NPE on put method in CacheServiceImpl when cache is full.

If cache is full, and try doing a put with no buffer space left available (i.e. cache is full) so a null pointer is returned from the memorymanager, this is then put into the hashmap holding keys and values, thus causing npe.

Solutions proposed to fix this:
When null is returned from memory manager on allocation we dont put into the key pointer map, and return null.

Questions to pose for how we want behaviour:
If key already exists in cache (i.e. update) do we want to remove existing key/pointer (as obviously user wants a new value), and thus avoids incorrect data presented.
Or by returning null to the user we expect them to know that update failed and existing pointer /data will be kept.
Or finally we allow behaviour configurable?