object-oriented application development using vb.net 1 chapter 13 introduction to data access...

Object-Oriented Application Development Using VB .NET

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Chapter 13

Introduction to Data Access Classes and

Persistence


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Objectives

In this chapter, you will:• Examine VB .NET input and output (I/O)• Make objects persistent• Design a data access class• Communicate with a data access class• Use a relational database with VB .NET


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Examining VB .NET Input and Output

• Stream– A collections of bytes that transfer data to and

from files• Stream classes with sequential file capabilities

– StreamWriter• Located in the System.IO namespace• Derived from the TextWriter class

– StreamReader• Located in the System.IO namespace• Derived from the TextReader class


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Examining VB .NET Input and Output

• In a file-based system– A file

• Represents the collection of all the data• A file consists of records

– A record• Represents a single entity

– A field• An attribute that will be included for each record in

the file• A record consists of fields


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Sequential File Processing

• Sequential file processing example– Text boxes

• Allow users to enter values for each record to be saved to a sequential file

• Names of the text boxes– txtStRegNo– txtLength– txtManufacturer– txtYear– txtCustPhoneNo


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• Sequential file processing example (continued)– Buttons

• Save To File button– Clicked to save the text box values to a sequential file

• List Records button– Clicked to extract the data from the file and display the

results in a multiline text box with a vertical scroll bar• Exit button

– Closes the program


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General Format for StreamWriter and StreamReader

• The boat attribute file example– Uses the following methods of the StreamWriter class

• Write• WriteLine• Close

– Uses the following methods of the StreamReader class• Read• ReadLine• ReadToEnd• Peek• Close


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General Format for StreamWriter and StreamReader


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Adding Toolbars and Dialog Controls

• Dialog boxes and toolbars– Used to build professional user interfaces

• To incorporate a toolbar with graphic buttons on a form– Place the graphic images into the ImageList control– Add the toolbar control to the form– Link the toolbar’s ImageList property to the

ImageList


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• ImageList control– Images property

• A collection of images• Used to open the Image Collection Editor

– Image Collection Editor is used to» Add an image to the collection» Change the order in which the images are displayed» Delete images from the collection


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Adding Toolbars and Dialog Controls• Toolbar control

– ImageList property• Used to link to the ImageList that holds the images


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• Toolbar control– Buttons property

• Used to open the ToolBarButton Collection Editor– ToolBarButton Collection Editor can be used to

» Add buttons to the toolbar


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Adding Dialogs to the Application

• Dialog boxes– Provide richness and standardization for common

user interface operations– For example

• Functions which should have a common look and feel, regardless of the application– Opening and saving files– Changing colors– Changing fonts

– To add a dialog box to an application• Double-click the dialog control in the Toolbox


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The Save File Dialog Box

• SaveFile dialog box opens a file for saving


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OpenFile Dialog Box

• OpenFile dialog box opens a file for reading


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Adding the Color Dialog Box

• Color dialog box is used to change the color in applications


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The FontDialog Box

• FontDialog control– Used to change fonts and font colors in

applications– Sets the Font property for controls


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Making Objects Persistent

• Object persistence– The capacity to store and retrieve information from

files• Two approaches to achieving persistence

– Attribute storage– Object storage


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• Attribute storage– Involves

• Retrieving attribute values from the instance• Writing attribute values to a file

– Disadvantage• It is necessary to re-create the object


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• Options for storing and retrieving objects:– Attribute storage and retrieval using the

StreamWriter and StreamReader– Object serialization– Attribute storage and retrieval using databases


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• Object serialization– An easy way to store and retrieve objects– Serialization process

• Transforms an object into a stream that can be saved to a sequential file

– Deserialization• Transforms the stream from the file back to the

object state before the object was stored– Advantage

• The object can be retrieved intact• It is not necessary to re-create the object


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• Databases– One or more files organized to help make queries– Can be used to make objects persistent– Organize data into tables that can be related to

each other• Each table column represents an attribute• Each row represents a record

– Structured Query Language (SQL)• A standard set of keywords and statements used to

access relational databases


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Designing a Data Access Class

• Purpose of a DA class– To provide methods that store and retrieve data– Make instances of a PD class persistent

• Reasons for placing data storage and retrieval tasks in a DA class– Data input and output code are isolated from other

classes• Can dramatically reduce maintenance

– Separate classes for each tier make deployment easier in a client-server environment • GUI, PD, and DA functions may reside on multiple

machines at various sites


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Designing a Data Access Class

• DA methods can be invoked only by the PD class– Services provided by the DA class appear to be

provided by the PD class– Hides the DA class from all other classes


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Data Access Methods

• A separate DA class is written for each PD class• Example:

– CustomerDA• Provides data storage and retrieval services for

customers• Methods can only be invoked by the PD class


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Data Access Methods

• Four basic tasks provided by CustomerDA and Customer:– Retrieve a customer– Store a customer– Change a customer’s data– Remove a customer


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Data Access Methods

• CustomerDA class– Find method

• Functionality implemented: retrieve a customer– AddNew method

• Functionality implemented: store a customer– Update method

• Functionality implemented: update a customer’s data– Delete method

• Functionality implemented: remove a customer


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Data Access Methods

• CustomerDA class (continued)– Additional methods

• Initialize• Terminate• GetAll

– Exception handler classes• NotFoundException• DuplicateException


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Communicating with a Data Access Class

• Customer class– Invokes methods in CustomerDA – Methods

• Find• AddNew• Find• Delete• Initialize• Terminate• GetAll


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Finding a Customer

• The PD Find method– Invokes the DA Find method

Public Shared Function Find(ByVal PhoneNo As String) As Customer Return CustomerDA.Find(PhoneNo)

