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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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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
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<title>Chapter 4. Using Entity Classes</title>
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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 Collections Tutorial" />
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<link rel="up" href="index.html" title="Berkeley DB Collections Tutorial" />
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<link rel="prev" href="retrievingbyindexkey.html" title="Retrieving Items by Index Key" />
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<link rel="next" href="creatingentitybindings.html" title="Creating Entity Bindings" />
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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 11.2.5.2</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 4.
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Using Entity Classes
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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="retrievingbyindexkey.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="creatingentitybindings.html">Next</a></td>
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</tr>
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</table>
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<hr />
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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="Entity"></a>Chapter 4.
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Using Entity Classes
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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="Entity.html#definingentityclasses">
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Defining Entity Classes
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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="creatingentitybindings.html">
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Creating 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="collectionswithentities.html">
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Creating Collections with 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="entitieswithcollections.html">
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Using Entities with 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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In the prior examples, the keys and values of each store were
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represented using separate classes. For example, a <code class="classname">PartKey</code>
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and a <code class="classname">PartData</code> class were used. Many times it is desirable
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to have a single class representing both the key and the value, for
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example, a <code class="classname">Part</code> class.
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</p>
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<p>
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Such a combined key and value class is called an <span class="emphasis"><em>entity
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class</em></span> and is used along with an <span class="emphasis"><em>entity binding</em></span>. Entity
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bindings combine a key and a value into an entity when reading a
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record from a collection, and split an entity into a key and a
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value when writing a record to a collection. Entity bindings are
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used in place of value bindings, and entity objects are used with
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collections in place of value objects.
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</p>
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<p>
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Some reasons for using entities are:
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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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When the key is a property of an entity object representing the
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record as a whole, the object's identity and concept are often
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clearer than with key and value objects that are disjoint.
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</p>
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</li>
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<li>
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<p>
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A single entity object per record is often more convenient to
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use than two objects.
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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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Of course, instead of using an entity binding, you could simply
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create the entity yourself after reading the key and value from a
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collection, and split the entity into a key and value yourself
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before writing it to a collection. But this would detract from the
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convenience of the using the Java collections API. It is convenient
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to obtain a <code class="classname">Part</code> object directly from
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<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>
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and to add a <code class="classname">Part</code> object using
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<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>.
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Collections having entity bindings can be used naturally without
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combining and splitting objects each time a collection method is
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called; however, an entity binding class must be defined by the
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application.
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</p>
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<p>
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In addition to showing how to use entity bindings, this example
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illustrates a key feature of all bindings: Bindings are independent
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of database storage parameters and formats. Compare this example to
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the prior Index example and you'll see that the <code class="classname">Sample</code> and
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<code class="classname">SampleViews</code> classes have been changed to use entity
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bindings, but the <code class="classname">SampleDatabase</code> class was not changed at
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all. In fact, the Entity program and the Index program can be used
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interchangeably to access the same physical database files. This
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demonstrates that bindings are only a "view" onto the physical
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stored data.
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</p>
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<p>
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<code class="classname">Warning:</code> When using multiple bindings for the same
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database, it is the application's responsibility to ensure that the
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same format is used for all bindings. For example, a serial binding
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and a tuple binding cannot be used to access the same records.
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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="definingentityclasses"></a>
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Defining Entity Classes
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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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As described in the prior section, <span class="emphasis"><em>entity classes</em></span> are
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combined key/value classes that are managed by entity bindings. In
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this example the <code class="classname">Part</code>, <code class="classname">Supplier</code> and <code class="classname">Shipment</code>
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classes are entity classes. These classes contain fields that are a
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union of the fields of the key and value classes that were defined
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earlier for each store.
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</p>
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<p>
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In general, entity classes may be defined in any way desired by
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the application. The entity binding, which is also defined by the
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application, is responsible for mapping between key/value objects
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and entity objects.
