mirror of
https://github.com/berkeleydb/je.git
synced 2024-11-15 01:46:24 +00:00
356 lines
12 KiB
HTML
356 lines
12 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 4. Using Entity Classes</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 Java Edition Collections Tutorial" />
|
||
<link rel="up" href="index.html" title="Berkeley DB Java Edition Collections Tutorial" />
|
||
<link rel="prev" href="retrievingbyindexkey.html" title="Retrieving Items by Index Key" />
|
||
<link rel="next" href="creatingentitybindings.html" title="Creating Entity Bindings" />
|
||
</head>
|
||
<body>
|
||
<div xmlns="" class="navheader">
|
||
<div class="libver">
|
||
<p>Library Version 12.2.7.5</p>
|
||
</div>
|
||
<table width="100%" summary="Navigation header">
|
||
<tr>
|
||
<th colspan="3" align="center">Chapter 4.
|
||
Using Entity Classes
|
||
</th>
|
||
</tr>
|
||
<tr>
|
||
<td width="20%" align="left"><a accesskey="p" href="retrievingbyindexkey.html">Prev</a> </td>
|
||
<th width="60%" align="center"> </th>
|
||
<td width="20%" align="right"> <a accesskey="n" href="creatingentitybindings.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="Entity"></a>Chapter 4.
|
||
Using Entity Classes
|
||
</h2>
|
||
</div>
|
||
</div>
|
||
</div>
|
||
<div class="toc">
|
||
<p>
|
||
<b>Table of Contents</b>
|
||
</p>
|
||
<dl>
|
||
<dt>
|
||
<span class="sect1">
|
||
<a href="Entity.html#definingentityclasses">
|
||
Defining Entity Classes
|
||
</a>
|
||
</span>
|
||
</dt>
|
||
<dt>
|
||
<span class="sect1">
|
||
<a href="creatingentitybindings.html">
|
||
Creating Entity Bindings
|
||
</a>
|
||
</span>
|
||
</dt>
|
||
<dt>
|
||
<span class="sect1">
|
||
<a href="collectionswithentities.html">
|
||
Creating Collections with Entity Bindings
|
||
</a>
|
||
</span>
|
||
</dt>
|
||
<dt>
|
||
<span class="sect1">
|
||
<a href="entitieswithcollections.html">
|
||
Using Entities with Collections
|
||
</a>
|
||
</span>
|
||
</dt>
|
||
</dl>
|
||
</div>
|
||
<p>
|
||
In the prior examples, the keys and values of each store were
|
||
represented using separate classes. For example, a <code class="classname">PartKey</code>
|
||
and a <code class="classname">PartData</code> class were used. Many times it is desirable
|
||
to have a single class representing both the key and the value, for
|
||
example, a <code class="classname">Part</code> class.
|
||
</p>
|
||
<p>
|
||
Such a combined key and value class is called an <span class="emphasis"><em>entity
|
||
class</em></span> and is used along with an <span class="emphasis"><em>entity binding</em></span>. Entity
|
||
bindings combine a key and a value into an entity when reading a
|
||
record from a collection, and split an entity into a key and a
|
||
value when writing a record to a collection. Entity bindings are
|
||
used in place of value bindings, and entity objects are used with
|
||
collections in place of value objects.
|
||
</p>
|
||
<p>
|
||
Some reasons for using entities are:
|
||
</p>
|
||
<div class="itemizedlist">
|
||
<ul type="disc">
|
||
<li>
|
||
<p>
|
||
When the key is a property of an entity object representing the
|
||
record as a whole, the object's identity and concept are often
|
||
clearer than with key and value objects that are disjoint.
|
||
</p>
|
||
</li>
|
||
<li>
|
||
<p>
|
||
A single entity object per record is often more convenient to
|
||
use than two objects.
