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691 lines
25 KiB
HTML
691 lines
25 KiB
HTML
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml">
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
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<title>Getting Records Using the Cursor</title>
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<link rel="stylesheet" href="gettingStarted.css" type="text/css" />
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<meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
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<link rel="start" href="index.html" title="Getting Started with Berkeley DB Java Edition" />
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<link rel="up" href="Cursors.html" title="Chapter 9. Using Cursors" />
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<link rel="prev" href="Cursors.html" title="Chapter 9. Using Cursors" />
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<link rel="next" href="PutEntryWCursor.html" title="Putting Records Using Cursors" />
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</head>
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<body>
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<div xmlns="" class="navheader">
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<div class="libver">
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<p>Library Version 12.2.7.5</p>
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</div>
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<table width="100%" summary="Navigation header">
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<tr>
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<th colspan="3" align="center">Getting Records Using the Cursor</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="Cursors.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 9. Using Cursors</th>
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<td width="20%" align="right"> <a accesskey="n" href="PutEntryWCursor.html">Next</a></td>
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</tr>
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</table>
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<hr />
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</div>
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<div class="sect1" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h2 class="title" style="clear: both"><a id="Positioning"></a>Getting Records Using the Cursor</h2>
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</div>
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</div>
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</div>
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<div class="toc">
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<dl>
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<dt>
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<span class="sect2">
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<a href="Positioning.html#diskorderedcursors">Disk Ordered Cursors</a>
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</span>
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</dt>
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<dt>
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<span class="sect2">
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<a href="Positioning.html#cursorsearch">Searching for Records</a>
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</span>
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</dt>
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<dt>
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<span class="sect2">
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<a href="Positioning.html#getdups">Working with Duplicate Records</a>
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</span>
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</dt>
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</dl>
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</div>
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<p>
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To iterate over database records, from the first record to
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the last, simply open the cursor and then use the
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<code class="methodname">Cursor.getNext()</code>
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method.
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For example:
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</p>
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<a id="je_cursor3"></a>
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<pre class="programlisting">package je.gettingStarted;
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import com.sleepycat.je.Cursor;
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import com.sleepycat.je.Database;
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import com.sleepycat.je.DatabaseEntry;
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import com.sleepycat.je.DatabaseException;
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import com.sleepycat.je.LockMode;
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import com.sleepycat.je.OperationStatus;
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...
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Cursor cursor = null;
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try {
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...
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// Database and environment open omitted for brevity
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...
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// Open the cursor.
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cursor = myDatabase.openCursor(null, null);
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// Cursors need a pair of DatabaseEntry objects to operate. These hold
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// the key and data found at any given position in the database.
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DatabaseEntry foundKey = new DatabaseEntry();
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DatabaseEntry foundData = new DatabaseEntry();
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// To iterate, just call getNext() until the last database record has
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// been read. All cursor operations return an OperationStatus, so just
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// read until we no longer see OperationStatus.SUCCESS
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while (cursor.getNext(foundKey, foundData, LockMode.DEFAULT) ==
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OperationStatus.SUCCESS) {
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// getData() on the DatabaseEntry objects returns the byte array
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// held by that object. We use this to get a String value. If the
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// DatabaseEntry held a byte array representation of some other
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// data type (such as a complex object) then this operation would
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// look considerably different.
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String keyString = new String(foundKey.getData(), "UTF-8");
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String dataString = new String(foundData.getData(), "UTF-8");
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System.out.println("Key | Data : " + keyString + " | " +
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dataString + "");
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}
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} catch (DatabaseException de) {
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System.err.println("Error accessing database." + de);
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} finally {
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// Cursors must be closed.
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cursor.close();
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}</pre>
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<p>
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To iterate over the database from the last record to the first,
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instantiate the cursor, and then
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use <code class="methodname">Cursor.getPrev()</code> until you read the first record in
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the database. For example:
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</p>
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<a id="je_cursor4"></a>
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<pre class="programlisting">package je.gettingStarted;
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import com.sleepycat.je.Cursor;
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import com.sleepycat.je.Database;
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import com.sleepycat.je.DatabaseEntry;
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import com.sleepycat.je.DatabaseException;
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import com.sleepycat.je.LockMode;
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import com.sleepycat.je.OperationStatus;
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...
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Cursor cursor = null;
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try {
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...
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// Database and environment open omitted for brevity
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...
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// Open the cursor.
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cursor = myDatabase.openCursor(null, null);
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// Get the DatabaseEntry objects that the cursor will use.
