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Programming with Java
1
Chapter 13
Storing Information, Object
Serialization, and JDBC
© 2002 The McGraw-Hill Companies, Inc. All rights reserved.
Programming with Java
2
Objectives
•
Understand the purpose of streaming classes.
•
Use the System class for input and output to standard devices.
•
Save an object to a file.
•
Retrieve an object from a file.
•
Retrieve information from a database using the JDBC API.
•
Write SQL Select statements to create a ResultSet.
•
Use SQL action queries to update an existing database.
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Programming with Java
3
Streams
•
The most basic of input/output with Java is done with
streams.
•
The stream can flow from the program to the screen, from a
keyboard to the program, to/from a disk file, other storage
media, to a printer, or even to a network or the Web.
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Programming with Java
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Input Stream and Output
Stream
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Programming with Java
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Output to the Screen
•
•
•
•
•
You can send an output stream to the "standard output device" or
read an input stream from the "standard input device."
The standard input device is normally the keyboard and the
standard output device is your monitor.
Java's System class contains utilities for reading from the
standard input device and writing to the standard output device.
Two methods to send text to the screen; println method-automatically adds a carriage return, and print method--does not
add a carriage return.
Java, like C and C++, has three standard IO (input/output)
objects: in, out, and err.
© 2002 The McGraw-Hill Companies, Inc. All rights reserved.
Programming with Java
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Saving an Object in File
•
A common term in programming is persistence.
•
How can you make data persist, or still be available, from one
run of the program to the next?
•
If you want to save data from one program execution to the
next, you must have some way to store the data.
•
Java has been able to store bytes of information and the
contents of individual variables into a disk file.
•
Using this technique, the programmer is responsible for writing
all data into a disk file and reading it again for the next
program execution.
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Programming with Java
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Applet vs. Applications
•
Although you can perform disk reading and writing from some
versions of the applet viewer, you cannot access disk files
from an applet running in a browser.
© 2002 The McGraw-Hill Companies, Inc. All rights reserved.
Programming with Java
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Object Serialization
•
From JDK 1.1, Java added a powerful tool for persisting an
entire object as a single unit.
•
Using object serialization, you can use an output stream for
writing objects.
•
When you store an object, you need to keep the contents of
the variables but not the methods.
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Programming with Java
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Object Serialization
Continued
•
You can create a file that stores the contents of the class
variables for multiple instances of a class.
•
The program that later inputs the information from the file can
read the values for the class variables into an instance of the
class, or multiple instances, one for each object stored in the
file.
•
If your object contains a reference to another object you also
must store the data of the referenced object.
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Programming with Java
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Creating a Serializable Class
•
You must import java.io.* and implement the interface
Serializable.
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Programming with Java
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Steps of Writing an Object
•
Create a FileOutputStream object.
•
Create an ObjectOutputStream object.
•
Obtain the data from the text fields (after the user enters them).
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Create an event for storing the data for one object.
•
After all objects have been stored, close the ObjectOutputStream
object.
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Programming with Java
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FileOutputStream Object
•
The FileOutputStream class establishes a link from the
program to an actual disk file.
•
If the file does not already exist, this class creates the file.
•
Declare the output object at the class level - FileOutputStream
outputEmployee;
•
Instantiate the object and assign the file name in a method
inside the application - outputEmployee = new
FileOutputStream("Employee.txt");.
•
You must place this statement in a try/catch block, catching
Exception.
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Programming with Java
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The ObjectOutputStream Object
•
•
•
•
You can use the FileOutputStream object for any type of output
stream.
To save an object using the stream, you must also set up an
ObjectOutputStream class.
You declare the object as a class variable, and then assign the
ObjectOutputStream object in a class method.
The argument for the ObjectOutputStream constructor is the
name of a FileOutputStream object.
//Declare the object output stream
ObjectOutputStream objSaveEmployee;
//Instantiate the object stream and associate it to the file stream
objSaveEmployee = new ObjectOutputStream(outputEmployee);
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Programming with Java
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Event for Storing Data
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The writeObject method of the ObjectOutputStream class
saves the contents of the named object.
