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337 lines
14 KiB
HTML
337 lines
14 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>Retrieving Multiple Objects</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_access.html" title="Chapter 5. Saving and Retrieving Objects" />
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<link rel="prev" href="simpleget.html" title="Retrieving Objects from an Entity Store" />
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<link rel="next" href="dpl_entityjoin.html" title="Join 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">Retrieving Multiple Objects</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="simpleget.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 5. Saving and Retrieving Objects</th>
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<td width="20%" align="right"> <a accesskey="n" href="dpl_entityjoin.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="getmultiple"></a>Retrieving Multiple Objects</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="getmultiple.html#dpl_cursor_initialize">Cursor Initialization</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="getmultiple.html#dpl_dups">Working with Duplicate Keys</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="getmultiple.html#dpl_cursor_range">Key Ranges</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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It is possible to iterate over every object referenced
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by a specific index. You may want to do this if, for
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example, you want to examine or modify every object
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accessible by a specific primary index.
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</p>
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<p>
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In addition, some indexes result in the retrieval of multiple
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objects. For example, <code class="literal">MANY_TO_ONE</code>
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secondary indexes can result in more than one object for any given
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key (also known as <span class="emphasis"><em>duplicate keys</em></span>).
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When this is the case, you must iterate
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over the resulting set of objects in order to examine
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each object in turn.
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</p>
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<p>
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There are two ways to iterate over a collection of
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objects as returned by an index. One is to use a
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standard Java <code class="classname">Iterator</code>, which you
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obtain using an <code class="classname">EntityCursor</code>,
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which in turn you can obtain from a <code class="classname">PrimaryIndex</code>:
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</p>
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<pre class="programlisting">PrimaryIndex<String,SimpleEntityClass> pi =
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store.getPrimaryIndex(String.class, SimpleEntityClass.class);
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EntityCursor<SimpleEntityClass> pi_cursor = pi.entities();
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try {
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Iterator<SimpleEntityClass> i = pi_cursor.iterator();
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while (i.hasNext()) {
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// Do something here
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}
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} finally {
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// Always close the cursor
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pi_cursor.close();
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} </pre>
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<p>
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Alternatively, you can use a Java "foreach" statement
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to iterate over object set:
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</p>
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<pre class="programlisting">PrimaryIndex<String,SimpleEntityClass> pi =
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store.getPrimaryIndex(String.class, SimpleEntityClass.class);
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EntityCursor<SimpleEntityClass> pi_cursor = pi.entities();
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try {
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for (SimpleEntityClass seci : pi_cursor) {
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// do something with each object "seci"
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}
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// Always make sure the cursor is closed when we are done with it.
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} finally {
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pi_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="dpl_cursor_initialize"></a>Cursor Initialization</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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When a cursor is first opened, it is not
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positioned to any value; that is,
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it is not <span class="emphasis"><em>initialized</em></span>.
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Most of the <code class="classname">EntityCursor</code>
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methods that move a cursor will initialize it
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to either the first or last object, depending
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on whether the operation is moving the cursor
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forward (all <code class="literal">next...</code>
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methods) or backwards (all
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<code class="literal">prev...</code>) methods.
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</p>
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<p>
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You can also force a cursor, whether it is
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initialized or not, to return the first object
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by calling
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<code class="methodname">EntityCursor.first()</code>.
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Similarly, you can force a return of the last
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object using
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<code class="methodname">EntityCursor.last()</code>.
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</p>
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<p>
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Operations that do not move the cursor (such as
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<code class="methodname">EntityCursor.current()</code>
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or <code class="methodname">EntityCursor.delete()</code>
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will throw an
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<code class="classname">IllegalStateException</code>
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when used on an uninitialized cursor.
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</p>
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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="dpl_dups"></a>Working with Duplicate Keys</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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If you have duplicate secondary keys, you can return an
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<code class="classname">EntityIndex</code> class object for them
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using <code class="methodname">SecondaryIndex.subIndex()</code>
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Then, use that object's
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<code class="methodname">entities()</code>
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method to obtain an <code class="classname">EntityCursor</code>
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instance.