|
||
</p>
|
||
</li>
|
||
</ul>
|
||
</div>
|
||
<p>
|
||
Of course, instead of using an entity binding, you could simply
|
||
create the entity yourself after reading the key and value from a
|
||
collection, and split the entity into a key and value yourself
|
||
before writing it to a collection. But this would detract from the
|
||
convenience of the using the Java collections API. It is convenient
|
||
to obtain a <code class="classname">Part</code> object directly from
|
||
<a class="ulink" href="http://download.oracle.com/javase/1.5.0/docs/api/java/util/Map.html#get(java.lang.Object)" target="_top">Map.get</a>
|
||
|
||
and to add a <code class="classname">Part</code> object using
|
||
<a class="ulink" href="http://download.oracle.com/javase/1.5.0/docs/api/java/util/Set.html#add(E)" target="_top">Set.add</a>.
|
||
Collections having entity bindings can be used naturally without
|
||
combining and splitting objects each time a collection method is
|
||
called; however, an entity binding class must be defined by the
|
||
application.
|
||
</p>
|
||
<p>
|
||
In addition to showing how to use entity bindings, this example
|
||
illustrates a key feature of all bindings: Bindings are independent
|
||
of database storage parameters and formats. Compare this example to
|
||
the prior Index example and you'll see that the <code class="classname">Sample</code> and
|
||
<code class="classname">SampleViews</code> classes have been changed to use entity
|
||
bindings, but the <code class="classname">SampleDatabase</code> class was not changed at
|
||
all. In fact, the Entity program and the Index program can be used
|
||
interchangeably to access the same physical database files. This
|
||
demonstrates that bindings are only a "view" onto the physical
|
||
stored data.
|
||
</p>
|
||
<p>
|
||
<code class="classname">Warning:</code> When using multiple bindings for the same
|
||
database, it is the application's responsibility to ensure that the
|
||
same format is used for all bindings. For example, a serial binding
|
||
and a tuple binding cannot be used to access the same records.
|
||
</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="definingentityclasses"></a>
|
||
Defining Entity Classes
|
||
</h2>
|
||
</div>
|
||
</div>
|
||
</div>
|
||
<p>
|
||
As described in the prior section, <span class="emphasis"><em>entity classes</em></span> are
|
||
combined key/value classes that are managed by entity bindings. In
|
||
this example the <code class="classname">Part</code>, <code class="classname">Supplier</code> and <code class="classname">Shipment</code>
|
||
classes are entity classes. These classes contain fields that are a
|
||
union of the fields of the key and value classes that were defined
|
||
earlier for each store.
|
||
</p>
|
||
<p>
|
||
In general, entity classes may be defined in any way desired by
|
||
the application. The entity binding, which is also defined by the
|
||
application, is responsible for mapping between key/value objects
|
||
and entity objects.
|
||
</p>
|
||
<p>
|
||
The <code class="classname">Part</code>, <code class="classname">Supplier</code> and <code class="classname">Shipment</code>
|
||
entity classes are
|
||
defined below.
|
||
</p>
|
||
<p>
|
||
An important difference between the entity classes defined here
|
||
and the key and value classes defined earlier is that the entity
|
||
classes are not serializable (do not implement the
|
||
<a class="ulink" href="http://download.oracle.com/javase/1.5.0/docs/api/java/io/Serializable.html" target="_top">Serializable</a>
|
||
|
||
interface). This is because the entity classes are not directly
|
||
stored. The entity binding decomposes an entity object into key and
|
||
value objects, and only the key and value objects are serialized
|
||
for storage.
|
||
</p>
|
||
<p>
|
||
One advantage of using entities can already be seen in the
|
||
<code class="methodname">toString()</code> method of the classes below. These return debugging
|
||
output for the combined key and value, and will be used later to
|
||
create a listing of the database that is more readable than in the
|
||
prior examples.
