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10 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>Chapter 6. Using Serializable Entities</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="sortedcollections.html" title="Using Sorted Collections" />
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<link rel="next" href="transientfieldsinbinding.html" title="Using Transient Fields in an Entity Binding" />
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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 6.
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Using Serializable Entities
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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="sortedcollections.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="transientfieldsinbinding.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="SerializableEntity"></a>Chapter 6.
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Using Serializable Entities
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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="SerializableEntity.html#transientfieldsinclass">
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Using Transient Fields in an Entity Class
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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="transientfieldsinbinding.html">
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Using Transient Fields in an Entity Binding
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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="removingredundantvalueclasses.html">
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Removing the Redundant Value Classes
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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 that used entities (the Entity and Tuple examples) you
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may have noticed the redundancy between the serializable value
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classes and the entity classes. An entity class by definition
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contains all properties of the value class as well as all
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properties of the key class.
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</p>
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<p>
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When using serializable values it is possible to remove this
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redundancy by changing the entity class in two ways:
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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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Make the entity class serializable, so it can be used in place
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of the value class.
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</p>
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</li>
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<li>
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<p>
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Make the key fields transient, so they are not redundantly
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stored in the record.
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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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The modified entity class can then serve double-duty: It can be
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serialized and stored as the record value, and it can be used as
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the entity class as usual along with the Java collections API. The
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<code class="classname">PartData</code>, <code class="classname">SupplierData</code> and <code class="classname">ShipmentData</code>
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classes can then be removed.
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</p>
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<p>
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Transient fields are defined in Java as fields that are not
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stored in the serialized form of an object. Therefore, when an
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object is deserialized the transient fields must be explicitly
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initialized. Since the entity binding is responsible for creating
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entity objects, it is the natural place to initialize the transient
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key fields.
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</p>
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<p>
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Note that it is not strictly necessary to make the key fields of
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a serializable entity class transient. If this is not done, the key
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will simply be stored redundantly in the record's value. This extra
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storage may or may not be acceptable to an application. But since
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we are using tuple keys and an entity binding class must be
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implemented anyway to extract the key from the entity, it is
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sensible to use transient key fields to reduce the record size. Of
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course there may be a reason that transient fields are not desired;
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for example, if an application wants to serialize the entity
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objects for other purposes, then using transient fields should be
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avoided.
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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="transientfieldsinclass"></a>
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Using Transient Fields in an Entity Class
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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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The entity classes in this example are redefined such that they
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can be used both as serializable value classes and as entity
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classes. Compared to the prior example there are three changes to
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the <code class="classname">Part</code>, <code class="classname">Supplier</code> and <code class="classname">Shipment</code> entity
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classes:
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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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Each class now implements the <code class="classname">Serializable</code>
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interface.
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</p>
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</li>
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<li>
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<p>
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The key fields in each class are declared as <code class="literal">transient</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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A package-private <code class="methodname">setKey()</code> method is added to each class
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for initializing the transient key fields. This method will be
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called from the entity bindings.
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</p>
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</li>
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</ul>
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</div>
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<a id="sentity_part"></a>
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<pre class="programlisting"><strong class="userinput"><code>import java.io.Serializable;</code></strong>
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...
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public class Part <strong class="userinput"><code>implements Serializable</code></strong>
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{
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private <strong class="userinput"><code>transient</code></strong> 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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<strong class="userinput"><code> final void setKey(String number)
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{
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this.number = number;
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}</code></strong>
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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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}
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...
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public class Supplier <strong class="userinput"><code>implements Serializable</code></strong>
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{
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private <strong class="userinput"><code>transient</code></strong> 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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<strong class="userinput"><code> void setKey(String number)
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{
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this.number = number;
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}</code></strong>
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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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}
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...
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public class Shipment <strong class="userinput"><code>implements Serializable</code></strong>
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{
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private <strong class="userinput"><code>transient</code></strong> String partNumber;
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private <strong class="userinput"><code>transient</code></strong> 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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<strong class="userinput"><code> void setKey(String partNumber, String supplierNumber)
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{
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this.partNumber = partNumber;
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this.supplierNumber = supplierNumber;
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} </code></strong>
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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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}
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</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="sortedcollections.html">Prev</a> </td>
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||
<td width="20%" align="center"> </td>
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||
<td width="40%" align="right"> <a accesskey="n" href="transientfieldsinbinding.html">Next</a></td>
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||
</tr>
|
||
<tr>
|
||
<td width="40%" align="left" valign="top">
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||
Using Sorted Collections
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||
</td>
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||
<td width="20%" align="center">
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<a accesskey="h" href="index.html">Home</a>
|
||
</td>
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<td width="40%" align="right" valign="top">
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Using Transient Fields in an Entity Binding
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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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