by ghfick » Tue May 15, 2018 10:46 pm
I can also recommend Sudoku Explainer [SE] by Nicolas Juillerat. Also in Java and its last update was in 2007. Nevertheless, it has a very extensive set of very elaborate Forcing Chain strategies. These strategies are still being explored by enthusiasts. Further, SE has been adapted to provide a [still] definitive rating of puzzles based on the complexity of the Forcing Chains.
I also very much recommend Phil's Folly [PF] by Philip Beeby. This solver has some more recently established strategies like [parts of ] JE, SK Loops, MSLS along with both bidirectional and unidirectional chains and much more.