Btree4j is from scratch implementation of a disk-based Prefix B+-tree written in Pure Java. It's pretty fast and 100k ops/sec is expected even on laptop.
Apache Hivemall is a scalable machine learning library that runs on Apache Hive and Spark. Apache Hivemall offers a variety of functionalities including regression, classification, recommendation, anomaly detection, k-nearest neighbor, and feature engineering. Won [IDG's InfoWorld 2014 Bossie Awards 2014, the best open source big data tools](https://www.infoworld.com/article/2688074/big-data-164727-bossie-awards-2014-the-best-open-source-big-data-tools.html).
XBird is a light-weight XQuery processor and database system written in Java. The light-weight means reasonably fast and embeddable. Impmented fully Function Programming lanuguage for XML, [XQuery](https://en.wikipedia.org/wiki/XQuery) using [JavaCC](https://javacc.github.io/javacc/). It passes about 91% of the minimal conformance of [XQuery Test Suite](https://dev.w3.org/2006/xquery-test-suite/PublicPagesStagingArea/).
Developed a multi-functional XML database environment using PostgreSQL as a graduate student project. Extended PostgreSQL functionality using [Server Programming Interface](https://www.postgresql.org/docs/current/spi.html). Awarded [super creater award at IPA mitoh youth](https://www.ipa.go.jp/jinzai/esp/mitoipedia/spc/year/2002ysc.html), a goverment program to educate young engineers.