Open-Source Java Object-Orientated Database 
Getting started
API documentation

Tiger Objectbase is a replacement for traditional relational databases. The main feature in Tiger Objectbase is storing Java objects on disk, by serialization. Every persistent object has an associated ID object that uniquely identifies the object. The ID object is then used for finding the persistant object.

Like relational databases you can define relationships between objects, and this relationship can be used for finding objects. Tiger also uses the concepts of aggregates for identifying relationships.

Tiger has the concept of indexes like relational databases. When a persistent object is being saved, index values from the object are stored in an index. These values can be used for finding objects.

After a set of objects are retrieved from storage they can be sorted or filtered.