen in other languages. NET requires you explicitly use the Overrides and Inherits keywords.
Most languages do not mandate this and are smart enough to know on their own. I believe this is a good thing. It provides self documentation and instant visual clue as to whether a method is overridden or not simplifying maintenance and debugging. Polymorphism means many forms. It gives classes the ability to define the same operation but provide unique implementations of that operation. In other words, each of the class' methods is different. You can use polymorphism with either interface or implementation inheritance.
Polymorphism is more than overriding a superclass's operations which is a small part of polymorphism's power. For an operation to be polymorphic it must implement an operation defined in an interface it implements or it must implement an operation defined in a superclass. The superclass' operation is typically abstract, which means it has no implementation code.
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For example, you might encounter the getInformation operation defined in Product as abstract. If another object refers to either a Stereo or Items object and wants to send it a message, it is best not to hard code that class name:. The input parameter's data type is Product.
VB is smart enough to know whether the function is asking Stereo or Items to get its information. VB lets you declare attributes and behaviors with the Private keyword inside a class module. This hides them from the world outside of the class. Conversly, if declared with the Public keyword other objects can access those properties and methods yet know nothing of their actual implementation.
Object-Oriented Programming with Visual Basic .NET
Overloading lets a function vary its behavior based on its input parameters. NET lets you have multiple functions with the same name but with different input parameters. The language knows which version to execute based on what is pased in. An example:. Sometimes you need to provide behavior on behalf of all objects in a particular class and maintaining attributes that all instances of a class can have access to.
NET uses shared members to resolves this issue. Other languages call these feature static or instance operations. If the Product class needs to maintain an attribute and operation that are available to all of its instances, you might implement the Product class like this:.
These attributes do not require an object instance to use them. In VB6 you can create a variable in a. This achieves the same function but is not very object-oriented. Constructors Constructors allow you to create an object and provide it with an initial set of data so it can initialize properly.
Without constructors, you create an object, then need to all a separate method to initialize it. Now that we have defined the properties of our class, the next task is to define methods that will give us access to our data. Since we have declared our class data properties to be private, we need to provide methods which will give us access to those properties from our code. This is achieved using Get and Set methods. The syntax for get and Set methods is as follows:. To demonstrate this we will add to our class to provide methods to get and set the account number and name.
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Note that after you type in the Public Property line of the declaration, Visual Studio automatically creates a template for the Get and Set methods, which should be filled in as follows:. Now that we have defined our getter and setter methods we can add our own method to perform a task. This is the same as writing a Visual Basic function. The purpose of our method will be to subtract the account fee from the current balance:.
When called, the above function subtracts the account fee from the balance, assigns the new balance to the intBalance variable and returns the new total.
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Now that we have added some functionality to our class it is time to instantiate objects from this class blueprint. The process of creating an object from the class 'blueprint' is called instantiation. Essentially, you instantiate an instance of the class and give that instance a name by which you will refer to it when accessing members and calling methods. You can create as many object instances of a class as you desire.