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Programming with Java
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Entering Information
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Programming with Java
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Reading an Object
•
The steps for reading an object are
 Create a FileInputStream object.
 Create an ObjectInputStream object.
 Obtain the data for one object from the file.
 Create an event to display the object's data in text fields.
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Programming with Java
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The FileInputStream
andObjectInput Stream Objects
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The FileInputStream object associates the project with a file.
•
And the ObjectInputStream enables you to read the entire object
with one read command.
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Declare the FileInputStream and ObjectInputStream objects as
class variables.
•
Then instantiate the objects in a class method.
© 2002 The McGraw-Hill Companies, Inc. All rights reserved.
Programming with Java
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Event for Displaying Data
•
You can create an Employee object and read the information
from the disk file.
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Read the information using the readObject method of the
ObjectInputStream; you must cast the result into an Employee
object.
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Then you can use the Employee object's get methods to display
the data to the screen.
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You can check for the end of file by catching an EOFException.
© 2002 The McGraw-Hill Companies, Inc. All rights reserved.
Programming with Java
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DataBase Connection with the
JDBC API
•
The JDBC API is a set of classes available in JDK 1.1 or higher,
in the java.sql package.
•
The classes and methods allow Java applets and applications to
interface with data that is stored in many formats including
relational, hierarchical, and network databases.
•
JDK 1.2 (Java 2) greatly improved database handling, providing
many additional features, such as scrollable recordsets.
© 2002 The McGraw-Hill Companies, Inc. All rights reserved.
Programming with Java
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DataBase Connection with the
JDBC API Continued
•
•
•
Many issues can complicate accessing databases in Java.
 If you are using an applet, security is an issue.
 You can easily access a database on the local machine, but
to access a database on a server, you must properly set up
security and sign the applet (see Chapter 15).
 For optimal database performance on the Web, you must
download and install drivers that are specific to the
database format.
The JDBC classes handle communication between your Java
program and a database driver.
Java 2 comes with JDBC, but not the manufacturer-specific
database drivers.
© 2002 The McGraw-Hill Companies, Inc. All rights reserved.
Programming with Java
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DataBase Connection with the
JDBC API Continued
•
The only driver that does come with Java is the JDBC-ODBC
Bridge, which we will use in this chapter.
•
The JDBC-ODBC Bridge takes advantage of Open DataBase
Connectivity (ODBC), which is Microsoft's implementation of
universal data access for diverse database formats.
•
Nearly all database formats provide ODBC drivers.
•
If an ODBC driver is available to access a database format, files
in that format can also be accessed from Java.
•
The following sections introduce you to JDBC and SQL
(structured query language).
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Programming with Java
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Setting up DSN
•
A Data Source Name (DSN) creates a link between a physical
data file and a name that you use in a program.
•
Before using JDBC in a Windows environment, you must
create a DSN, which registers your database file on the
computer.
•
You use the ODBC icon on the control panel to register
databases.
•
Follow the six steps in the book to register your database file.
•
You set up the DSN only once for a database file in a given
location.
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Programming with Java
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Loading the JDBC-ODBC Bridge
Driver
•
•
•
In your code, you must connect to the driver using the
Class.forName method.
Connect to Sun's JDBC-ODBC Bridge driver using this
statement.
Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");
If you are using Microsoft's J++ or Internet Explorer, you
cannot load Sun's driver, but must load Microsoft's version of
the Bridge instead:
Class.forName("com.ms.jdbc.odbc.JdbcOdbcDriver");
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Programming with Java
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Loading the JDBC-ODBC Bridge
Driver
Continued
•
If you need to be able to load either driver, you can use try/catch
blocks and attempt to load one driver.
•
If that fails, load the other one. If both fail, a ClassNotFound
exception occurs.
© 2002 The McGraw-Hill Companies, Inc. All rights reserved.
Programming with Java
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Connecting to the Database
•
Your next step is to connect to the database using the name that
you registered as a DSN.
•
This name is included in a string literal.
