CollectionMap is an implementation of a Map that allows you to store multiple values for a single key. This is done by using a Map that keeps Collections of the type you actually want to store.
You have to subclass CollectionMap and implement createContentsMap() and createCollection(). The former will be called by the constructor to create the Map that will hold all values. createCollection() will be called whenever a value is added to a key that has been empty. If it returns a thread safe implemention of Collection then CollectionMap will also be thread safe.