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<th width="60%" align="center">Chapter 9. Using Cursors</th>
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<div class="sect1" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both"><a id="Positioning"></a>Getting Records Using the Cursor</h2>
</div>
</div>
<div></div>
</div>
<p>
To iterate over database records, from the first record to
the last, simply open the cursor and then use the
<tt class="methodname">Cursor.getNext()</tt>
method.
For example:
</p>
<a id="java_cursor3"></a>
<pre class="programlisting">package db.GettingStarted;
import com.sleepycat.db.Database;
import com.sleepycat.db.DatabaseEntry;
import com.sleepycat.db.DatabaseException;
import com.sleepycat.db.Cursor;
import com.sleepycat.db.LockMode;
import com.sleepycat.db.OperationStatus;
...
Cursor cursor = null;
try {
...
Database myDatabase = null;
// Database open omitted for brevity
...
// Open the cursor.
cursor = myDatabase.openCursor(null, null);
// Cursors need a pair of DatabaseEntry objects to operate. These hold
// the key and data found at any given position in the database.
DatabaseEntry foundKey = new DatabaseEntry();
DatabaseEntry foundData = new DatabaseEntry();
// To iterate, just call getNext() until the last database record has been
// read. All cursor operations return an OperationStatus, so just read
// until we no longer see OperationStatus.SUCCESS
while (cursor.getNext(foundKey, foundData, LockMode.DEFAULT) ==
OperationStatus.SUCCESS) {
// getData() on the DatabaseEntry objects returns the byte array
// held by that object. We use this to get a String value. If the
// DatabaseEntry held a byte array representation of some other data
// type (such as a complex object) then this operation would look
// considerably different.
String keyString = new String(foundKey.getData(), "UTF-8");
String dataString = new String(foundData.getData(), "UTF-8");
System.out.println("Key | Data : " + keyString + " | " +
dataString + "");
}
} catch (DatabaseException de) {
System.err.println("Error accessing database." + de);
} finally {
// Cursors must be closed.
cursor.close();
}</pre>
<p>
To iterate over the database from the last record to the first,
instantiate the cursor, and then
use <tt class="methodname">Cursor.getPrev()</tt> until you read the first record in
the database. For example:
</p>
<a id="java_cursor4"></a>
<pre class="programlisting">package db.GettingStarted;
import com.sleepycat.db.Cursor;
import com.sleepycat.db.Database;
import com.sleepycat.db.DatabaseEntry;
import com.sleepycat.db.DatabaseException;
import com.sleepycat.db.LockMode;
import com.sleepycat.db.OperationStatus;
...
Cursor cursor = null;
Database myDatabase = null;
try {
...
// Database open omitted for brevity
...
// Open the cursor.
cursor = myDatabase.openCursor(null, null);
// Get the DatabaseEntry objects that the cursor will use.
DatabaseEntry foundKey = new DatabaseEntry();
DatabaseEntry foundData = new DatabaseEntry();
// Iterate from the last record to the first in the database
while (cursor.getPrev(foundKey, foundData, LockMode.DEFAULT) ==
OperationStatus.SUCCESS) {
String theKey = new String(foundKey.getData(), "UTF-8");
String theData = new String(foundData.getData(), "UTF-8");
System.out.println("Key | Data : " + theKey + " | " + theData + "");
}
} catch (DatabaseException de) {
System.err.println("Error accessing database." + de);
} finally {
// Cursors must be closed.
cursor.close();
}</pre>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="cursorsearch"></a>Searching for Records</h3>
</div>
</div>
<div></div>
</div>
<p>
You can use cursors to search for database records. You can search based
on just a key, or you can search based on both the key and the data.
You can also perform partial matches if your database supports sorted
duplicate sets. In all cases, the key and data parameters of these
methods are filled with the key and data values of the database record
to which the cursor is positioned as a result of the search.
</p>
<p>
Also, if the search fails, then cursor's state is left unchanged
and
<tt class="literal">OperationStatus.NOTFOUND</tt>
is returned.
</p>
<p>
The following <tt class="classname">Cursor</tt> methods allow you to
perform database searches:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
<tt class="methodname">Cursor.getSearchKey()</tt>
</p>
<p>
Moves the cursor to the first record in the database with
the specified key.
</p>
</li>
<li>
<p>
<tt class="methodname">Cursor.getSearchKeyRange()</tt>
</p>
<p>
<span>Identical to
<tt class="methodname">Cursor.getSearchKey()</tt>
unless you are using the BTree access. In this case, the cursor
moves</span>
to the first record in the database whose
key is greater than or equal to the specified key. This comparison
is determined by the
<span>comparator</span>
that you provide for the database. If no
<span>comparator</span>
is provided, then the default
lexicographical sorting is used.