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DatabaseEntry foundKey = new DatabaseEntry();
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DatabaseEntry foundData = new DatabaseEntry();
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// Iterate from the last record to the first in the database
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while (cursor.getPrev(foundKey, foundData, LockMode.DEFAULT) ==
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OperationStatus.SUCCESS) {
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String theKey = new String(foundKey.getData(), "UTF-8");
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String theData = new String(foundData.getData(), "UTF-8");
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System.out.println("Key | Data : " + theKey + " | " +
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theData + "");
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}
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} catch (DatabaseException de) {
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System.err.println("Error accessing database." + de);
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} finally {
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// Cursors must be closed.
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cursor.close();
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}</pre>
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<div class="sect2" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h3 class="title"><a id="diskorderedcursors"></a>Disk Ordered Cursors</h3>
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</div>
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</div>
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</div>
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<p>
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The previous example shows how to scan through the records in
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your database sequentially; that is, in the record's sort order.
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This is mostly determined by the value contained in the records' keys
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(additional sorting is required in the case of duplicate
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records). However, you can use cursors to retrieve records based
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on how they are stored on disk. This can improve retrieval times,
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and is useful if your application needs to scan all the records
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in the database quickly, without concern for key sort order.
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You do this using the <code class="classname">DiskOrderedCursor</code>
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class.
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</p>
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<p>
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<code class="classname">DiskOrderedClass</code> works in the same way as
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a regular cusor: you simply open the cursor, then retrieve
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records one after another using the
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<code class="methodname">DiskOrderedClass.getNext()</code>
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method.
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</p>
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<p>
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You open a <code class="classname">DiskOrderedCursor</code>
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using the <code class="methodname">Database.openCursor()</code>
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method. This version of the method takes a single parameter: an
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instance of the
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<code class="classname">DiskOrderedCursorConfig</code>
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class, which can be used to configure various aspects of the
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<code class="classname">DiskOrderedCursor</code>
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class.
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</p>
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<a id="je_diskorderedcursor1"></a>
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<pre class="programlisting">package je.gettingStarted;
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import com.sleepycat.je.Database;
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import com.sleepycat.je.DatabaseEntry;
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import com.sleepycat.je.DatabaseException;
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import com.sleepycat.je.DiskOrderedCursor;
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import com.sleepycat.je.DiskOrderedCursorConfig;
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import com.sleepycat.je.LockMode;
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import com.sleepycat.je.OperationStatus;
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...
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DiskOrderedCursor dcursor = null;
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DiskOrderedCursorConfig docc = new DiskOrderedCursorConfig();
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try {
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...
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// Database and environment open omitted for brevity
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...
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// Open the cursor.
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dcursor = myDatabase.openCursor(docc);
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// Cursors need a pair of DatabaseEntry objects to operate. These hold
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// the key and data found at any given position in the database.
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DatabaseEntry foundKey = new DatabaseEntry();
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DatabaseEntry foundData = new DatabaseEntry();
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// To iterate, just call getNext() until the last database record has
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// been read. All cursor operations return an OperationStatus, so just
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// read until we no longer see OperationStatus.SUCCESS
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while (dcursor.getNext(foundKey, foundData, LockMode.DEFAULT) ==
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OperationStatus.SUCCESS) {
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// getData() on the DatabaseEntry objects returns the byte array
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// held by that object. We use this to get a String value. If the
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// DatabaseEntry held a byte array representation of some other
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// data type (such as a complex object) then this operation would
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// look considerably different.
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String keyString = new String(foundKey.getData(), "UTF-8");
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String dataString = new String(foundData.getData(), "UTF-8");
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System.out.println("Key | Data : " + keyString + " | " +
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dataString + "");
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}
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} catch (DatabaseException de) {
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System.err.println("Error accessing database." + de);
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} finally {
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// Cursors must be closed.
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cursor.close();
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}</pre>
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</div>
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<div class="sect2" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h3 class="title"><a id="cursorsearch"></a>Searching for Records</h3>
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</div>
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</div>
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</div>
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<p>
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You can use cursors to search for database records. You can search based
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on just a key, or you can search based on both the key and the data.
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You can also perform partial matches if your database supports sorted
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duplicate sets. In all cases, the key and data parameters of these
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methods are filled with the key and data values of the database record
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to which the cursor is positioned as a result of the search.
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</p>
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<p>
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Also, if the search fails, then cursor's state is left unchanged
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and
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<code class="literal">OperationStatus.NOTFOUND</code>
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is returned.
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</p>
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<p>
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The following <code class="classname">Cursor</code> methods allow you to
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perform database searches:
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</p>
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<div class="itemizedlist">
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<ul type="disc">
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<li>
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<p>
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<code class="methodname">Cursor.getSearchKey()</code>
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</p>
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<p>
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Moves the cursor to the first record in the database with
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the specified key.
