JTransforms is the first, open source, multithreaded FFT library
written in pure Java. Currently, four types of transforms are
available: Discrete Fourier Transform (DFT), Discrete Cosine Transform
(DCT), Discrete Sine Transform (DST) and Discrete Hartley Transform
(DHT).  The code is derived from General Purpose FFT Package written
by Takuya Ooura and from Java FFTPack written by Baoshe Zhang.