</p>
<p>
For example, suppose you have database records that use the
following
<span>Strings</span>
as keys:
</p>
<pre class="programlisting">Alabama
Alaska
Arizona</pre>
<p>
Then providing a search key of <tt class="literal">Alaska</tt> moves the
cursor to the second key noted above. Providing a key of
<tt class="literal">Al</tt> moves the cursor to the first key (<tt class="literal">Alabama</tt>), providing
a search key of <tt class="literal">Alas</tt> moves the cursor to the second key
(<tt class="literal">Alaska</tt>), and providing a key of <tt class="literal">Ar</tt> moves the
cursor to the last key (<tt class="literal">Arizona</tt>).
</p>
</li>
<li>
<p>
<tt class="methodname">Cursor.getSearchBoth()</tt>
</p>
<p>
Moves the cursor to the first record in the database that uses
the specified key and data.
</p>
</li>
<li>
<p>
<tt class="methodname">Cursor.getSearchBothRange()</tt>
</p>
<p>
Moves the cursor to the first record in the database whose key matches the specified
key and whose data is
greater than or equal to the specified data. If the database supports
duplicate records, then on matching the key, the cursor is moved to
the duplicate record with the smallest data that is greater than or
equal to the specified data.
</p>
<p>
For example,
<span>suppose your database uses BTree
and it has </span>
database records that use the following key/data pairs:
</p>
<pre class="programlisting">Alabama/Athens
Alabama/Florence
Alaska/Anchorage
Alaska/Fairbanks
Arizona/Avondale
Arizona/Florence </pre>
<p>then providing:</p>
<div class="informaltable">
<table border="1" width="80%">
<colgroup>
<col />
<col />
<col />
</colgroup>
<thead>
<tr>
<th>a search key of ...</th>
<th>and a search data of ...</th>
<th>moves the cursor to ...</th>
</tr>
</thead>
<tbody>
<tr>
<td>Alaska</td>
<td>Fa</td>
<td>Alaska/Fairbanks</td>
</tr>
<tr>
<td>Arizona</td>
<td>Fl</td>
<td>Arizona/Florence</td>
</tr>
<tr>
<td>Alaska</td>
<td>An</td>
<td>Alaska/Anchorage</td>
</tr>
</tbody>
</table>
</div>
</li>
</ul>
</div>
<p>
For example, assuming a database containing sorted duplicate records of
U.S. States/U.S Cities key/data pairs (both as
<span>Strings),</span>
then the following code fragment can be used to position the cursor
to any record in the database and print its key/data values:
</p>
<a id="java_cursor5"></a>
<pre class="programlisting">package db.GettingStarted;
import com.sleepycat.db.Cursor;
import com.sleepycat.db.Database;
import com.sleepycat.db.DatabaseEntry;
import com.sleepycat.db.DatabaseException;
import com.sleepycat.db.LockMode;
import com.sleepycat.db.OperationStatus;
...
// For this example, hard code the search key and data
String searchKey = "Alaska";
String searchData = "Fa";
Cursor cursor = null;
Database myDatabase = null;
try {
...
// Database open omitted for brevity
...
// Open the cursor.
cursor = myDatabase.openCursor(null, null);
DatabaseEntry theKey =
new DatabaseEntry(searchKey.getBytes("UTF-8"));
DatabaseEntry theData =
new DatabaseEntry(searchData.getBytes("UTF-8"));
// Open a cursor using a database handle
cursor = myDatabase.openCursor(null, null);
// Perform the search
OperationStatus retVal = cursor.getSearchBothRange(theKey, theData,
LockMode.DEFAULT);
// NOTFOUND is returned if a record cannot be found whose key
// matches the search key AND whose data begins with the search data.
if (retVal == OperationStatus.NOTFOUND) {
System.out.println(searchKey + "/" + searchData +
" not matched in database " +
myDatabase.getDatabaseName());
} else {
// Upon completing a search, the key and data DatabaseEntry
// parameters for getSearchBothRange() are populated with the
// key/data values of the found record.
String foundKey = new String(theKey.getData(), "UTF-8");
String foundData = new String(theData.getData(), "UTF-8");
System.out.println("Found record " + foundKey + "/" + foundData +
"for search key/data: " + searchKey +
"/" + searchData);
}
} catch (Exception e) {
// Exception handling goes here
} finally {
// Make sure to close the cursor
cursor.close();
}</pre>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="getdups"></a>Working with Duplicate Records</h3>
</div>
</div>
<div></div>
</div>
<p>
A record is a duplicate of another record if the two records share the
same key. For duplicate records, only the data portion of the record is unique.
</p>
<p>
Duplicate records are supported only for the BTree or Hash access methods.
For information on configuring your database to use duplicate records,
see <a href="btree.html#duplicateRecords">Allowing Duplicate Records</a>.
</p>
<p>
If your database supports duplicate records, then it can potentially
contain multiple records that share the same key.
