While the Key interface in javax.state provides some flexibility towards different implementations, the question, whether the key couldn't simply be a Generic type should be explored.
Similar to the pattern in java.util.Map<K,V> with K as the key and V already used for value in many places.
As the only method Key provides at the moment is a String value, this could e.g. be the toString() method in Object, if the generic value needs a most common base type. BasicKey already substitutes toString() with the key value. As of now, the get() method in Key does not seem referenced anywhere.