Transcript Intro. to Data Access Classes and Persistence

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
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txtStRegNo
txtLength
txtManufacturer
txtYear
txtCustPhoneNo
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Sequential File Processing
• 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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Sequential File Processing
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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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Adding Toolbars and Dialog Controls
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Adding Toolbars and Dialog Controls
• 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
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Adding Toolbars and Dialog Controls
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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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Adding Toolbars and Dialog Controls
• 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 Toolbars and Dialog Controls
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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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Making Objects Persistent
• 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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Making Objects Persistent
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Making Objects Persistent
• 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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Making Objects Persistent
• 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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Making Objects Persistent
• 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:
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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
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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 ArrayList
Return 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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Implementing Persistence with a
Sequential File
• 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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Implementing Persistence with a
Sequential File
• 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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Implementing Persistence with a
Sequential File
• 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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Implementing Persistence with a
Sequential File
• 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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Implementing Persistence with a
Sequential File
• 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
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–
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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
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