One convenient way to understand the source code is through modelling. It is possible to use Eclipse plugin called ObjectAid. Here we only consider the class modeller, which is provided for free.
The object modeller helps to visualize the code, that has already been written. It also can update the models automatically, when the source code was changed. Therefore, it is not a modelling tool, but rather the source code visualizer. A model is essentially a limited view onto the source code, as it only contains a subset of classes that are relevant for the depicted functionality and structure.
For further installation instructions, see: http://www.objectaid.com/