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261 lines
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HTML
261 lines
10 KiB
HTML
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<?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>Using Comparators</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="DBEntry.html" title="Chapter 8. Database Records" />
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<link rel="prev" href="bindAPI.html" title="Using the BIND APIs" />
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<link rel="next" href="dbtUsage.html" title="Database Record Example" />
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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">Using Comparators</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="bindAPI.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 8. Database Records</th>
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<td width="20%" align="right"> <a accesskey="n" href="dbtUsage.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="comparator"></a>Using Comparators</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="comparator.html#writeCompare">Writing Comparators</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="comparator.html#setCompare">Setting Comparators</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>Internally, JE databases are organized as BTrees.
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This means that most database operations
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(inserts, deletes, reads, and so forth) involve BTree node
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comparisons. This comparison most frequently occurs based on database
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keys, but if your database supports duplicate records then
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comparisons can also occur based on the database data.
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</p>
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<p>
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By default, JE performs all such comparisons using a byte-by-byte
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lexicographic comparison. This mechanism works well for most data.
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However, in some cases you may need to specify your own comparison
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routine. One frequent reason for this is to perform a language sensitive
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lexical ordering of string keys.
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</p>
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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="writeCompare"></a>Writing Comparators</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 override the default comparison function by providing a Java
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<code class="classname">Comparator</code> class to the database.
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The Java <code class="classname">Comparator</code> interface requires you to implement the
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<code class="methodname">Comparator.compare()</code> method
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(see <a class="ulink" href="http://java.sun.com/j2se/1.4.2/docs/api/java/util/Comparator.html" target="_top">http://java.sun.com/j2se/1.4.2/docs/api/java/util/Comparator.html</a> for details).
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</p>
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<p>
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JE passes your <code class="methodname">Comparator.compare()</code> method
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the <code class="literal">byte</code> arrays that you stored in the database. If
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you know how your data is organized in the <code class="literal">byte</code>
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array, then you can write a comparison routine that directly examines
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the contents of the arrays. Otherwise, you have to reconstruct your
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original objects, and then perform the comparison.
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</p>
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<p>
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For example, suppose you want to perform unicode lexical comparisons
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instead of UTF-8 byte-by-byte comparisons. Then you could provide a
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comparator that uses <code class="methodname">String.compareTo()</code>,
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which performs a Unicode comparison of two strings (note that for
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single-byte roman characters, Unicode comparison and UTF-8
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byte-by-byte comparisons are identical – this is something you
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would only want to do if you were using multibyte unicode characters
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with JE). In this case, your comparator would look like the
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following:
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</p>
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<a id="je_dbt14"></a>
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<pre class="programlisting">package je.gettingStarted;
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import java.util.Comparator;
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public class MyDataComparator implements Comparator {
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public MyDataComparator() {}
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public int compare(Object d1, Object d2) {
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byte[] b1 = (byte[])d1;
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byte[] b2 = (byte[])d2;
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String s1 = new String(b1, "UTF-8");
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String s2 = new String(b2, "UTF-8");
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return s1.compareTo(s2);
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}
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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="setCompare"></a>Setting Comparators</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 specify a <code class="classname">Comparator</code> using the following
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methods. Note that by default these methods can only be used at database
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creation time, and they are ignored for normal database opens. Also,
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note that JE uses the no-argument constructor for these comparators.
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Further, it is not allowable for there to be a mutable state in these
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comparators or else unpredictable results will occur.
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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">DatabaseConfig.setBtreeComparator()</code>
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</p>
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<p>
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Sets the Java <code class="classname">Comparator</code> class used
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to compare two keys in the database.
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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">DatabaseConfig.setDuplicateComparator()</code>
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</p>
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<p>
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Sets the Java <code class="classname">Comparator</code> class used to compare the
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data on two duplicate records in the database. This comparator is
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used only if the database supports duplicate records.
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</p>
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</li>
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</ul>
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</div>
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<p>
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You can use the above methods to set a database's comparator after
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database creation time if you explicitly indicate that the comparator
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is to be overridden. You do this by using the following methods:
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</p>
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<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
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<h3 class="title">Note</h3>
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<p>
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If you override your comparator, the new comparator must preserve the
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sort order implemented by your original comparator. That is, the new
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comparator and the old comparator must return the same value for the
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comparison of any two valid objects. Failure to observe this constraint
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will cause unpredictable results for your application.
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</p>
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<p>
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If you want to change the fundamental sort order for your database, back
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up the contents of the database, delete the database, recreate it, and
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then reload its data.
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</p>
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</div>
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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">DatabaseConfig.setOverrideBtreeComparator()</code>
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</p>
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<p>
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If set to <code class="literal">true</code>, causes the database's Btree
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comparator to be overridden with the
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<code class="classname">Comparator</code> specified on
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<code class="methodname">DatabaseConfig.setBtreeComparator()</code>. This
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method can be used to change the comparator post-environment
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creation.
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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">DatabaseConfig.setOverrideDuplicateComparator()</code>
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</p>
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<p>
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If set to <code class="literal">true</code>, causes the database's
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duplicates comparator to be overridden with the
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<code class="classname">Comparator</code> specified on
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<code class="methodname">DatabaseConfig.setDuplicateComparator()</code>.
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</p>
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</li>
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</ul>
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</div>
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<p>For example, to use the <code class="classname">Comparator</code>
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described in the previous section:</p>
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<a id="je_dbt15"></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.DatabaseConfig;
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import com.sleepycat.je.DatabaseException;
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import java.util.Comparator;
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...
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// Environment open omitted for brevity
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try {
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// Get the database configuration object
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DatabaseConfig myDbConfig = new DatabaseConfig();
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myDbConfig.setAllowCreate(true);
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// Set the duplicate comparator class
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myDbConfig.setDuplicateComparator(MyDataComparator.class);
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// Open the database that you will use to store your data
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myDbConfig.setSortedDuplicates(true);
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Database myDatabase = myDbEnv.openDatabase(null, "myDb", myDbConfig);
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} catch (DatabaseException dbe) {
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// Exception handling goes here
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}</pre>
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</div>
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</div>
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<div class="navfooter">
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<hr />
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<table width="100%" summary="Navigation footer">
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<tr>
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<td width="40%" align="left"><a accesskey="p" href="bindAPI.html">Prev</a> </td>
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|||
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<td width="20%" align="center">
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|||
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<a accesskey="u" href="DBEntry.html">Up</a>
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|||
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</td>
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|||
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<td width="40%" align="right"> <a accesskey="n" href="dbtUsage.html">Next</a></td>
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</tr>
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<tr>
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<td width="40%" align="left" valign="top">Using the BIND APIs </td>
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<td width="20%" align="center">
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<a accesskey="h" href="index.html">Home</a>
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</td>
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<td width="40%" align="right" valign="top"> Database Record Example</td>
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</tr>
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</table>
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</div>
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</body>
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</html>
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