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</p>
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<p>
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For example:
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</p>
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<pre class="programlisting">PrimaryIndex<String,SimpleEntityClass> pi =
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store.getPrimaryIndex(String.class, SimpleEntityClass.class);
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SecondaryIndex<String,String,SimpleEntityClass> si =
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store.getSecondaryIndex(pi, String.class, "sKey");
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EntityCursor<SimpleEntityClass> sec_cursor =
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si.subIndex("skeyone").entities();
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try {
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for (SimpleEntityClass seci : sec_cursor) {
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// do something with each object "seci"
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}
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// Always make sure the cursor is closed when we are done with it.
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} finally {
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sec_cursor.close(); } </pre>
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<p>
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Note that if you are working with duplicate keys, you can
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control how cursor iteration works by using the following
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<code class="classname">EntityCursor</code> methods:
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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">nextDup()</code>
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</p>
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<p>
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Moves the cursor to the next object with the
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same key as the cursor is currently
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referencing. That is, this method returns the
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next duplicate object. If no such object
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exists, this method returns
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<code class="literal">null</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">prevDup()</code>
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</p>
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<p>
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Moves the cursor to the previous object with the
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same key as the cursor is currently
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referencing. That is, this method returns the
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previous duplicate object in the cursor's set
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of objects. If no such object exists, this method returns
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<code class="literal">null</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">nextNoDup()</code>
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</p>
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<p>
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Moves the cursor to the next object in the
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cursor's set that has a key which is different
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than the key that the cursor is currently
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referencing. That is, this method skips all
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duplicate objects and returns the
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next non-duplicate object in the cursor's set
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of objects. If no such object exists, this method returns
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<code class="literal">null</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">prevNoDup()</code>
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</p>
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<p>
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Moves the cursor to the previous object in the
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cursor's set that has a key which is different
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than the key that the cursor is currently
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referencing. That is, this method skips all
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duplicate objects and returns the
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previous non-duplicate object in the cursor's set
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of objects. If no such object exists, this method returns
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<code class="literal">null</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>
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For example:
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</p>
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<pre class="programlisting">PrimaryIndex<String,SimpleEntityClass> pi =
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store.getPrimaryIndex(String.class, SimpleEntityClass.class);
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SecondaryIndex<String,String,SimpleEntityClass> si =
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store.getSecondaryIndex(pi, String.class, "sKey");
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EntityCursor<SimpleEntityClass> sec_cursor =
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si.subIndex("skeyone").entities();
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try {
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SimpleEntityClass sec;
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Iterator<SimpleEntityClass> i = sec_cursor.iterator();
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while (sec = i.nextNoDup() != null) {
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// Do something here
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}
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// Always make sure the cursor is closed when we are done with it.
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} finally {
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sec_cursor.close(); } </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="dpl_cursor_range"></a>Key Ranges</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 restrict the scope of a cursor's movement
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by specifying a <span class="emphasis"><em>range</em></span> when you
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create the cursor. The cursor can then never be
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positioned outside of the specified range.
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</p>
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<p>
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When specifying a range, you indicate whether a
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range bound is <span class="emphasis"><em>inclusive</em></span> or
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<span class="emphasis"><em>exclusive</em></span> by providing a
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boolean value for each range.
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<code class="literal">true</code> indicates that the provided
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bound is inclusive, while <code class="literal">false</code>
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indicates that it is exclusive.
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</p>
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<p>
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You provide this information when you call
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<code class="classname">PrimaryIndex.entities()</code>
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or
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<code class="classname">SecondaryIndex.entities()</code>.
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For example, suppose you had a class indexed by
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numerical information. Suppose further that you
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wanted to examine only those objects with indexed
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values of 100 - 199. Then (assuming the numerical
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information is the primary index), you can bound
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your cursor as follows:
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</p>
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<pre class="programlisting">
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EntityCursor<SomeEntityClass> cursor =
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primaryIndex.entities(100, true, 200, false);
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try {
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for (SomeEntityClass sec : cursor {
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// Do something here to objects ranged from 100 to 199
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}
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// Always make sure the cursor is closed when we are done with it.
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} finally {
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cursor.close(); } </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="simpleget.html">Prev</a> </td>
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||
<td width="20%" align="center">
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<a accesskey="u" href="persist_access.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="dpl_entityjoin.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">Retrieving Objects from an Entity Store </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"> Join Cursors</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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