Direct conversion from Android .apk apps to working Java .jar applications is most likely not possible. You can try to decompile it to manually review the code, but simple conversion from apk to jar wil not work.
Nevertheless, it should be possible to convert Android APK packages to JAR with decompiling the APK file to DEX format with programs such as Dex2Jar and then compiling it to JAR with JavaDecompiler.