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</p>
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</li>
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<li>
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<p>
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<code class="methodname">Cursor.getSearchKeyRange()</code>
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</p>
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<p>
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<span>Moves the cursor</span>
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to the first record in the database whose
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key is greater than or equal to the specified key. This comparison
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is determined by the
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<span>comparator</span>
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that you provide for the database. If no
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<span>comparator</span>
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is provided, then the default
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<span> unsigned byte-by-byte </span>
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lexicographical sorting is used.
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</p>
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<p>
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For example, suppose you have database records that use the
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following
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<span>Strings</span>
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as keys:
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</p>
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<pre class="programlisting">Alabama
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Alaska
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Arizona</pre>
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<p>
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Then providing a search key of <code class="literal">Alaska</code> moves the
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cursor to the second key noted above. Providing a key of
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<code class="literal">Al</code> moves the cursor to the first key (<code class="literal">Alabama</code>), providing
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a search key of <code class="literal">Alas</code> moves the cursor to the second key
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(<code class="literal">Alaska</code>), and providing a key of <code class="literal">Ar</code> moves the
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cursor to the last key (<code class="literal">Arizona</code>).
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</p>
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</li>
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<li>
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<p>
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<code class="methodname">Cursor.getSearchBoth()</code>
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</p>
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<p>
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Moves the cursor to the first record in the database that uses
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the specified key and data.
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</p>
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</li>
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<li>
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<p>
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<code class="methodname">Cursor.getSearchBothRange()</code>
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</p>
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<p>
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Moves the cursor to the first record in the database whose key matches the specified
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key and whose data is
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greater than or equal to the specified data. If the database supports
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duplicate records, then on matching the key, the cursor is moved to
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the duplicate record with the smallest data that is greater than or
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equal to the specified data.
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</p>
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<p>
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For example,
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<span>suppose you have</span>
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database records that use the following key/data pairs:
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</p>
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<pre class="programlisting">Alabama/Athens
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Alabama/Florence
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Alaska/Anchorage
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Alaska/Fairbanks
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Arizona/Avondale
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Arizona/Florence </pre>
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<p>then providing:</p>
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<div class="informaltable">
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<table border="1" width="80%">
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<colgroup>
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<col />
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<col />
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<col />
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</colgroup>
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<thead>
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<tr>
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<th>a search key of ...</th>
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<th>and a search data of ...</th>
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<th>moves the cursor to ...</th>
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</tr>
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</thead>
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<tbody>
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<tr>
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<td>Alaska</td>
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<td>Fa</td>
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<td>Alaska/Fairbanks</td>
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</tr>
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<tr>
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<td>Arizona</td>
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<td>Fl</td>
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<td>Arizona/Florence</td>
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</tr>
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<tr>
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<td>Alaska</td>
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<td>An</td>
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<td>Alaska/Anchorage</td>
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</tr>
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</tbody>
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</table>
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</div>
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</li>
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</ul>
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</div>
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<p>
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For example, assuming a database containing sorted duplicate records of
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U.S. States/U.S Cities key/data pairs (both as
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<span>Strings),</span>
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then the following code fragment can be used to position the cursor
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to any record in the database and print its key/data values:
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<span>
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||
</span>
|
||
</p>
|
||
<a id="je_cursor5"></a>
|
||
<pre class="programlisting">package je.gettingStarted;
|
||
|
||
import com.sleepycat.je.Cursor;
|
||
import com.sleepycat.je.Database;
|
||
import com.sleepycat.je.DatabaseEntry;
|
||
import com.sleepycat.je.DatabaseException;
|
||
import com.sleepycat.je.LockMode;
|
||
import com.sleepycat.je.OperationStatus;
|
||
|
||
...
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||
|
||
// For this example, hard code the search key and data
|
||
String searchKey = "Alaska";
|
||
String searchData = "Fa";
|
||
|
||
Cursor cursor = null;
|
||
try {
|
||
...
|
||
// Database and environment 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>
|
||
<p>
|
||
If your database supports duplicate records, then it can potentially
|
||
contain multiple records that share the same key.
|
||
|
||
<span>
|
||
Using normal database get operations, you can only ever obtain the
|
||
first such record in a set of duplicate records. To access subsequent
|
||
duplicates, use a cursor.
