/*
 * 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.ElementType.METHOD;
import static java.lang.annotation.RetentionPolicy.RUNTIME;

Specifies the map key for associations of type java.util.Map when the map key is itself the primary key or a persistent field or property of the entity that is the value of the map. 

If a persistent field or property other than the primary key is used as a map key then it is expected to
have a uniqueness constraint associated with it.
 The MapKeyClass annotation is not used when MapKey is specified and vice versa.

   Example 1:
   @Entity
   public class Department {
       ...
       @OneToMany(mappedBy="department")
       @MapKey  // map key is primary key
       public Map<Integer, Employee> getEmployees() {... }
       ...
   }
   @Entity
   public class Employee {
       ...
       @Id Integer getEmpId() { ... }
       @ManyToOne
       @JoinColumn(name="dept_id")
       public Department getDepartment() { ... }
       ...
   }
   Example 2:
   @Entity
       public class Department {
       ...
       @OneToMany(mappedBy="department")
       @MapKey(name="name")
       public Map<String, Employee> getEmployees() {... }
       ...
   }
   @Entity
       public class Employee {
       @Id public Integer getEmpId() { ... }
       ...
       @ManyToOne
       @JoinColumn(name="dept_id")
       public Department getDepartment() { ... }
       ...
   }

Since:
Java Persistence 1.0/**
 * Specifies the map key for associations of type {@link java.util.Map java.util.Map} when the map key is
 * itself the primary key or a persistent field or property of the entity that is the value of the map.
 * <p>
 * If a persistent field or property other than the primary key is used as a map key then it is expected to
 * have a uniqueness constraint associated with it.
 * <p>
 * The {@link MapKeyClass} annotation is not used when <code>MapKey</code> is specified and vice versa.
 * <p>
 * <pre>
 *
 *    Example 1:
 *
 *    &#064;Entity
 *    public class Department {
 *        ...
 *        &#064;OneToMany(mappedBy="department")
 *        &#064;MapKey  // map key is primary key
 *        public Map&#060;Integer, Employee&#062; getEmployees() {... }
 *        ...
 *    }
 *
 *    &#064;Entity
 *    public class Employee {
 *        ...
 *        &#064;Id Integer getEmpId() { ... }
 *        &#064;ManyToOne
 *        &#064;JoinColumn(name="dept_id")
 *        public Department getDepartment() { ... }
 *        ...
 *    }
 *
 *    Example 2:
 *
 *    &#064;Entity
 *        public class Department {
 *        ...
 *        &#064;OneToMany(mappedBy="department")
 *        &#064;MapKey(name="name")
 *        public Map&#060;String, Employee&#062; getEmployees() {... }
 *        ...
 *    }
 *
 *    &#064;Entity
 *        public class Employee {
 *        &#064;Id public Integer getEmpId() { ... }
 *        ...
 *        &#064;ManyToOne
 *        &#064;JoinColumn(name="dept_id")
 *        public Department getDepartment() { ... }
 *        ...
 *    }
 * </pre>
 *
 * @since Java Persistence 1.0
 */
@Target({METHOD, FIELD})
@Retention(RUNTIME)
public @interface MapKey {

  
(Optional) The name of the persistent field or property of the associated entity that is used as the
map key.

Default: If the name element is not specified, the primary key of the associated entity is
used as the map key. If the primary key is a composite primary key and is mapped as
IdClass, an instance of the primary key class is used as the key.
Returns:
The name/**
   * (Optional) The name of the persistent field or property of the associated entity that is used as the
   * map key.
   * <p>
   * Default: If the <code>name</code> element is not specified, the primary key of the associated entity is
   * used as the map key. If the primary key is a composite primary key and is mapped as
   * <code>IdClass</code>, an instance of the primary key class is used as the key.
   *
   * @return The name
   */
  String name() default "";
}