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124 lines
5.5 KiB
HTML
124 lines
5.5 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>Saving and Retrieving Data</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="persist_first.html" title="Chapter 3. Direct Persistence Layer First Steps" />
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<link rel="prev" href="persistobject.html" title="Persistent Objects" />
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<link rel="next" href="persist_index.html" title="Chapter 4. Working with Indices" />
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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">Saving and Retrieving Data</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="persistobject.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 3. Direct Persistence Layer First Steps</th>
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<td width="20%" align="right"> <a accesskey="n" href="persist_index.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="saveret"></a>Saving and Retrieving Data</h2>
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</div>
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</div>
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</div>
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<p>
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All data stored using the DPL has one primary index and
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zero or more secondary indices associated with it.
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(Sometimes these are referred to as the primary and
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secondary <span class="emphasis"><em>keys</em></span>.) So to store data under the DPL, you must:
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</p>
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<div class="orderedlist">
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<ol type="1">
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<li>
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<p>
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Declare a class to be an entity class.
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</p>
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</li>
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<li>
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<p>
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Identify the features on the class which
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represent indexed material.
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</p>
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</li>
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<li>
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<p>
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Retrieve the store's primary index for a
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given class using the
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<code class="methodname">EntityStore.getPrimaryIndex()</code>
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method.
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</p>
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</li>
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<li>
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<p>
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Put class objects to the store using the
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<code class="methodname">PrimaryIndex.put()</code>
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method.
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</p>
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</li>
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</ol>
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</div>
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<p>
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In order to retrieve an object from the store, you use
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the index that is most convenient for your purpose. This
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may be the primary index, or it may be some other
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secondary index that you declared on your entity class.
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</p>
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<p>
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You obtain a primary index in the same was as when you
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put the object to the store: using
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<code class="methodname">EntityStore.getPrimaryIndex()</code>.
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You can get a secondary index for the store using the
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<code class="methodname">EntityStore.getSecondaryIndex()</code>
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method. Note that
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<code class="methodname">getSecondaryIndex()</code> requires you
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to provide a <code class="classname">PrimaryIndex</code> class
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instance when you call it, so a class's primary index is
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always required when retrieving objects from an entity
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store.
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</p>
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<p>
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Usually all of the activity surrounding saving and
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retrieving data is organized within a class or classes
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specialized to that purpose. We describe the construction
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of these data accessor classes in <a class="xref" href="simpleda.html" title="SimpleDA.class">SimpleDA.class</a>. But before you perform
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any entity store activity, you need to understand
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indexes. We therefore describe them in the next chapter.
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</p>
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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="persistobject.html">Prev</a> </td>
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<td width="20%" align="center">
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<a accesskey="u" href="persist_first.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="persist_index.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">Persistent Objects </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"> Chapter 4. Working with Indices</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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