April 2003
Beginner to intermediate
380 pages
12h 25m
English
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Creating and Initializing a Query
The PersistenceManager
interface contains a set of Query
factory methods used to construct Query instances. They mainly differ in which
query components are initialized. Query instances may be constructed at any time
before a PersistenceManager is
closed.
The following PersistenceManager method constructs an empty
Query instance with none of the
components initialized:
Query newQuery( );
The following PersistenceManager methods construct a
Query instance with an Extent as the collection of candidate
instances:
Query newQuery(Extent candidates); Query newQuery(Extent candidates, String filter);
The candidate class is initialized with the class of the Extent. The second method also initializes the
query filter. We used this second method when we constructed the
Query on line [3] in our example.
Alternatively, a collection can serve as the set of candidate
instances in a query. The following PersistenceManager methods construct a
Query instance with a Collection as the set of candidate
instances:
Query newQuery(Class candidateClass, Collection candidates); Query newQuery(Class candidateClass, Collection candidates, String filter);
When performing a query on a collection, it is necessary to specify the class of the candidate instances explicitly.
The elements in the collection should be persistent instances
associated with the same PersistenceManager as the Query instance. If the collection contains
instances associated with another PersistenceManager, a JDOUserException ...
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.