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</p>
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<p>
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The <code class="classname">Part</code>, <code class="classname">Supplier</code> and <code class="classname">Shipment</code>
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entity classes are
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defined below.
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</p>
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<p>
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An important difference between the entity classes defined here
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and the key and value classes defined earlier is that the entity
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classes are not serializable (do not implement the
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<a class="ulink" href="http://download.oracle.com/javase/1.5.0/docs/api/java/io/Serializable.html" target="_top">Serializable</a>
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interface). This is because the entity classes are not directly
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stored. The entity binding decomposes an entity object into key and
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value objects, and only the key and value objects are serialized
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for storage.
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</p>
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<p>
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One advantage of using entities can already be seen in the
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<code class="methodname">toString()</code> method of the classes below. These return debugging
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output for the combined key and value, and will be used later to
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create a listing of the database that is more readable than in the
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prior examples.
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</p>
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<a id="entity_part"></a>
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<pre class="programlisting"><strong class="userinput"><code>public class Part
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{
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private String number;
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private String name;
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private String color;
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private Weight weight;
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private String city;
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public Part(String number, String name, String color, Weight weight,
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String city)
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{
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this.number = number;
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this.name = name;
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this.color = color;
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this.weight = weight;
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this.city = city;
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}
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public final String getNumber()
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{
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return number;
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}
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public final String getName()
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{
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return name;
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}
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public final String getColor()
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{
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return color;
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}
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public final Weight getWeight()
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{
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return weight;
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}
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public final String getCity()
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{
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return city;
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}
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public String toString()
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{
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return "Part: number=" + number +
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" name=" + name +
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" color=" + color +
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" weight=" + weight +
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" city=" + city + '.';
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}
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}</code></strong> </pre>
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<a id="entity_supplier"></a>
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<pre class="programlisting"><strong class="userinput"><code>public class Supplier
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{
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private String number;
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private String name;
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private int status;
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private String city;
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public Supplier(String number, String name, int status, String city)
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{
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this.number = number;
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this.name = name;
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this.status = status;
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this.city = city;
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}
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public final String getNumber()
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{
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return number;
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}
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public final String getName()
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{
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return name;
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}
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public final int getStatus()
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{
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return status;
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}
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public final String getCity()
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{
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return city;
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}
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public String toString()
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{
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return "Supplier: number=" + number +
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" name=" + name +
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" status=" + status +
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" city=" + city + '.';
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}
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} </code></strong> </pre>
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<a id="entity_shipment"></a>
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<pre class="programlisting"><strong class="userinput"><code>public class Shipment
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{
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private String partNumber;
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private String supplierNumber;
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private int quantity;
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public Shipment(String partNumber, String supplierNumber, int quantity)
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{
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this.partNumber = partNumber;
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this.supplierNumber = supplierNumber;
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this.quantity = quantity;
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}
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public final String getPartNumber()
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{
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return partNumber;
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}
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public final String getSupplierNumber()
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{
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return supplierNumber;
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}
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public final int getQuantity()
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{
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return quantity;
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}
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public String toString()
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{
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return "Shipment: part=" + partNumber +
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" supplier=" + supplierNumber +
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" quantity=" + quantity + '.';
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}
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} </code></strong> </pre>
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</div>
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|||
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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">
|
|||
|
|
<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>
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|||
|
|
</tr>
|
|||
|
|
<tr>
|
|||
|
|
<td width="40%" align="left" valign="top">
|
|||
|
|
Retrieving Items by Index Key
|
|||
|
|
</td>
|
|||
|
|
<td width="20%" align="center">
|
|||
|
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<a accesskey="h" href="index.html">Home</a>
|
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|
|
</td>
|
|||
|
|
<td width="40%" align="right" valign="top">
|
|||
|
|
Creating Entity Bindings
|
|||
|
|
</td>
|
|||
|
|
</tr>
|
|||
|
|
</table>
|
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|
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</div>
|
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|
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</body>
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</html>
|