End Function


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Adding a Customer

• The PD AddNew method– Invokes the AddNew method in the DA class to

store a new customer instance

Public Sub AddNew()CustomerDA.AddNew(Me)

End Sub


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Changing a Customer

• The PD Update method– Invokes the Update method in the DA class

Public Sub Update()CustomerDA.Update(Me)

End Sub


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Deleting a Customer

• The PD Delete method– Invokes the Delete method in the DA class

Public Sub Delete()CustomerDA.Delete(Me)

End Sub


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Additional Problem Domain Methods

• Additional PD methods– Initialize

Public Shared Sub Initialize()CustomerDA.Initialize()

End Sub

– Terminate

Public Shared Sub Terminate() CustomerDA.Terminate()End Sub


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Additional Problem Domain Methods

• Additional PD methods (continued)– GetAll

Public Shared Function GetAll() As ArrayListReturn CustomerDA.GetAll

End Function


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Implementing Persistence with a Sequential File

• Attribute storage– Used for sequential access files

• The Initialize method– Reads customer’s attribute values from the

sequential file– Creates customer instances– Stores references for these instances in an

ArrayList


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• Terminate method– Creates a file that

• Contains attribute values for all the customer instances referenced by the ArrayList customers

• Find method– Seeks a customer instance with a phone number

that matches the value received by the method• If the customer is found

– Its reference is returned• If the customer is not found

– A NotFoundException is thrown


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• AddNew method– Adds the new reference to the ArrayList after

checking for a duplicate phone number• A duplicate phone number is assumed to be a

duplicate customer


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• Update method– Contains no code– Updates are made to the customer instance by

invoking accessor methods• setName• setAddress• setPhoneNo

– Later• Terminate method

– Writes the attribute values for all the instances to a sequential file


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• Delete method– Removes a customer from the system by

• Removing the customer from the ArrayList customers

– Later• Terminate method

– Saves all the customers in the ArrayList customers to the sequential file


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• GetAll method– Returns a reference to the customers ArrayList

• DuplicateException and NotFoundException classes– Inform the invoking client of a problem


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Testing CustomerDA for Sequential File Implementation

• The tester application will:– Create two customer instances– Invoke the Initialize method– Invoke the AddNew method to add the two new

customers to storage– Retrieve a reference to the first customer by

invoking the Find method


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Testing CustomerDA for Sequential File Implementation

• The tester application will (continued):– Invoke the GetAll method to retrieve references to

both customers– Invoke the Delete method for the second customer

and verify the deletion– Change the first customer’s address and verify the

change using the Update method– Invoke the Terminate method


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Implementing Persistence with Object Serialization

• Persistence using object serialization– Uses object storage– Complete object instances are written to and read

from a file– Instances are retrieved intact

• There is no need to re-create instances – Uses the FileStream class


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Implementing Persistence with Object Serialization

• To change from sequential access file processing to object serialization– Add the attribute <Serializable()> to the Customer

definition header– Change the following methods in CustomerDA

• Initialize• Terminate


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Using Relational Databases with VB .NET

• Relational databases– Referred to as database management systems (DBMSs)– Provide tools for organizing data into tables

• In a DBMS– Each column represents a field– Each row represents a record– Primary key: a field used to uniquely identify a record


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Structured Query Language

• Structured Query Language (SQL)– A standardized language used to manage and

query relational databases– Used by DA methods

• Find method uses the SQL SELECT statement to– Retrieve a specific customer’s record from the

database• AddNew method uses the SQL INSERT INTO

statement to– Add a new customer’s record to the database


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Structured Query Language

• Uses of SQL by DA methods (continued)– Update method uses the SQL UPDATE statement

to• Change the contents of one or more fields in a

customer’s record– Delete method executes the SQL DELETE

statement• SQL DELETE statement specifies the key value of

the customer to be deleted


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Accessing a DBMS with VB .NET

• VB .NET options for accessing data– ADO (Active-X Data Objects)

• Maintains a constant connection to the server• Data is kept and processed in a recordset

– ADO .NET• Only connects to the server when

– Data is needed from the data store– Updates back to the data store are required

• Relies on the concept of datasets as the basis for processing data


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VB .NET Database Access Classes

• VB .NET managed providers for accessing data:– The OleDb data provider

• Designed for accessing Microsoft Access databases and other non-SQLServer data sources

• Namespace: OleDb– The SQLServer data provider

• Designed for accessing Microsoft SQLServer databases

• Namespace: SQLClient• Object database connectivity (ODBC)

– Can be used to access most other databases


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VB .NET Database Access Classes


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VB .NET and DBMS Example

• Example– Project involves working with data from a relational

database• Selecting, adding, updating, and deleting rows from

a table– The GUI includes

• A data grid• Four buttons

– Add Record– Update Record– Delete Record– Find


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VB .NET and DBMS Example


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Implementing Object Persistence with a Database

• To implement object persistence with a relational database– Initialize method establishes a connection to the

database– Find, AddNew, Update, Delete, and GetAll methods

access the database directly– Terminate method closes the database connection


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Summary

• Object persistence: storing instance data for future retrieval

• Persistence can be achieved by storing either attribute values or entire instances

• A data access (DA) class provides methods that store and retrieve data

• Methods of a DA class are invoked only by the methods of the corresponding PD class

• Methods of a DA class: Find, AddNew, Update, Delete, GetAll, Initialize, and Terminate


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Summary

• VB .NET’s stream approach to I/O views data input and output as a flow of bytes

• Persistence can be achieved using sequential files, object serialization, or relational databases

• A relational database is used to organize data into tables

• In a relational database, each column represents a field and each row represents a record

• SQL is a popular, standard language used to access relational databases

object-oriented application development using vb.net 1 chapter 13 introduction to data access...

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