/*
 * Copyright (c) 2008, 2009, 2011 Oracle, Inc. All rights reserved.
 *
 * This program and the accompanying materials are made available under the
 * terms of the Eclipse Public License v1.0 and Eclipse Distribution License v. 1.0
 * which accompanies this distribution.  The Eclipse Public License is available
 * at http://www.eclipse.org/legal/epl-v10.html and the Eclipse Distribution License
 * is available at http://www.eclipse.org/org/documents/edl-v10.php.
 */
package javax.persistence;

import java.lang.annotation.Retention;
import java.lang.annotation.Target;

import static java.lang.annotation.ElementType.FIELD;
import static java.lang.annotation.RetentionPolicy.RUNTIME;

Specifies the type of the map key for associations of type java.util.Map. The map key can be a
basic type, an embeddable class, or an entity. If the map is specified using Java generics, the
MapKeyClass annotation and associated type need not be specified; otherwise they must be
specified.

The MapKeyClass annotation is used in conjunction with ElementCollection or one
of the collection-valued relationship annotations (OneToMany or ManyToMany). The
MapKey annotation is not used when MapKeyClass is specified and vice versa.

   Example 1:
   @Entity
   public class Item {
      @Id int id;
      ...
      @ElementCollection(targetClass=String.class)
      @MapKeyClass(String.class)
      Map images;  // map from image name to image filename
      ...
   }
   Example 2:
   // MapKeyClass and target type of relationship can be defaulted
   @Entity
   public class Item {
      @Id int id;
      ...
      @ElementCollection
      Map<String, String> images;
       ...
    }
    Example 3:
    @Entity
    public class Company {
       @Id int id;
       ...
       @OneToMany(targetEntity=com.example.VicePresident.class)
       @MapKeyClass(com.example.Division.class)
       Map organization;
    }
    Example 4:
    // MapKeyClass and target type of relationship are defaulted
    @Entity
    public class Company {
       @Id int id;
       ...
       @OneToMany
       Map<Division, VicePresident> organization;
    }

See Also:
ElementCollection
OneToMany
ManyToMany
Since:
Java Persistence 2.0/**
 * Specifies the type of the map key for associations of type <code>java.util.Map</code>. The map key can be a
 * basic type, an embeddable class, or an entity. If the map is specified using Java generics, the
 * <code>MapKeyClass</code> annotation and associated type need not be specified; otherwise they must be
 * specified.
 * <p>
 * The <code>MapKeyClass</code> annotation is used in conjunction with <code>ElementCollection</code> or one
 * of the collection-valued relationship annotations (<code>OneToMany</code> or <code>ManyToMany</code>). The
 * <code>MapKey</code> annotation is not used when <code>MapKeyClass</code> is specified and vice versa.
 * <p>
 * <pre>
 *
 *    Example 1:
 *
 *    &#064;Entity
 *    public class Item {
 *       &#064;Id int id;
 *       ...
 *       &#064;ElementCollection(targetClass=String.class)
 *       &#064;MapKeyClass(String.class)
 *       Map images;  // map from image name to image filename
 *       ...
 *    }
 *
 *    Example 2:
 *
 *    // MapKeyClass and target type of relationship can be defaulted
 *
 *    &#064;Entity
 *    public class Item {
 *       &#064;Id int id;
 *       ...
 *       &#064;ElementCollection
 *       Map&#060;String, String&#062; images;
 *        ...
 *     }
 *
 *     Example 3:
 *
 *     &#064;Entity
 *     public class Company {
 *        &#064;Id int id;
 *        ...
 *        &#064;OneToMany(targetEntity=com.example.VicePresident.class)
 *        &#064;MapKeyClass(com.example.Division.class)
 *        Map organization;
 *     }
 *
 *     Example 4:
 *
 *     // MapKeyClass and target type of relationship are defaulted
 *
 *     &#064;Entity
 *     public class Company {
 *        &#064;Id int id;
 *        ...
 *        &#064;OneToMany
 *        Map&#060;Division, VicePresident&#062; organization;
 *     }
 *
 * </pre>
 *
 * @see ElementCollection
 * @see OneToMany
 * @see ManyToMany
 * @since Java Persistence 2.0
 */
@Target({FIELD})
@Retention(RUNTIME)
public @interface MapKeyClass {
  
(Required) The type of the map key.
Returns:
The type/**
   * (Required) The type of the map key.
   *
   * @return The type
   */
  Class value();
}