•
For example, if we registered the DSN as MerryPeas the string
becomes “jdbc:odbc:MerryPeas“.
•
Use this string with the DriverManager.getConnection method
and assign to a Connection object.
•
Notice that the connection is placed inside of an exception
testing for an SQLException.
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Programming with Java
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Creating a ResultSet
•
A ResultSet object contains a collection of records from the
database.
•
To create a ResultSet, you must declare a Statement object
and call the Connection object's createStatement method.
•
Then you can use the Statement object to create a query
Statement cmdEmployees;
ResultSet rsEmployees;
•
You can create a SQL query.
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Programming with Java
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Creating a ResultSet Continued
•
The following example selects all fields and all records in the
Employees table of the MerryPeas database.
•
The SQL string is :
Select * from Employees;
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Programming with Java
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Creating a ResultSet Continued
•
Use this string as an argument for the executeQuery method of
the Statement object.
try
{
//Connect to the database
conEmployees =
DriverManager.getConnection("jdbc:odbc:MerryPeas");
//Create a ResultSet
Statement cmdEmployees = conEmployees.createStatement();
ResultSet rsEmployees = cmdEmployees.executeQuery(
"Select * from Employees");
}
catch(SQLException error)
{
System.err.println("Error:" + error.toString());
}
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Programming with Java
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Creating a ResultSet Continued
•
You can create an SQL statement to create a ResultSet that
matches a given condition.
•
You can allow the user to enter the desired name and substitute
the literal name with a string variable.
•
Let’s look at some examples:
ResultSet rsEmployees = cmdEmployees.executeQuery(
"Select * from Employees Where [Last Name] = 'Rigner'");
ResultSet rsEmployees = cmdEmployees.executeQuery(
"Select * from Employees Where [Last Name] = '" + strLastName + "'");
•
You can create a PreparedStatement outside of Java and call it
from the Java code.
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Programming with Java
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Employees table in MerryPeas
Database
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Retrieving a Record
•
When you first open a ResultSet, the current-record pointer is
located just prior to the first record.
•
You call the ResultSet's next method, which has a dual
purpose.
•
The next method moves the current-record pointer (also called
the cursor) to the first record and returns boolean true or false.
•
A true means that the next record exists and a false means that
no more records exist in the ResultSet (the operation was
successful or unsuccessful).
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Programming with Java
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Retrieving a Record Continued
•
You must have a current record before you can access the data
fields in a record.
•
This code moves to the first (next) record and adds the contents
of the Last Name field to a List component called lstNames.
•
The getString method retrieves the data for the specified
string field.
•
Make sure to enclose the next method in a try/catch block.
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Programming with Java
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Looping through a ResultSet
•
You can step through all records in a ResultSet using a loop.
•
The boolean condition (rsResultSet.next()) tests true as long as
records remain in the ResultSet, and tests false when no more
records remain.
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Programming with Java
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Accessing the Data Fields
•
Access the data in database fields by using the appropriate
method for the data type.
•
Use getString for string fields; use getFloat or getInt for float
or int fields.
•
You must give the correct field name, including spacing. For
example :
lblFirstName.setText(rsEmployees.getString("First Name"));
lblSSN.setText(rsEmployees.getString("SSN"));
lblPhone.setText(rsEmployees.getString("Phone Number"));
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Programming with Java
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Accessing the Data Fields
Continued
•
If you declare a ResultSet object at the class level, you cannot
access the ResultSet in any method other than the one in which
it was created.
•
You must actually pass the ResultSet to any other method that
needs to reference the data.
•
Failure to do this results in a null pointer error.
© 2002 The McGraw-Hill Companies, Inc. All rights reserved.
Programming with Java
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Closing the Connection
•
Make sure that the database connection is closed at the
termination of the program.
•
Place the statements in the stop method of an applet.
•
If you are writing an application, place the code in your exit
routine.
•
You can avoid an exception for attempting to close a connection
that doesn't exist by testing for null.
© 2002 The McGraw-Hill Companies, Inc. All rights reserved.
Programming with Java
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SQL
•
Java's JDBC uses Structured Query Language (SQL) to create a
ResultSet.
