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Retrieving Database Items
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<h2 class="title" style="clear: both"><a id="retrievingdatabaseitems"></a>
Retrieving Database Items
</h2>
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<p>
Retrieving information from the database is accomplished via the
standard Java collections API. In the example, the
<a href="http://java.sun.com/j2se/1.3/docs/api/java/util/Set.html#iterator" target="_top">Set.iterator</a>
method is used to iterate all
<a href="http://java.sun.com/j2se/1.3/docs/api/java/util/Map.Entry.html" target="_top">Map.Entry</a>
objects for each store. All standard Java methods for retrieving
objects from a collection may be used with the Sleepycat Java Collections
API.
</p>
<p>
The <tt class="methodname">PrintDatabase.doWork()</tt> method calls
<tt class="methodname">printEntries()</tt>
to print the map entries for each database store. It is called via
the
<a href="../../java/com/sleepycat/collections/TransactionRunner.html" target="_top">TransactionRunner</a>
class and was outlined in the previous section.
</p>
<a id="cb_printdatabase"></a>
<pre class="programlisting">import com.sleepycat.collections.StoredIterator;
import java.util.Iterator;
...
public class Sample
{
...
private SampleViews views;
...
private class PrintDatabase implements TransactionWorker
{
public void doWork()
throws Exception
{
<b class="userinput"><tt> printEntries("Parts",
views.getPartEntrySet().iterator());
printEntries("Suppliers",
views.getSupplierEntrySet().iterator());
printEntries("Shipments",
views.getShipmentEntrySet().iterator());</tt></b>
}
}
...
<b class="userinput"><tt> private void printEntries(String label, Iterator iterator)
{
}</tt></b>
...
} </pre>
<p>
The
<a href="http://java.sun.com/j2se/1.3/docs/api/java/util/Set.html" target="_top">Set</a>
of
<a href="http://java.sun.com/j2se/1.3/docs/api/java/util/Map.Entry.html" target="_top">Map.Entry</a>
objects for each store is obtained from the <tt class="classname">SampleViews</tt>
object. This set can also be obtained by calling the
<a href="http://java.sun.com/j2se/1.3/docs/api/java/util/Map.html#entrySet" target="_top">Map.entrySet</a>
method of a stored map.
</p>
<p>
The <tt class="methodname">printEntries()</tt> prints the map entries for any stored
map. The
<a href="http://java.sun.com/j2se/1.3/docs/api/java/lang/Object.html#toString" target="_top">Object.toString</a>
method of each key and value is called to
obtain a printable representation of each object.
</p>
<a id="cb_printentries"></a>
<pre class="programlisting"> private void printEntries(String label, Iterator iterator)
{
<b class="userinput"><tt> System.out.println("\n--- " + label + " ---");
try
{
while (iterator.hasNext())
{
Map.Entry entry = (Map.Entry) iterator.next();
System.out.println(entry.getKey().toString());
System.out.println(entry.getValue().toString());
}
}
finally
{
StoredIterator.close(iterator);
}</tt></b>
} </pre>
<p>
It is very important that all iterators for stored collections
are explicitly closed. To ensure they are closed, a <tt class="literal">finally</tt>
clause should be used as shown above. If the iterator is not
closed, the underlying Berkeley DB cursor is not closed either and
the store may become unusable.
</p>
<p>
If the iterator is cast to
<a href="../../java/com/sleepycat/collections/StoredIterator.html" target="_top">StoredIterator</a>
then its
<a href="../../java/com/sleepycat/collections/StoredIterator.html#close()" target="_top">StoredIterator.close()</a>
method can be called. Or, as shown above, the static
<a href="../../java/com/sleepycat/collections/StoredIterator.html#close(java.util.Iterator)" target="_top">StoredIterator.close()</a>
method can be called
to avoid casting. The static form of this method can be called
safely for any
<a href="http://java.sun.com/j2se/1.3/docs/api/java/util/Iterator.html" target="_top">Iterator</a>.
If an iterator for a non-stored collection is passed, it is simply
ignored.
</p>
<p>
This is one of a small number of behavioral differences between
standard Java collections and stored collections. For a complete
list see
<a href="UsingStoredCollections.html">
Using Stored Collections
</a>.
</p>
<p>
The output of the example program is shown below.
</p>
<pre class="programlisting">Adding Suppliers
Adding Parts
Adding Shipments
--- Parts ---
PartKey: number=P1
PartData: name=Nut color=Red weight=[12.0 grams] city=London
PartKey: number=P2
PartData: name=Bolt color=Green weight=[17.0 grams] city=Paris
PartKey: number=P3
PartData: name=Screw color=Blue weight=[17.0 grams] city=Rome
PartKey: number=P4
PartData: name=Screw color=Red weight=[14.0 grams] city=London
PartKey: number=P5
PartData: name=Cam color=Blue weight=[12.0 grams] city=Paris
PartKey: number=P6
PartData: name=Cog color=Red weight=[19.0 grams] city=London
--- Suppliers ---
SupplierKey: number=S1
SupplierData: name=Smith status=20 city=London
SupplierKey: number=S2
SupplierData: name=Jones status=10 city=Paris
SupplierKey: number=S3
SupplierData: name=Blake status=30 city=Paris
SupplierKey: number=S4
SupplierData: name=Clark status=20 city=London
SupplierKey: number=S5
SupplierData: name=Adams status=30 city=Athens
--- Shipments ---
ShipmentKey: supplier=S1 part=P1
ShipmentData: quantity=300
ShipmentKey: supplier=S2 part=P1
ShipmentData: quantity=300
ShipmentKey: supplier=S1 part=P2
ShipmentData: quantity=200
ShipmentKey: supplier=S2 part=P2
ShipmentData: quantity=400
ShipmentKey: supplier=S3 part=P2
ShipmentData: quantity=200
ShipmentKey: supplier=S4 part=P2
ShipmentData: quantity=200
ShipmentKey: supplier=S1 part=P3
ShipmentData: quantity=400
ShipmentKey: supplier=S1 part=P4
ShipmentData: quantity=200
ShipmentKey: supplier=S4 part=P4
ShipmentData: quantity=300
ShipmentKey: supplier=S1 part=P5
ShipmentData: quantity=100
ShipmentKey: supplier=S4 part=P5
ShipmentData: quantity=400
ShipmentKey: supplier=S1 part=P6
ShipmentData: quantity=100 </pre>
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