javax.persistence

## Annotation Type MapKey

    @Target(value={METHOD,FIELD})
     @Retention(value=RUNTIME)
    public @interface MapKey

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

### Optional Element Summary

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

### Element Detail

#### name

    public abstract java.lang.String name

(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.

Default:
  ""