The JTS Topology Suite is an API of 2D spatial predicates and functions. It has the following design goals:

• JTS conforms to the Simple Features Specification for SQL published by the Open GIS Consortium
• JTS provides a complete, consistent, robust implementation of fundamental 2D spatial algorithms
• JTS is fast enough for production use
• JTS is written in 100% pure JavaTM
• JTS is open source (under the LGPL license)

http://www.vividsolutions.com/jts/JTSHome.htm