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5.1.3 Collection types
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So far, we ve looked at the plumbing that goes into creating persistent collections, concentrating on the Set interface. However, Hibernate supports all of the major Java Collections interfaces, each with special characteristics and capabilities. This section examines each collection type.
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Sets
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Since we ve been using Sets in most of our examples, we don t need to spend much time with them here. In short, Sets in Hibernate retain the semantics of the Java interface: Sets are a collection of unique objects. A Set cannot contain duplicate elements, and Sets do not require an <index> element, since they are unordered.
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Collections and custom types
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Lists and Arrays
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Unlike their Set counterparts, Lists can contain duplicate elements. Since Lists are indexed, meaning that contained elements are stored at a specific location in the List, you need to define a <list-index> element:
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<list name="speakers"> <list-index column="speaker_index"/> <key column="event_id"/> <one-to-many class="Speaker"/> </list>
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When a persistent list is retrieved from the database, the elements are ordered according to the index value. This also applies to Arrays:
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<array name="speakers"> <list-index column="speaker_index"/> <key column="event_id"/> <one-to-many class="Speaker"/> </array>
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The <list-index> element defines the column storing the object s index in the collection or array. The index column type is an integer for Lists and Arrays. An integer is used because it corresponds to the primitive type used to refer to a specific element in an Array or a List. For instance, the variable i is an integer in the code snippet shown here:
Object o = myObjectArray[i];
as well as this one:
Object o = myList.get(i);
Persistent arrays behave in the same manner as Lists. We ve never had a reason to use a persistent array since Lists are much more flexible. In addition to requiring an index column, Lists cannot be mapped inversely to the parent object. In sections 5.1.6 and 5.1.7, we explain this problem and show how to work around it with Bags.
Persisting collections and arrays
Maps
Maps are probably the most distinctive of the persistent collections because they behave exactly like their Java counterparts. Maps store entries in key/value pairs, meaning that you retrieve the value by looking up the associated key. Maps are also called dictionaries and associative arrays. Let s look at two method signatures from the java.util.Map interface:
Object Map.get(Object key) Object Map.put(Object key, Object value)
The get( ) method returns the value object for the given key, if any. The put( ) method stores the value in the map under the specified key. If an object is already stored under the specified key, it is returned. This means you can have only one value per key. The keys are stored in an index column. use the index column to store the key for an entry in the map. In Hibernate 3, indexes for Maps are defined using map-key elements. Since keys for a java.util.Map can be of any type, you can specify just about any type for the index value, including composite types. The only type that can t be used as a Map index is another collection.
key Maps
To define a Map index, you d use the type attribute to declare the mapas a String:
<map-key column="attendee_index" type="string" length="20"/>
This snippet defines a VARCHAR(20) column for a Map. This assumes your map will use Strings for keys. Let s look at the full mapping definition that assumes the Event class stores Speaker instances in a Map instead of a List:
<map name="speakers"> <map-key column="speakers_index" type="string" length="20"/> <key column="event_id"/> <one-to-many class="Speaker"/> </map>
Collections and custom types
When inserting elements into the map, use a String for the key:
Map speakers = new HashMap(); speakers.put("speaker1", new Speaker()); speakers.put("speaker2", new Speaker()); Event event = new Event(); event.setSpeakers(speakers);
However, Maps provide another option to the standard <index> element. If your collection is a Map, you can also use an entity for the index. To do that, you need to use the <map-key-many-to-many> element instead of the <index> element:
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