•
SQL is an industry-standard language that you can use to access
data in nearly every relational database format.
•
Although the basic SQL statements are standardized, some
vendors include extensions that are non-standard.
•
You will see several ways to use an SQL statement in code to
create a ResultSet or to directly update a database.
© 2002 The McGraw-Hill Companies, Inc. All rights reserved.
Programming with Java
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Types of SQL Statements
•
SQL Select statements select data from a database and return
that data to the program.
•
You can specify which fields from which table or tables, and
select only certain records based on criteria.
•
You can also write SQL statements that perform actions on the
database rather than just select data.
•
The actions that you can perform include inserting records,
deleting and updating records, as well as modifying the
structure of a database, such as adding tables and fields.
© 2002 The McGraw-Hill Companies, Inc. All rights reserved.
Programming with Java
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Writing SQL Select Statements
•
Multiple-word field names must be enclosed in square
brackets or accent grave marks.
•
The optional Distinct drops out duplicates so that no two
records are alike.
© 2002 The McGraw-Hill Companies, Inc. All rights reserved.
Programming with Java
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The SQL Select Statement—
General Form
SELECT [Distinct] Field(s) FROM Table(s) [IN Database]
[WHERE Criteria]
[GROUP BY Field(s)]
[HAVING GroupCriteria]
[ORDER BY Field(s)];
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Programming with Java
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The SQL Select Statement—
Examples
SELECT [Last Name], [First Name], Phone FROM Patient
ORDER BY [Last Name], [First Name];
SELECT DISTINCT `Last Name` FROM Patient
ORDER BY `Last Name`;
SELECT * FROM Patient
WHERE [Last Name] = "'" + txtSearch.getText() + "';"
SELECT * FROM Patient, Insurance
WHERE Patient.[Insurance Company Code] =
InsuranceCompany.Code;
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Programming with Java
42
Writing SQL Select Statements
Continued
•
Note that the last example joins the Patient and Insurance tables
so that the actual name of the company, not just the Code, is
included in the results.
•
This easy method for joining tables creates a ResultSet that is
nonupdateable and does not include any patients without a
matching entry in the InsuranceCompany table, including those
with no insurance.
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Programming with Java
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Writing SQL Select Statements
Continued
•
To make a joined ResultSet updateable and complete, you must
use the Join clause of the SQL Select statement.
•
The closing semi-colon (;) is specified in the SQL standards.
Many versions of SQL do not require the semi-colon, but some
do.
© 2002 The McGraw-Hill Companies, Inc. All rights reserved.
Programming with Java
44
SQL Queries
•
A few simple queries:
•
Example :SELECT [Last Name], [First Name] FROM
Employee;
•
The ResultSet created from this SQL statement will include
only the Last Name and First Name fields from the Employee
table.
•
Example: SELECT * FROM Employee;
•
This ResultSet will include all fields (* = all fields) from the
Employee table.
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Programming with Java
45
SQL Queries Continued
•
Example:
SELECT Employee.[Last Name], Employee.[First Name],
InsuranceCompany.Name
FROM Employee, InsuranceCompany
WHERE Employee.[Insurance Company Code] =
InsuranceCompany.Code;
•
This ResultSet will include three fields: Last Name and First
Name from the Employee table and the Name field from the
InsuranceCompany table.
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Programming with Java
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SQL Queries Continued
•
It joins the two tables, in order to retrieve the company name
that matches the Insurance Company Code field from the
Employee table.
•
For field names that contain a space, you must enclose the
name with either square brackets or the accent grave symbol
(`).
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Programming with Java
47
Where Clause
•
You can use a Where clause to join tables.
"WHERE Employee.[Insurance Company Code] =
InsuranceCompany.Code"
•
You can also use a Where clause to select only those records
that meet specific criteria.
"WHERE Employee.[Insurance Company Code] = ‘ABC‘ "
"WHERE Employee.[Insurance Company Code] = '" +
strSelectedCompany + "'"
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Programming with Java
48
Where Clause Continued
•
Note the single quotes and double quotes.