|
||
</span>
|
||
|
||
|
||
|
||
The following
|
||
<span><code class="methodname">Cursor</code> methods</span>
|
||
|
||
|
||
are interesting when working with databases that support duplicate records:
|
||
</p>
|
||
<div class="itemizedlist">
|
||
<ul type="disc">
|
||
<li>
|
||
<p>
|
||
<span>
|
||
<code class="methodname">Cursor.getNext()</code>,
|
||
<code class="methodname">Cursor.getPrev()</code>
|
||
</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 class="xref" href="Positioning.html" title="Getting Records Using the Cursor">Getting Records Using the Cursor</a>.
|
||
</p>
|
||
</li>
|
||
<li>
|
||
<p>
|
||
<code class="methodname">Cursor.getSearchBothRange()</code>
|
||
|
||
</p>
|
||
<p>
|
||
Useful for seeking the cursor to a specific record, regardless of
|
||
whether it is a duplicate record. See <a class="xref" href="Positioning.html#cursorsearch" title="Searching for Records">Searching for Records</a> for more
|
||
information.
|
||
</p>
|
||
</li>
|
||
<li>
|
||
<p>
|
||
<span>
|
||
<code class="methodname">Cursor.getNextNoDup()</code>,
|
||
<code class="methodname">Cursor.getPrevNoDup()</code>
|
||
</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><code class="methodname">Cursor.getPrevNoDup()</code>,</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 <code class="literal">Alaska/Fairbanks</code>,
|
||
and you then call
|
||
<span><code class="methodname">Cursor.getPrevNoDup()</code>,</span>
|
||
|
||
then the cursor is positioned to Alabama/Florence. Similarly, if
|
||
you call
|
||
<span><code class="methodname">Cursor.getNextNoDup()</code>,</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
|
||
<code class="literal">OperationStatus.NOTFOUND</code>
|
||
|
||
is returned, and the cursor is left unchanged.
|
||
</p>
|
||
</li>
|
||
<li>
|
||
<p>
|
||
<span>
|
||
<code class="methodname">Cursor.getNextDup()</code>,
|
||
<code class="methodname">Cursor.getPrevDup()</code>
|
||
</span>
|
||
|
||
</p>
|
||
<p>
|
||
|
||
Gets the
|
||
<span>next/previous</span>
|
||
|
||
record that shares the current key. If the
|
||
cursor is positioned at the last record in the duplicate set and
|
||
you call
|
||
<span><code class="methodname">Cursor.getNextDup()</code>,</span>
|
||
|
||
|
||
then
|
||
<code class="literal">OperationStatus.NOTFOUND</code>
|
||
|
||
is returned and the cursor is left unchanged.
|
||
<span>
|
||
Likewise, if you call
|
||
<code class="methodname">getPrevDup()</code> and the
|
||
cursor is positioned at the first record in the duplicate set, then
|
||
<code class="literal">OperationStatus.NOTFOUND</code> is returned and the
|
||
cursor is left unchanged.
|
||
</span>
|
||
</p>
|
||
</li>
|
||
<li>
|
||
<p>
|
||
<code class="methodname">Cursor.count()</code>
|
||
</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, if the key contains duplicate records,
|
||
displays all the 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="je_cursor6"></a>
|
||
<pre class="programlisting">package je.gettingStarted;
|
||
|
||
import com.sleepycat.je.Cursor;
|
||
import com.sleepycat.je.Database;
|
||
import com.sleepycat.je.DatabaseEntry;
|
||
import com.sleepycat.je.DatabaseException;
|
||
import com.sleepycat.je.LockMode;
|
||
import com.sleepycat.je.OperationStatus;
|
||
|
||
...
|
||
|
||
Cursor cursor = null;
|
||
try {
|
||
...
|
||
// Database and environment 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>
|
||
</div>
|
||
</div>
|
||
<div class="navfooter">
|
||
<hr />
|
||
<table width="100%" summary="Navigation footer">
|
||
<tr>
|
||
<td width="40%" align="left"><a accesskey="p" href="Cursors.html">Prev</a> </td>
|
||
<td width="20%" align="center">
|
||
<a accesskey="u" href="Cursors.html">Up</a>
|
||
</td>
|
||
<td width="40%" align="right"> <a accesskey="n" href="PutEntryWCursor.html">Next</a></td>
|
||
</tr>
|
||
<tr>
|
||
<td width="40%" align="left" valign="top">Chapter 9. Using Cursors </td>
|
||
<td width="20%" align="center">
|
||
<a accesskey="h" href="index.html">Home</a>
|
||
</td>
|
||
<td width="40%" align="right" valign="top"> Putting Records Using Cursors</td>
|
||
</tr>
|
||
</table>
|
||
</div>
|
||
</body>
|
||
</html>
|