JDBM3 provides TreeMap, HashMap and other collections backed up by disk storage. Now you can handle billions of items without ever running out of memory. JDBM is probably the fastest and the simpliest pure Java database.
JDBM is tiny (160KB nodeps jar), but packed with features such as transactions, instance cache and space efficient serialization. It also has outstanding performance. It is tightly optimized and has minimal overhead. It scales well from Android phone to multi-terabyte data sets.