GeometryCore is a java library to quickly develop prototypes for geometric algorithms and tools.
It has five primary foci: (1) stand-alone and portable; (2) basic geometric objects in 2D; (3) easily set up a basic GUI; (4) ease of importing from and exporting to IPE files; (5) abstraction from rendering code between GUI and file formats.
Refer to the Wiki pages on GitHub for more details and basic tutorials.