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7. Map


Definition:
A map is a simple name-value pair list stored on a first rank collection.

Scenario:
First rank class Foo has a map containing people's ages indexed by their names

Map Foo.getAges()   //  returns a collection of String name-value pairs

Hibernate Mapping:
In Hibernate, this could be mapped as follows:

<class name="Foo" table="foo">
     ...
     <map role="ages">
         <key column="id"/>
         <index column="name" type="string"/>
         <element column="age" type="string"/>
     </map>
 </class>

Table Schema:

Foo

id

Ages

id

name

age


A simple extra table, Ages, is used to store the name and age string-value pair. Note that the map needs its own identity column too: id.

Bidirectionality:
Bidirectionality has no meaning for a map.

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