Chapter 14 Designing Data Access Classes

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Chapter 14Designing Data Access Classes
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Making Objects Persistent

• Use object persistence to store instances or
  attribute values for later retrieval
• Two approaches to achieving persistence
• Attribute storage store the objects attributes
  individually
• Object storage store the entire object
• Advantage of object storage is that you dont
  need to recreate the object when you retrieve it

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Making Objects Persistent
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Designing a Data Access Class

• The purpose of the Data Access (DA) class is to
  provide methods to store and retrieve data and
  make instances of a Problem Domain (PD) class
  persistence
• Three tier OO design
• GUI class
• PD class for business entities
• DA class for data storage and retrieval
• DA class isolates data storage and retrieval
• DA class supports three-tier architecture

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Data Access Methods

• Since you will not have instances of the DA class
  all methods will be static
• Four basic methods are provided
• Retrieve find method
• Store addNew method
• Change update method
• Remove delete method

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Data Access Methods
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Communicating with DA Class
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• The Customer class invokes methods in CustomerDA
  class
• Isolates the DA class from everything but PD
  class
• DA class methods are not tied to data storage
  method

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Finding a Customer
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• The purpose of the PD find method is to invoke
  the DA find method
• Its a static method because its not tied to a
  specific customer instance
• Throws a NotFoundException if the customer is not
  found

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Adding a Customer

• The purpose of the PD addNew method is to invoke
  the DA addNew method
• Its a non-static method because its invoked for
  the new customer being added
• Throws a DuplicateException if the customer
  already exists

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Changing a Customer

• The purpose of the PD update method is to invoke
  the DA update method
• Its a non-static method because its invoked for
  the customer being updated
• Throws a NotFoundException if the customer is not
  found

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Deleting a Customer

• The purpose of the PD delete method is to invoke
  the DA delete method
• Its a non-static method because its invoked for
  the customer being deleted
• Throws a NotFoundException if the customer is not
  found

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Understanding Java I/O
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• Java views data input and output as a flow of
  bytes
• Two different data streams
• Byte stream
• Character stream

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Understanding Java I/O
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Understanding Java I/O
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• Record is a collection of related variables
• File is a collection of related records
• Sequential files contains records that are stored
  and processed in sequential order
• Random access files are organized so you can
  access a record by specifying its record number
• Database is one or more files that help make
  queries
• Relational database organized in tables and rows

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Implementing Persistence with a Sequential File
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• Reads the customer records, creates a customer
  instance for each record, and places instances in
  a Vector
• Uses the java.io package import java.io.

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Initialize Method
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• Reads the attribute values for all of the
  customers from the sequential file
• Uses BufferReader class to read the file
  sequentially

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Terminate Method
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• Responsible for creating a file that contains
  attribute values for all the customer instances
  in the Vector
• Attributes written to the file using the println
  method

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Find Method

• The find method simply iterates the customers
  Vector, seeking a customer instance with a phone
  number that matches the value received by the
  method
• If the customer is not found a NotFoundException
  is thrown

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AddNew Method

• Simply adds the new reference to the Vector
• If a duplicate is found throws a
  DuplicateException error

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Update Method
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• Contains no code since the file is written when
  done

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Delete Method
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• Remove a customer from the system
• Removes the reference from the Vector
• If the customer is not found a NotFoundException
  is thrown

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getAll Method

• The Vector already contains all of the customers
• Simply returns this Vector

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Testing CustomerDA Class
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• Create two customer instances
• Invoke initialize
• Invoke addNew to add two customers
• Retrieve reference to first customer
• Invoke getAll
• Invoke delete
• Change first customers address and verify
• Invoke the terminate method

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Implementing Persistence with Random Access

• Change the file specification to Customer.ran
• Use the RandomAccessFile class
• Use the writeBytes method

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Implementing Persistence with Object Serialization

• Store the entire Customer instances
• Change the file specification to Customer.obj
• Use the ObjectInputStream and the readObject
  method
• Use the ObjectOutputStream and the writeObject
  method

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Designing a Relational Database
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• Provide tools to organize data into tables
• Each column represents a field
• Each row represents a record
• Each customer is identified by their phone number
  primary key
• Protocols required to access Microsoft Access
• Java Database Connectivity (JDBC)
• Open Database Connectivity (ODBC)

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Designing a Relational Database
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Understanding SQL
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• Structured Query Language (SQL) used to access
  relational databases
• SQL SELECT is used to retrieve a specific
  customer record
• SQL INSERT is used to add a new customer record
• SQL UPDATE is used to change a customer record
• SQLQ DELETE is used to delete a customer record
• Java.sql package contains the database access
  method

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Understanding SQL
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