External database t implement tree like / hash map structure

Question

What external database can I use to work with my java application except Microsoft Access? I need a tree like structure or a hashmap and not relational tables.

Answer 1

For HashMap like i.e. Key,Value pair -> MongoDB would be a good choice since you store documents in it in BSON (Binary JSON) structure.

For Tree/Graph like structures -> Neo4j would be a good choice I guess.