Lecture 12 - Shared Class Resources (VB 2008)

8/14/2019 Lecture 12 - Shared Class Resources (VB 2008)

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Lecture 12: Shared Class Resources


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Outline In the last lecture, we continued our discussion of Overriding

With Polymorphism, a simple but powerful concept Form 1 : Overriding Base Class Methods and Properties. Form 2 : The Ability of Methods to Accept Related Classes.

And then illustrated with a generalized Multiple-JTrain Example That uses both forms of Polymorphism.

We also learned the basics of creating our own custom Collections:

Similar to Collections.ArrayList, but defined by us

By inheriting from the .NET System Class, CollectionBase. And defined our own Collection called JTrainCollection.

In this lecture, we first continue by updating Module1: Add two utility Methods:

AddTrain()

FindTrain() Update ourMain() Method to use these with JTrainCollection

Then, we discuss SharedMembers and Methods Which allow us to share resources amongst all Object Instances.

Example (JFreightTrain) : A Shared WagonWeight Member


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Last time, we had just added the

CommandTrain() Method


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Next, add the AddTrain() Method


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The FindTrain() Method


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Shared Resources

It is often useful to define a Shared Class resource:

A Member, Property, or Method which is associated with a Class. But not to any particular instance. Such shared resources can be useful for:

Book-keeping SPACE efficiency.

To make a resource shared, we use the Shared keyword.

Some JTrain Examples: A Shared WagonWeight Memberfor Class JFreightTrain

That sets the weight ofall wagons for all instances (once only). This lets us change the LoadWeight at once for all Trains!

Note: We are really setting their ideal full-load (not actual) weights

A Shared Counterfor Class JTrain: That keeps track of how many instances exist, total.

We simply add JTrain.Counter Then increment on creation, using the Base Constructors


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Ex.: Shared WagonWeight Member


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Example (cont.)


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Shared Members (Assignment 1)

(*) As an exercise, do the same for aShared JTrain Counter

1. Add a Shared Membercalled Counter. Note: (*) You must also update the Bases New() methods, as well!

To increment Counter each time a new JTrain or JFreightTrain is created.

2. Update the Main() Method to let the user check it!

I will check this, when you turn in your Final Project.


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Shared Methods

Shared Methods work in the same way...

Such Methods are shared by ALL Class instances.

However, Shared Methods have a natural limitation : They may only access Members of the class that are also Shared.

Otherwise, it is unclear which instance of the Class is being acted on.


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Overview / Forward

In this lecture, we have continued our discussion of Objects

By finishing our our own custom Collection: JTrainCollection Similar to Collections.ArrayList, but defined by us

By inheriting from the .NET System Class, CollectionBase. As illustrated its use, with a generalized Multiple-JTrain Example.

We then learned about Shared Members and Methods Which allow us to share resources amongst all Object Instances.

Ex. 1 (JFreightTrain) : A Shared WagonWeight Member Ex. 2 (JTrain): A Shared Counter Member

Next Lecture, we will continue with a discussion of Objects: The NET Framework

The .NET Framework Classes

Namespaces

The Object Browser, etc

Finally, I will assign yourFINAL PROJECT for the course.

Lecture 12 - Shared Class Resources (VB 2008)

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