Legacy JRPN has been superceded by JRPN (written in Flutter). Legacy JRPN is an adaptation of Emmet Gray's wonderful little WRPN calculator, modeled after the HP 16-C. The original is available at http://www.wrpn.emmet-gray.com/. I started with version 6.0.8, and re-wrote part of the UI to use rendered fonts rather than images. This makes the text on the calculator face nice and crisp, no matter what resolution it's scaled up to. Click on the screenshot, above, for a full-screen grab.
Source repository, desktop Java: http://github.com/zathras/JRPN_Java/
Executable Releases, desktop Java: http://github.com/zathras/JRPN_Java/releases/
Source repository, Android: http://github.com/zathras/JRPN_Android/
Executable Release, Android: http://github.com/zathras/JRPN_Android/releases/ or on the Google Play Store or F-Droid.