By "Java Database" you may mean a number of things:
A database written in java maybe to allow embedding in a java application
A java mechanism to access a database would typically mean using something called
JDBC to connect to a "data source" and retrieve data using some query language
like SQL.
But maybe you just need somewhere to store data using java. This introduces the more basic concept of "persistence".
Persistence in the world of computer software indicates that some data has some longevity - that it persists.
To enable persistence the data must be remembered in some retrievable way. This may range from storing in a named variable to enable persistent access within even a program fragment, to writing the data to non-volatile media such as a hard disk drive.
Conventionally, it is this last category that is meant by persistence.
Absolutely. Without persistence at various levels computer systems would not make sense or do anything useful.
A java-based persistence mechanism is central to the Cut The Crap Software.
The core package is GPO - Generic Persistent Object - and this
provides more than simple persistence, but a powerful generic object model with
scalable persistence designed in from the beginning.
Please check out the main website for more information and free downloads.