Other NoSQL database systems
As mentioned earlier, MongoDB is currently the top ranked NoSQL database system. (see http://www.kdnuggets.com/2016/06/top-nosql-database-engines.html) It is followed by Apache Cassandra, Redis, Apache HBase, and Neoj4.
MongoDB uses the document data model: a database is a set of collections, each of which is a set of documents, each of which is a set of key-value pairs. Each document is stored as a BSON file.
Cassandra and HBase use the column data model: each data element is a triple: a key, its value, and a timestamp. Cassandra has its own query language, called CQL, that looks like SQL.
Redis uses the key-value data model: a database is a set of dictionaries, each of which is a set of key-value records, where the value is a sequence of fields.
Neoj4 uses the graph data model: a database is a graph whose nodes contain the data. It supports the Cypher query language. Other graph DBS that support Java include GraphBase and OrientDB.
These all have Java APIs, so...