|
||
</p>
|
||
<a id="entity_part"></a>
|
||
<pre class="programlisting"><strong class="userinput"><code>public class Part
|
||
{
|
||
private String number;
|
||
private String name;
|
||
private String color;
|
||
private Weight weight;
|
||
private String city;
|
||
|
||
public Part(String number, String name, String color, Weight weight,
|
||
String city)
|
||
{
|
||
this.number = number;
|
||
this.name = name;
|
||
this.color = color;
|
||
this.weight = weight;
|
||
this.city = city;
|
||
}
|
||
|
||
public final String getNumber()
|
||
{
|
||
return number;
|
||
}
|
||
|
||
public final String getName()
|
||
{
|
||
return name;
|
||
}
|
||
|
||
public final String getColor()
|
||
{
|
||
return color;
|
||
}
|
||
|
||
public final Weight getWeight()
|
||
{
|
||
return weight;
|
||
}
|
||
|
||
public final String getCity()
|
||
{
|
||
return city;
|
||
}
|
||
|
||
public String toString()
|
||
{
|
||
return "Part: number=" + number +
|
||
" name=" + name +
|
||
" color=" + color +
|
||
" weight=" + weight +
|
||
" city=" + city + '.';
|
||
}
|
||
}</code></strong> </pre>
|
||
<a id="entity_supplier"></a>
|
||
<pre class="programlisting"><strong class="userinput"><code>public class Supplier
|
||
{
|
||
private String number;
|
||
private String name;
|
||
private int status;
|
||
private String city;
|
||
|
||
public Supplier(String number, String name, int status, String city)
|
||
{
|
||
this.number = number;
|
||
this.name = name;
|
||
this.status = status;
|
||
this.city = city;
|
||
}
|
||
|
||
public final String getNumber()
|
||
{
|
||
return number;
|
||
}
|
||
|
||
public final String getName()
|
||
{
|
||
return name;
|
||
}
|
||
|
||
public final int getStatus()
|
||
{
|
||
return status;
|
||
}
|
||
|
||
public final String getCity()
|
||
{
|
||
return city;
|
||
}
|
||
|
||
public String toString()
|
||
{
|
||
return "Supplier: number=" + number +
|
||
" name=" + name +
|
||
" status=" + status +
|
||
" city=" + city + '.';
|
||
}
|
||
} </code></strong> </pre>
|
||
<a id="entity_shipment"></a>
|
||
<pre class="programlisting"><strong class="userinput"><code>public class Shipment
|
||
{
|
||
private String partNumber;
|
||
private String supplierNumber;
|
||
private int quantity;
|
||
|
||
public Shipment(String partNumber, String supplierNumber, int quantity)
|
||
{
|
||
this.partNumber = partNumber;
|
||
this.supplierNumber = supplierNumber;
|
||
this.quantity = quantity;
|
||
}
|
||
|
||
public final String getPartNumber()
|
||
{
|
||
return partNumber;
|
||
}
|
||
|
||
public final String getSupplierNumber()
|
||
{
|
||
return supplierNumber;
|
||
}
|
||
|
||
public final int getQuantity()
|
||
{
|
||
return quantity;
|
||
}
|
||
|
||
public String toString()
|
||
{
|
||
return "Shipment: part=" + partNumber +
|
||
" supplier=" + supplierNumber +
|
||
" quantity=" + quantity + '.';
|
||
}
|
||
} </code></strong> </pre>
|
||
</div>
|
||
</div>
|
||
<div class="navfooter">
|
||
<hr />
|
||
<table width="100%" summary="Navigation footer">
|
||
<tr>
|
||
<td width="40%" align="left"><a accesskey="p" href="retrievingbyindexkey.html">Prev</a> </td>
|
||
<td width="20%" align="center"> </td>
|
||
<td width="40%" align="right"> <a accesskey="n" href="creatingentitybindings.html">Next</a></td>
|
||
</tr>
|
||
<tr>
|
||
<td width="40%" align="left" valign="top">
|
||
Retrieving Items by Index Key
|
||
</td>
|
||
<td width="20%" align="center">
|
||
<a accesskey="h" href="index.html">Home</a>
|
||
</td>
|
||
<td width="40%" align="right" valign="top">
|
||
Creating Entity Bindings
|
||
</td>
|
||
</tr>
|
||
</table>
|
||
</div>
|
||
</body>
|
||
</html>
|