<span>By default, normal database
get operations will only return the first such record in a set
of duplicate records. Typically, subsequent duplicate records are
accessed using a cursor.
</span>
The following
<span><tt class="methodname">Cursor</tt> methods</span>
are interesting when working with databases that support duplicate records:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
<span>
<tt class="methodname">Cursor.getNext()</tt>,
<tt class="methodname">Cursor.getPrev()</tt>
</span>
</p>
<p>
Shows the next/previous record in the database, regardless of
whether it is a duplicate of the current record. For an example of
using these methods, see <a href="Positioning.html">Getting Records Using the Cursor</a>.
</p>
</li>
<li>
<p>
<tt class="methodname">Cursor.getSearchBothRange()</tt>
</p>
<p>
Useful for seeking the cursor to a specific record, regardless of
whether it is a duplicate record. See <a href="Positioning.html#cursorsearch">Searching for Records</a> for more
information.
</p>
</li>
<li>
<p>
<span>
<tt class="methodname">Cursor.getNextNoDup()</tt>,
<tt class="methodname">Cursor.getPrevNoDup()</tt>
</span>
</p>
<p>
Gets the next/previous non-duplicate record in the database. This
allows you to skip over all the duplicates in a set of duplicate
records. If you call
<span><tt class="methodname">Cursor.getPrevNoDup()</tt>,</span>
then the cursor is positioned to the last record for the previous
key in the database. For example, if you have the following records
in your database:
</p>
<pre class="programlisting">Alabama/Athens
Alabama/Florence
Alaska/Anchorage
Alaska/Fairbanks
Arizona/Avondale
Arizona/Florence</pre>
<p>
and your cursor is positioned to <tt class="literal">Alaska/Fairbanks</tt>,
and you then call
<span><tt class="methodname">Cursor.getPrevNoDup()</tt>,</span>
then the cursor is positioned to Alabama/Florence. Similarly, if
you call
<span><tt class="methodname">Cursor.getNextNoDup()</tt>,</span>
then the cursor is positioned to the first record corresponding to
the next key in the database.
</p>
<p>
If there is no next/previous key in the database, then
<tt class="literal">OperationStatus.NOTFOUND</tt>
is returned, and the cursor is left unchanged.
</p>
</li>
<li>
<p>
</p>
<p>
Gets the
<span>next</span>
record that shares the current key. If the
cursor is positioned at the last record in the duplicate set and
you call
<span><tt class="methodname">Cursor.getNextDup()</tt>,</span>
then
<tt class="literal">OperationStatus.NOTFOUND</tt>
is returned and the cursor is left unchanged.
<span>
Likewise, if you call
<tt class="methodname">getPrevDup()</tt> and the
cursor is positioned at the first record in the duplicate set, then
<tt class="literal">OperationStatus.NOTFOUND</tt> is returned and the
cursor is left unchanged.
</span>
</p>
</li>
<li>
<p>
<tt class="methodname">Cursor.count()</tt>
</p>
<p>Returns the total number of records that share the current key.</p>
</li>
</ul>
</div>
<p>
For example, the following code fragment positions a cursor to a key
<span>and displays it and all its
duplicates.</span>
<span>Note that the following code fragment assumes that the database contains
only String objects for the keys and data.</span>
</p>
<a id="java_cursor6"></a>
<pre class="programlisting">package db.GettingStarted;
import com.sleepycat.db.Cursor;
import com.sleepycat.db.Database;
import com.sleepycat.db.DatabaseEntry;
import com.sleepycat.db.DatabaseException;
import com.sleepycat.db.LockMode;
import com.sleepycat.db.OperationStatus;
...
Cursor cursor = null;
Database myDatabase = null;
try {
...
// Database open omitted for brevity
...
// Create DatabaseEntry objects
// searchKey is some String.
DatabaseEntry theKey = new DatabaseEntry(searchKey.getBytes("UTF-8"));
DatabaseEntry theData = new DatabaseEntry();
// Open a cursor using a database handle
cursor = myDatabase.openCursor(null, null);
// Position the cursor
// Ignoring the return value for clarity
OperationStatus retVal = cursor.getSearchKey(theKey, theData,
LockMode.DEFAULT);
// Count the number of duplicates. If the count is greater than 1,
// print the duplicates.
if (cursor.count() > 1) {
while (retVal == OperationStatus.SUCCESS) {
String keyString = new String(theKey.getData(), "UTF-8");
String dataString = new String(theData.getData(), "UTF-8");
System.out.println("Key | Data : " + keyString + " | " +
dataString + "");
retVal = cursor.getNextDup(theKey, theData, LockMode.DEFAULT);
}
}
} catch (Exception e) {
// Exception handling goes here
} finally {
// Make sure to close the cursor
cursor.close();
}</pre>
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