A simple transformation is uni-directional from a source data structure to a target data structure.
SiTra is (in my biased opinion) the easiest way to write a simple uni-directional model transformation.
SiTra is a simple Java library for supporting a programming approach to writing transformations aiming to, firstly use Java for writing transformations, and secondly, to provide a minimal framework for the execution of transformations. SiTra consists of two interfaces and a class that implements a transformation algorithm. The aim is to facilitate a style of programming that incorporates the concept of transformation rules.