The Java EE 6 Tutorial, Volume I

Collection Member Expressions

The [NOT] MEMBER [OF] collection member expression determines whether a value is a member of a collection. The value and the collection members must have the same type.

If either the collection-valued or single-valued path expression is unknown, then the collection member expression is unknown. If the collection-valued path expression designates an empty collection, then the collection member expression is FALSE.

The OF keyword is optional.

The following example tests whether a line item is part of an order:

 FROM Order o
 WHERE :lineItem MEMBER OF o.lineItems