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        Using Entity Classes	
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          <th colspan="3" align="center">Chapter 4. 
        Using Entity Classes	
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          <div>
            <h2 class="title"><a id="Entity"></a>Chapter 4. 
        Using Entity Classes	
	</h2>
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        <div></div>
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      <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>
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          </dt>
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      <p>
    In the prior examples, the keys and values of each store were
	represented using separate classes. For example, a <tt class="classname">PartKey</tt>
	and a <tt class="classname">PartData</tt> class were used. Many times it is desirable
	to have a single class representing both the key and the value, for
	example, a <tt class="classname">Part</tt> 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 <tt class="classname">Part</tt> object directly from 
	<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/Map.html#get" target="_top">Map.get</a>
	
	and to add a <tt class="classname">Part</tt> object using 
	<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/Set.html#add" 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 <tt class="classname">Sample</tt> and
	<tt class="classname">SampleViews</tt> classes have been changed to use entity
	bindings, but the <tt class="classname">SampleDatabase</tt> 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>
    <tt class="classname">Warning:</tt> 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></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 <tt class="classname">Part</tt>, <tt class="classname">Supplier</tt> and <tt class="classname">Shipment</tt>
	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 <tt class="classname">Part</tt>, <tt class="classname">Supplier</tt> and <tt class="classname">Shipment</tt> 
    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 href="http://java.sun.com/j2se/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
	<tt class="methodname">toString()</tt> 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"><b class="userinput"><tt>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 + '.';
    }
}</tt></b> </pre>
        <a id="entity_supplier"></a>
        <pre class="programlisting"><b class="userinput"><tt>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 + '.';
    }
} </tt></b> </pre>
        <a id="entity_shipment"></a>
        <pre class="programlisting"><b class="userinput"><tt>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 + '.';
    }
} </tt></b> </pre>
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