index() - JPA ListJoin's method

Create an expression that corresponds to the index of the object in the referenced association or element collection. This method must only be invoked upon an object that represents an association or element collection for which an order column has been defined.
expression denoting the index
JPA 2.0

This documentation page is derived (with some adjustments) from the open source JPA 2 RI (EclipseLink)
and is available under the terms of the Eclipse Public License, v. 1.0 and Eclipse Distribution License, v. 1.0.
Object Relational Mapping (ORM) JPA 2 providers include Hibernate, EclipseLink, TopLink, OpenJPA and DataNucleus.
ObjectDB is not an ORM JPA implementation but an Object Database for Java with built in JPA 2 support.