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210 lines
7.6 KiB
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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>Chapter 5. Using Tuples</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="Berkeley DB Java Edition Collections Tutorial" />
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<link rel="up" href="index.html" title="Berkeley DB Java Edition Collections Tutorial" />
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<link rel="prev" href="entitieswithcollections.html" title="Using Entities with Collections" />
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<link rel="next" href="tupleswithkeycreators.html" title="Using Tuples with Key Creators" />
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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">Chapter 5.
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Using Tuples
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</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="entitieswithcollections.html">Prev</a> </td>
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<th width="60%" align="center"> </th>
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<td width="20%" align="right"> <a accesskey="n" href="tupleswithkeycreators.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="chapter" 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"><a id="Tuple"></a>Chapter 5.
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Using Tuples
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</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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<p>
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<b>Table of Contents</b>
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</p>
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<dl>
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<dt>
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<span class="sect1">
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<a href="Tuple.html#tupleformat">
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Using the Tuple Format
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</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="tupleswithkeycreators.html">
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Using Tuples with Key Creators
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</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="tuplekeybindings.html">
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Creating Tuple Key Bindings
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</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="tuple-serialentitybindings.html">
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Creating Tuple-Serial Entity Bindings
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</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="sortedcollections.html">
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Using Sorted Collections
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</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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JE JE Collections API <span class="emphasis"><em>tuples</em></span> are sequences of
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primitive Java data types, for example, integers and strings. The
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<span class="emphasis"><em>tuple format</em></span> is a binary format for tuples that can be used
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to store keys and/or values.
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</p>
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<p>
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Tuples are useful as keys because they have a meaningful sort
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order, while serialized objects do not. This is because the binary
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data for a tuple is written in such a way that its raw byte
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ordering provides a useful sort order. For example, strings in
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tuples are written with a null terminator rather than with a
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leading length.
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</p>
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<p>
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Tuples are useful as keys <span class="emphasis"><em>or</em></span> values when reducing the
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record size to a minimum is important. A tuple is significantly
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smaller than an equivalent serialized object. However, unlike
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serialized objects, tuples cannot contain complex data types and
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are not easily extended except by adding fields at the end of the
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tuple.
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</p>
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<p>
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Whenever a tuple format is used, except when the key or value
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class is a Java primitive wrapper class, a <span class="emphasis"><em>tuple binding</em></span> class must
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be implemented to map between the Java object and the tuple fields.
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Because of this extra requirement, and because tuples are not
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easily extended, a useful technique shown in this example is to use
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tuples for keys and serialized objects for values. This provides
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compact ordered keys but still allows arbitrary Java objects as
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values, and avoids implementing a tuple binding for each value
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class.
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</p>
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<p>
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Compare this example to the prior Entity example and you'll see
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that the <code class="classname">Sample</code> class has not changed. When changing a
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database format, while new bindings are needed to map key and value
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objects to the new format, the application using the objects often
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does not need to be modified.
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</p>
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<p>
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The complete source of the final version of the example program
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is included in the Berkeley DB distribution.
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</p>
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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="tupleformat"></a>
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Using the Tuple Format
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</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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Tuples are sequences of primitive Java values that can be
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written to, and read from, the raw data bytes of a stored record.
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The primitive values are written or read one at a time in sequence,
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using the JE JE Collections API
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<a class="ulink" href="../../java/com/sleepycat/bind/tuple/TupleInput.html" target="_top">TupleInput</a>
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and
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<a class="ulink" href="../../java/com/sleepycat/bind/tuple/TupleOutput.html" target="_top">TupleOutput</a>
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classes. These classes are very similar to the standard Java
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<a class="ulink" href="http://download.oracle.com/javase/1.5.0/docs/api/java/io/DataInput.html" target="_top">DataInput</a>
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and
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<a class="ulink" href="http://download.oracle.com/javase/1.5.0/docs/api/java/io/DataOutput.html" target="_top">DataOutput</a>
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interfaces. The primary difference is the binary format of the
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data, which is designed for sorting in the case of tuples.
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</p>
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<p>
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For example, to read and write a tuple containing two string
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values, the following code snippets could be used.
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</p>
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<a id="tuple_tuplefragment"></a>
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<pre class="programlisting"><strong class="userinput"><code>import com.sleepycat.bind.tuple.TupleInput;
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import com.sleepycat.bind.tuple.TupleOutput;
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...
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TupleInput input;
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TupleOutput output;
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...
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String partNumber = input.readString();
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String supplierNumber = input.readString();
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...
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output.writeString(partNumber);
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output.writeString(supplierNumber); </code></strong> </pre>
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<p>
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Since a tuple is defined as an ordered sequence, reading and
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writing order must match. If the wrong data type is read (an
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integer instead of string, for example), an exception may be thrown
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or at minimum invalid data will be read.
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</p>
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<p>
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When the tuple format is used, bindings and key creators must
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read and write tuples using the tuple API as shown above. This will
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be illustrated in the next two sections.
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</p>
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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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|
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<tr>
|
|||
|
|
<td width="40%" align="left"><a accesskey="p" href="entitieswithcollections.html">Prev</a> </td>
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|||
|
|
<td width="20%" align="center"> </td>
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|||
|
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<td width="40%" align="right"> <a accesskey="n" href="tupleswithkeycreators.html">Next</a></td>
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</tr>
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|||
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<tr>
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|||
|
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<td width="40%" align="left" valign="top">
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|
|
Using Entities with Collections
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|||
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</td>
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<td width="20%" align="center">
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|
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<a accesskey="h" href="index.html">Home</a>
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|
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</td>
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|||
|
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<td width="40%" align="right" valign="top">
|
|||
|
|
Using Tuples with Key Creators
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</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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