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The Java Persistence API (JPA) : a standard interface-based Java model abstraction of persistence, developed by the JCP.

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/*
 * 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.
 */

// $Id: $

package javax.persistence;

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

import static java.lang.annotation.ElementType.ANNOTATION_TYPE;
import static java.lang.annotation.ElementType.FIELD;
import static java.lang.annotation.ElementType.METHOD;
import static java.lang.annotation.RetentionPolicy.RUNTIME;
import static javax.persistence.EnumType.ORDINAL;

/**
 * Specifies the enum type for a map key whose basic type is an enumerated type. The
 * MapKeyEnumerated annotation can be applied to an element collection or relationship of type
 * java.util.Map, in conjunction with the ElementCollection, OneToMany,
 * or ManyToMany annotation. If the enumerated type is not specified or the
 * MapKeyEnumerated annotation is not used, the enumerated type is assumed to be
 * ORDINAL.
 *
 *  *   Example:
 * 
 *   public enum ProjectStatus {COMPLETE, DELAYED, CANCELLED, IN_PROGRESS}
 * 
 *   public enum SalaryRate {JUNIOR, SENIOR, MANAGER, EXECUTIVE}
 * 
 *   @Entity public class Employee {
 *       @ManyToMany
 *       public Projects<ProjectStatus, Project> getProjects() {...}
 * 
 *       @OneToMany
 *       @MapKeyEnumerated(STRING)
 *       public Map<SalaryRate, Employee> getEmployees() {...}
 *       ...
 *   }
 * 
 * @see ElementCollection
 * @see OneToMany
 * @see ManyToMany
 * @since Java Persistence 2.0
 */
@Target({ANNOTATION_TYPE, METHOD, FIELD})
@Retention(RUNTIME)
public @interface MapKeyEnumerated
{

    /**
     * (Optional) The type used in mapping a map key enum type.
     * @return The type
     */
    EnumType value() default ORDINAL;
}