•
You can include multiple conditions in a Where clause.
"WHERE Employee.[Insurance Company Code] =
InsuranceCompany.Code " + "AND Employee.[Insurance
Company Code] = '" + strSelectedCompany + "'"
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Programming with Java
49
Comparing Database Fields to
Java Fields
•
The syntax of the criteria in a Where clause depends on the
location of the data.
•
You must specify database fields differently from program
variables.
•
And string fields must be compared only to string data;
numeric fields compared only to numeric data.
•
Otherwise your program will fail.
© 2002 The McGraw-Hill Companies, Inc. All rights reserved.
Programming with Java
50
Comparing Database Fields to
Java Fields Continued
•
Look at the following examples:
//Compare a string field to a string variable
"WHERE [Last Name] = '" + strName + "'"
//Compare a string field to a string from a text component
"WHERE [Last Name] Like '" + txtSearchName.getText() + "'"
//Compare a string field to a string constant
"WHERE [Last Name] = 'Jones'"
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Programming with Java
51
Comparing Database Fields to
Java Fields Continued
//Compare a numeric field to a numeric variable
"WHERE [Duration] = " + intSearchMinutes
//Compare a numeric field to a property
"WHERE [Duration] = " +
Float.valueOf(txtNumber.getText()).floatValue()
//Compare a numeric field to a numeric constant
"WHERE [Duration] = 15"
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Programming with Java
52
The Order by Clause
•
It is incredibly easy to sort your ResultSet in SQL—just use
the Order By clause.
•
You can order by one or more fields, in ascending or
descending sequence.
•
If you don't specify the direction, ascending is assumed.
"ORDER BY [Last Name], [First Name]"
"ORDER BY InsuranceCompany.Name ASC"
"ORDER BY DateDue DESC"
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Programming with Java
53
Updating a Database
•
You can also update a database from a Java program.
•
You use the Statement object's executeUpdate method, which
works with SQL statements.
•
Many different errors can occur while updating a database.
•
If you get a "null pointer exception," it is likely that the update
or query has failed.
•
With an update this could be a result of many different factors—
a mandatory or key field left blank, duplicate records, incorrect
data type, or not a valid format.
© 2002 The McGraw-Hill Companies, Inc. All rights reserved.
Programming with Java
54
SQL Updates
•
You can use your validation techniques from Chapter 6 to avoid
some of the problems; others problems may not be easy to find.
•
You will see a different way to program, using methods to
directly update a database.
•
You will write SQL action queries, such as Insert, Delete, and
Update, and execute the actions using the executeUpdate method
of the Statement object.
© 2002 The McGraw-Hill Companies, Inc. All rights reserved.
Programming with Java
55
SQL Updates
•
Action queries operate directly on the database, not on any
open ResultSet.
•
To execute an SQL action query on a database, you must have
an open Connection.
•
The user can add new records, delete records, or edit (modify)
existing records.
•
For the delete and edit options, the user first displays the
desired record and then clicks the appropriate button.
© 2002 The McGraw-Hill Companies, Inc. All rights reserved.
Programming with Java
56
Adding a Record
•
Adding a record is a two-step process.
•
When the user clicks the Add button, you must clear the text
fields for data entry and set the focus in the first text field.
•
After the user enters the data for the new record, he or she will
click the OK (or Cancel) button.
•
To actually add a new record to the database, you use the SQL
Insert statement.
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57
The SQL Insert Statement—
General Format
Insert Into TableName (Fieldlist) VALUES (ListOfValues);
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Programming with Java
58
The SQL Insert Statement—
Examples
Insert Into Employees ([Last Name], [First Name]) Values ('Berry', 'Terry');
strSQL = "Insert Into Employees ([Last Name], [First Name]) "
+ "VALUES ('"+ txtLastName.getText() + "', '"
+ txtFirstName.getText() + "');"
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User Interface for the Update
Program
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60
The Add Method
•
In the Add method, you must clear the contents of the text fields
so that the user can enter new values.
