Users trying to convert Java archives (.jar) to Android packages (.apk) may encounter many obstacles with jar to apk conversion. This is typically the case when users try to convert older Java apps or games for their Android phones.
If this conversion may work, the Java app needs to be written with Android Java libraries in mind. Such .jar files can be then converted to .apk packages with for example online JAR/JAD to APK converter.
Older Java apps would need to be manually rewritten by a programmer so they use Android Java libraries instead the generic Java ones. This is needed because Java for Android is somewhat limited and does not support all the stuff from desktop platforms.