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<h2 class="title" style="clear: both"><a id="saveret"></a>Saving and Retrieving Data</h2>
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<p>
All data stored using the DPL has one primary index and
zero or more secondary indices associated with it.
(Sometimes these are referred to as the primary and
secondary <span class="emphasis"><em>keys</em></span>.) So to store data under the DPL, you must:
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<ol type="1">
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<p>
Declare a class to be an entity class.
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</li>
<li>
<p>
Identify the features on the class which
represent indexed material.
</p>
</li>
<li>
<p>
Retrieve the store's primary index for a
given class using the
<code class="methodname">EntityStore.getPrimaryIndex()</code>
method.
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</li>
<li>
<p>
Put class objects to the store using the
<code class="methodname">PrimaryIndex.put()</code>
method.
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In order to retrieve an object from the store, you use
the index that is most convenient for your purpose. This
may be the primary index, or it may be some other
secondary index that you declared on your entity class.
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<p>
You obtain a primary index in the same was as when you
put the object to the store: using
<code class="methodname">EntityStore.getPrimaryIndex()</code>.
You can get a secondary index for the store using the
<code class="methodname">EntityStore.getSecondaryIndex()</code>
method. Note that
<code class="methodname">getSecondaryIndex()</code> requires you
to provide a <code class="classname">PrimaryIndex</code> class
instance when you call it, so a class's primary index is
always required when retrieving objects from an entity
store.
</p>
<p>
Usually all of the activity surrounding saving and
retrieving data is organized within a class or classes
specialized to that purpose. We describe the construction
of these data accessor classes in <a class="xref" href="simpleda.html" title="SimpleDA.class">SimpleDA.class</a>. But before you perform
any entity store activity, you need to understand
indexes. We therefore describe them in the next chapter.
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