Decompiling

Decompiling Sonic the Hedgehog (J2ME) is pretty simplistic due to the fact that it is a basic JAR file. Which is essentially just a ZIP.

Extracting Files

To initially extract the files, simply use a tool like 7Zip to open the JAR as an archive.

Immediately all the files in their original form will present themselves and are ready to be modified. However, this is the old method of modding Sonic the Hedgehog (J2ME), using this decompilation is much easier.

Decompiling .CLASS Files

While the PNGs and MIDIs can be easily edited, the code is a bit more of a struggle as they are compiled Java files. A tool like FernFlower can decompile them easily.

Just as a heads up, other decompilers like Java Decompiler will cause internal errors on both b.class and c.class.

While the .CLASS files are now decompiled into .JAVA, all of the code is obfuscated, and most likely won't compile. A guide on deobfuscating would be incredibly tedious and long, so that process is up to you.