While I have decades experience as a programmer I have never played in the GIS sandbox.
The project is a standalone Java application employing Swing GUI and Derby_res and MariaDB databases.
I have no problem adding additional database connectivity. At this time I am rendering a heatmap
as a polar plot via JFreechart that displays different color icons for varying signal intensity
at different distances and bearings from the plot center, which is the user's location.
The need is to incorporate such a heat map in a geo cartography map.
It is not required but highly desired that the feature is available without Internet connectivity.
It is desirable to have the capability to center the map presentation on
a lat/lon point. I will never build a complex GIS. I just need to add this
limited mapping functionality in the most expeditious manner possible.
There appear to be three general categories of approach to the problem. Which approach would you suggest?
1) Utilize online resources. Fewer coding hours. Inability to utilize feature offline.
2) Offline solutions that seem to require many learning/coding hours like geotools and unfolding.
3) Rendering a map is too expensive in labor hours. Just stick with the heatmap plot.

Some candidate solutions that I've looked at and my observations include:

Geolite2: focused on IP geolocation. Requires Internet.

Google heatmaps: Requires javascript that I could employ via Rhino. Requires realtime access to Internet resources.

ggplot2: Appears to be limited to charting without the ability to underlay a map.

geotools: It appears that this will provide the necessary functionality, and can be used within my
preferred IDE of Netbeans. The downside is that it looks like many hours to achieve my heatmap.

unfolding: A brief review of this product leads me to suspect that I can produce my feature more quickly
using unfolding than with geotools. I would like to deploy on Windows/Linux/Mac.
A downside is that it seems to be a wrapper over native OpenGL with its attendant platform specific dependencies.

ArcGIS SDK for Java: Enormous functionality with a learning curve to match.