mirror of
https://github.com/berkeleydb/libdb.git
synced 2024-11-17 01:26:25 +00:00
210 lines
7.6 KiB
HTML
210 lines
7.6 KiB
HTML
|
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
|
|||
|
|
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
|
|||
|
|
<html xmlns="http://www.w3.org/1999/xhtml">
|
|||
|
|
<head>
|
|||
|
|
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
|
|||
|
|
<title>Chapter 5. Using Tuples</title>
|
|||
|
|
<link rel="stylesheet" href="gettingStarted.css" type="text/css" />
|
|||
|
|
<meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
|
|||
|
|
<link rel="start" href="index.html" title="Berkeley DB Collections Tutorial" />
|
|||
|
|
<link rel="up" href="index.html" title="Berkeley DB Collections Tutorial" />
|
|||
|
|
<link rel="prev" href="entitieswithcollections.html" title="Using Entities with Collections" />
|
|||
|
|
<link rel="next" href="tupleswithkeycreators.html" title="Using Tuples with Key Creators" />
|
|||
|
|
</head>
|
|||
|
|
<body>
|
|||
|
|
<div xmlns="" class="navheader">
|
|||
|
|
<div class="libver">
|
|||
|
|
<p>Library Version 11.2.5.2</p>
|
|||
|
|
</div>
|
|||
|
|
<table width="100%" summary="Navigation header">
|
|||
|
|
<tr>
|
|||
|
|
<th colspan="3" align="center">Chapter 5.
|
|||
|
|
Using Tuples
|
|||
|
|
</th>
|
|||
|
|
</tr>
|
|||
|
|
<tr>
|
|||
|
|
<td width="20%" align="left"><a accesskey="p" href="entitieswithcollections.html">Prev</a> </td>
|
|||
|
|
<th width="60%" align="center"> </th>
|
|||
|
|
<td width="20%" align="right"> <a accesskey="n" href="tupleswithkeycreators.html">Next</a></td>
|
|||
|
|
</tr>
|
|||
|
|
</table>
|
|||
|
|
<hr />
|
|||
|
|
</div>
|
|||
|
|
<div class="chapter" lang="en" xml:lang="en">
|
|||
|
|
<div class="titlepage">
|
|||
|
|
<div>
|
|||
|
|
<div>
|
|||
|
|
<h2 class="title"><a id="Tuple"></a>Chapter 5.
|
|||
|
|
Using Tuples
|
|||
|
|
</h2>
|
|||
|
|
</div>
|
|||
|
|
</div>
|
|||
|
|
</div>
|
|||
|
|
<div class="toc">
|
|||
|
|
<p>
|
|||
|
|
<b>Table of Contents</b>
|
|||
|
|
</p>
|
|||
|
|
<dl>
|
|||
|
|
<dt>
|
|||
|
|
<span class="sect1">
|
|||
|
|
<a href="Tuple.html#tupleformat">
|
|||
|
|
Using the Tuple Format
|
|||
|
|
</a>
|
|||
|
|
</span>
|
|||
|
|
</dt>
|
|||
|
|
<dt>
|
|||
|
|
<span class="sect1">
|
|||
|
|
<a href="tupleswithkeycreators.html">
|
|||
|
|
Using Tuples with Key Creators
|
|||
|
|
</a>
|
|||
|
|
</span>
|
|||
|
|
</dt>
|
|||
|
|
<dt>
|
|||
|
|
<span class="sect1">
|
|||
|
|
<a href="tuplekeybindings.html">
|
|||
|
|
Creating Tuple Key Bindings
|
|||
|
|
</a>
|
|||
|
|
</span>
|
|||
|
|
</dt>
|
|||
|
|
<dt>
|
|||
|
|
<span class="sect1">
|
|||
|
|
<a href="tuple-serialentitybindings.html">
|
|||
|
|
Creating Tuple-Serial Entity Bindings
|
|||
|
|
</a>
|
|||
|
|
</span>
|
|||
|
|
</dt>
|
|||
|
|
<dt>
|
|||
|
|
<span class="sect1">
|
|||
|
|
<a href="sortedcollections.html">
|
|||
|
|
Using Sorted Collections
|
|||
|
|
</a>
|
|||
|
|
</span>
|
|||
|
|
</dt>
|
|||
|
|
</dl>
|
|||
|
|
</div>
|
|||
|
|
<p>
|
|||
|
|
DB Java Collections API <span class="emphasis"><em>tuples</em></span> are sequences of
|
|||
|
|
primitive Java data types, for example, integers and strings. The
|
|||
|
|
<span class="emphasis"><em>tuple format</em></span> is a binary format for tuples that can be used
|
|||
|
|
to store keys and/or values.
|
|||
|
|
</p>
|
|||
|
|
<p>
|
|||
|
|
Tuples are useful as keys because they have a meaningful sort
|
|||
|
|
order, while serialized objects do not. This is because the binary
|
|||
|
|
data for a tuple is written in such a way that its raw byte
|
|||
|
|
ordering provides a useful sort order. For example, strings in
|
|||
|
|
tuples are written with a null terminator rather than with a
|
|||
|
|
leading length.
|
|||
|
|
</p>
|
|||
|
|
<p>
|
|||
|
|
Tuples are useful as keys <span class="emphasis"><em>or</em></span> values when reducing the
|
|||
|
|
record size to a minimum is important. A tuple is significantly
|
|||
|
|
smaller than an equivalent serialized object. However, unlike
|
|||
|
|
serialized objects, tuples cannot contain complex data types and
|
|||
|
|
are not easily extended except by adding fields at the end of the
|
|||
|
|
tuple.