•
Also, it's a good idea to give the user only two choices once the
Add operation starts: OK (which saves the new record) or
Cancel (in case of a mind change).
•
We will use the Add button for a dual purpose.
 Once the Add is in progress, the button's caption changes to
"OK".
 After the user clicks OK or Cancel, the caption of the Add
button changes back to "Add"
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The Save Method
•
The Save method, which is executed when the user clicks OK
at the end of an Add, must perform validation.
•
If the data passes the validation, then the record is added.
•
Use the Statement object's executeUpdate method to execute
the SQL Insert Into statement.
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Programming with Java
62
Modifying an Existing Record
•
To allow users to modify an existing record, you need to
display the record's current contents and allow them to make
changes.
•
In the example, the user selects a Last Name from the list and
the field contents display in the text fields.
•
Then the user should be able to change the contents of the
fields and click the Save button.
© 2002 The McGraw-Hill Companies, Inc. All rights reserved.
Programming with Java
63
The SQL Update Statement—
General Format
Update TableName
Set FieldName = FieldValue, FieldName = FieldValue, . . .
Where Criteria;
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Programming with Java
64
The SQL Update Statement—
Examples
Update Employee
Set [Last Name] = 'Bowser'
Where [Employee Number] = 500;
Update Visit
Set Date = #1/1/2000#
Where Date = #1/1/1900#;
strSQL = "Update Employees " +
"Set [Last Name] = '" +
txtLastName.getText() + "', " +
"[First Name] = '" + txtFirstName.getText() + "', " +
"Where [SSN] = " + txtSSN.getText();
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Programming with Java
65
The Edit Method
•
The Edit method executes when the user clicks the Save
button after making changes to an existing record.
•
What if the user clicks the Save button before displaying the
record first.
•
We need to first check if the record is displayed.
•
After validation, use the executeUpdate method of the
Statement object, passing it an SQL Update statement.
© 2002 The McGraw-Hill Companies, Inc. All rights reserved.
Programming with Java
66
Deleting a Record
•
Deleting a record is similar to modifying an existing record.
•
The user must display the record to delete and then click the
Delete button.
•
The Delete statement requires a Where clause that specifies
which record(s) to delete.
© 2002 The McGraw-Hill Companies, Inc. All rights reserved.
Programming with Java
67
The Delete Method
•
When writing the routine for handling the delete, consider the
tasks:
 Delete the record from the database.
 Delete the name from the list.
 Clear the record from the text fields.
•
To delete the record from the database, use the
executeUpdate method of the Statement object to
execute an SQL Delete Statement.
© 2002 The McGraw-Hill Companies, Inc. All rights reserved.
Programming with Java
68
Joins
•
One of the primary characteristics of relational databases is that
data are stored in multiple tables that are related to each other
by common fields.
•
Data can be stored once and used in many places by using the
relationships between tables.
•
You will often want to select some fields from one table and
other fields from another related table, maybe even fields from
several related tables.
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Programming with Java
69
Joins
•
Although you can join tables using the Where clause, the
resulting ResultSet is not updateable.
•
If you want the user to be able to update the data, you must use
a Join clause in the SQL Select statement.
•
Joins are of three types: inner join, left join, and right join.
•
The left join and right join are often called left outer join and
right outer join.
© 2002 The McGraw-Hill Companies, Inc. All rights reserved.
Programming with Java
70
SQL Joins
Join Type
Selects
Inner join
Only records that have matching records in both tables.
Left join
All records from the first table and only the matching records
from the second table.
Right join
All records from the second table and only the matching
records from the first table.
© 2002 The McGraw-Hill Companies, Inc. All rights reserved.
Programming with Java
71
The Join Clause
•
To code a Join clause, name only the first table in the From
clause (this becomes the left table).
•
Then specify the join type and the second table name; then
write the relationship for the join using the On clause.
•
Example:
Select [Last Name], [First Name], [Insurance Company Code], Name From
Patient
Left Join InsuranceCompany On
Patient.[Insurance Company Code] = InsuranceCompany.Code;
© 2002 The McGraw-Hill Companies, Inc. All rights reserved.