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<H1><A name="WorkingWithPaging-WorkingWithPaging"></A>WorkingWithPaging</H1>
<P>The RDB DAS provides a mechanism for working with the result of large queries one chunk (page) at a time. This capability is modeled after the behavior of web search engines. For example, if I Google "RDB DAS" I am presented with a page representing the first ten references. I am also told that there were close to 500,000 matches and I am given the opportunity to go to the next page or directly to pages two through ten. After moving from the first page I am also provided the opportunity to move back to some previous page.</P>
<P>The DAS "Pager" is a wrapper around a provided read command and provides the APIs for paging. Here is the Pager interface:</P>
<DIV class="code panel" style="border-width: 1px;"><DIV class="codeContent panelContent">
<PRE class="code-java">
<SPAN class="code-keyword">public</SPAN> <SPAN class="code-keyword">interface</SPAN> Pager {
/**
* Get the next page of data
*/
DataObject next();
/**
* Get the page prior to the last page returned
*/
DataObject previous();
/**
* Return a specific identified page.
*/
DataObject getPage(<SPAN class="code-object">int</SPAN> page);
}
</PRE>
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<P>A Pager is instantiated on a given read Command and the constructor is used to specify the desired page size (number of database rows per page). The following example illustrates construction and use of a pager: </P>
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<PRE class="code-java">
DAS das = DAS.FACTORY.createDAS(getConnection());
<SPAN class="code-comment">// Build command to read all customers
</SPAN> Command custCommand = das.createCommand(<SPAN class="code-quote">"select * from CUSTOMER order by ID"</SPAN>);
<SPAN class="code-comment">// Create a pager with the command
</SPAN> Pager pager = <SPAN class="code-keyword">new</SPAN> PagerImpl(custCommand, 2);
<SPAN class="code-comment">// Get and work with first page
</SPAN> DataObject root = pager.next();
DataObject customer1 = root.getDataObject(<SPAN class="code-quote">"CUSTOMER[1]"</SPAN>);
DataObject customer2 = root.getDataObject(<SPAN class="code-quote">"CUSTOMER[2]"</SPAN>);
<SPAN class="code-comment">// Get and work with the second page
</SPAN> root = pager.next();
customer1 = root.getDataObject(<SPAN class="code-quote">"CUSTOMER[1]"</SPAN>);
customer2 = root.getDataObject(<SPAN class="code-quote">"CUSTOMER[2]"</SPAN>);
<SPAN class="code-comment">// First page again
</SPAN> root = pager.previous();
customer1 = root.getDataObject(<SPAN class="code-quote">"CUSTOMER[1]"</SPAN>);
customer2 = root.getDataObject(<SPAN class="code-quote">"CUSTOMER[2]"</SPAN>);
</PRE>
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<P>The current Pager specification and implementation are very preliminary and will likely change in significant ways! For instance, the pager is intended to work in completely disconnected scenarios so there should be no connection context maintained between invocations of the pager. Currently the Pager references a Command that, in turn, references a connection (oops!). Some work must be done to make the pager lightweight as it will likely be stored in session.</P>
<P>We probably should also create a Factory rather than using the impl constructor. </P>
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