|
|||
|
|
</p>
|
|||
|
|
<p>
|
|||
|
|
Whenever a tuple format is used, except when the key or value
|
|||
|
|
class is a Java primitive wrapper class, a <span class="emphasis"><em>tuple binding</em></span> class must
|
|||
|
|
be implemented to map between the Java object and the tuple fields.
|
|||
|
|
Because of this extra requirement, and because tuples are not
|
|||
|
|
easily extended, a useful technique shown in this example is to use
|
|||
|
|
tuples for keys and serialized objects for values. This provides
|
|||
|
|
compact ordered keys but still allows arbitrary Java objects as
|
|||
|
|
values, and avoids implementing a tuple binding for each value
|
|||
|
|
class.
|
|||
|
|
</p>
|
|||
|
|
<p>
|
|||
|
|
Compare this example to the prior Entity example and you'll see
|
|||
|
|
that the <code class="classname">Sample</code> class has not changed. When changing a
|
|||
|
|
database format, while new bindings are needed to map key and value
|
|||
|
|
objects to the new format, the application using the objects often
|
|||
|
|
does not need to be modified.
|
|||
|
|
</p>
|
|||
|
|
<p>
|
|||
|
|
The complete source of the final version of the example program
|
|||
|
|
is included in the Berkeley DB distribution.
|
|||
|
|
</p>
|
|||
|
|
<div class="sect1" lang="en" xml:lang="en">
|
|||
|
|
<div class="titlepage">
|
|||
|
|
<div>
|
|||
|
|
<div>
|
|||
|
|
<h2 class="title" style="clear: both"><a id="tupleformat"></a>
|
|||
|
|
Using the Tuple Format
|
|||
|
|
</h2>
|
|||
|
|
</div>
|
|||
|
|
</div>
|
|||
|
|
</div>
|
|||
|
|
<p>
|
|||
|
|
Tuples are sequences of primitive Java values that can be
|
|||
|
|
written to, and read from, the raw data bytes of a stored record.
|
|||
|
|
The primitive values are written or read one at a time in sequence,
|
|||
|
|
using the DB Java Collections API
|
|||
|
|
<a class="ulink" href="../../java/com/sleepycat/bind/tuple/TupleInput.html" target="_top">TupleInput</a>
|
|||
|
|
|
|||
|
|
and
|
|||
|
|
<a class="ulink" href="../../java/com/sleepycat/bind/tuple/TupleOutput.html" target="_top">TupleOutput</a>
|
|||
|
|
|
|||
|
|
classes. These classes are very similar to the standard Java
|
|||
|
|
<a class="ulink" href="http://download.oracle.com/javase/1.5.0/docs/api/java/io/DataInput.html" target="_top">DataInput</a>
|
|||
|
|
|
|||
|
|
and
|
|||
|
|
<a class="ulink" href="http://download.oracle.com/javase/1.5.0/docs/api/java/io/DataOutput.html" target="_top">DataOutput</a>
|
|||
|
|
|
|||
|
|
interfaces. The primary difference is the binary format of the
|
|||
|
|
data, which is designed for sorting in the case of tuples.
|
|||
|
|
</p>
|
|||
|
|
<p>
|
|||
|
|
For example, to read and write a tuple containing two string
|
|||
|
|
values, the following code snippets could be used.
|
|||
|
|
</p>
|
|||
|
|
<a id="tuple_tuplefragment"></a>
|
|||
|
|
<pre class="programlisting"><strong class="userinput"><code>import com.sleepycat.bind.tuple.TupleInput;
|
|||
|
|
import com.sleepycat.bind.tuple.TupleOutput;
|
|||
|
|
...
|
|||
|
|
TupleInput input;
|
|||
|
|
TupleOutput output;
|
|||
|
|
...
|
|||
|
|
String partNumber = input.readString();
|
|||
|
|
String supplierNumber = input.readString();
|
|||
|
|
...
|
|||
|
|
output.writeString(partNumber);
|
|||
|
|
output.writeString(supplierNumber); </code></strong> </pre>
|
|||
|
|
<p>
|
|||
|
|
Since a tuple is defined as an ordered sequence, reading and
|
|||
|
|
writing order must match. If the wrong data type is read (an
|
|||
|
|
integer instead of string, for example), an exception may be thrown
|
|||
|
|
or at minimum invalid data will be read.
|
|||
|
|
</p>
|
|||
|
|
<p>
|
|||
|
|
When the tuple format is used, bindings and key creators must
|
|||
|
|
read and write tuples using the tuple API as shown above. This will
|
|||
|
|
be illustrated in the next two sections.
|
|||
|
|
</p>
|
|||
|
|
</div>
|
|||
|
|
</div>
|
|||
|
|
<div class="navfooter">
|
|||
|
|
<hr />
|
|||
|
|
<table width="100%" summary="Navigation footer">
|
|||
|
|
<tr>
|
|||
|
|
<td width="40%" align="left"><a accesskey="p" href="entitieswithcollections.html">Prev</a> </td>
|
|||
|
|
<td width="20%" align="center"> </td>
|
|||
|
|
<td width="40%" align="right"> <a accesskey="n" href="tupleswithkeycreators.html">Next</a></td>
|
|||
|
|
</tr>
|
|||
|
|
<tr>
|
|||
|
|
<td width="40%" align="left" valign="top">
|
|||
|
|
Using Entities with Collections
|
|||
|
|
</td>
|
|||
|
|
<td width="20%" align="center">
|
|||
|
|
<a accesskey="h" href="index.html">Home</a>
|
|||
|
|
</td>
|
|||
|
|
<td width="40%" align="right" valign="top">
|
|||
|
|
Using Tuples with Key Creators
|
|||
|
|
</td>
|
|||
|
|
</tr>
|
|||
|
|
</table>
|
|||
|
|
</div>
|
|||
|
|
</body>
|
|||
|
|
</html>
|