Accessing Databases with JDBC - Java: How to Program Early Objects - page 1067

constant integer from class Types (package java.sql ) indicating the type of a specified
column. Programs can use these values in a switch statement to invoke ResultSet
methods that return the column values as appropriate Java types. For example, if the type
of a column is Types.INTEGER , ResultSet method getInt can be used to get the column
value as an int . For simplicity, this example treats each value as an Object . We retrieve
each column value with ResultSet method getObject (line 40), then print the Object 's
String representation. ResultSet get methods typically receive as an argument either a
column number (as an int ) or a column name (as a String ) indicating which column's
value to obtain. Unlike array indices, ResultSet column numbers start at 1 .
Performance Tip 24.1
If a query specifies the exact columns to select from the database, the ResultSet contains
the columns in the specified order. In this case, using the column number to obtain the
column's value is more efficient than using the column name. The column number pro-
vides direct access to the specified column. Using the column name requires a search of the
column names to locate the appropriate column.
Error-Prevention Tip 24.1
Using column names to obtain values from a ResultSet produces code that is less error
prone than obtaining values by column number—you don't need to remember the col-
umn order. Also, if the column order changes, your code does not have to change.
Common Programming Error 24.6
Specifying column index 0 when obtaining values from a ResultSet causes a SQL-
Exception —the first column index in a ResultSet is always 1.
When the end of the try block is reached (line 43), the close method is called on the
ResultSet , Statement and Connection that were obtained by the try -with-resources
statement.
Common Programming Error 24.7
A SQLException occurs if you attempt to manipulate a ResultSet after closing the
Statement that created it. The ResultSet is discarded when the Statement is closed.
Software Engineering Observation 24.5
Each Statement object can open only one ResultSet object at a time. When a Statement
returns a new ResultSet , the Statement closes the prior ResultSet . To use multiple
ResultSet s in parallel, separate Statement objects must return the ResultSet s.
24.6.2 Querying the books Database
The next example (Figs. 24.25 and 24.28) allows the user to enter any query into the pro-
gram. The example displays the result of a query in a JTable , using a TableModel object
to provide the ResultSet data to the JTable . A JTable is a swing GUI component that
can be bound to a database to display the results of a query. Class ResultSetTableModel
(Fig. 24.25) performs the connection to the database via a TableModel and maintains the
ResultSet . Class DisplayQueryResults (Fig. 24.28) creates the GUI and specifies an in-
stance of class ResultSetTableModel to provide data for the JTable .