You can use this project https://github.com/Berico-Technologies/Geo-Coordinate-Conversion-Java/, adding it to your existing pom.xml using jitpack.
I've successfully been able to convert UTM coordinates (30N, this is, 30 zone and northern hemisphere) to Latitude and Longitude. See my example below:
public void setPunto(Point punto) {
this.punto = punto;
LatLon latlon = UTMCoord.locationFromUTMCoord(30, AVKey.NORTH, punto.getX(), punto.getY());
this.latitud = latlon.getLatitude().degrees;
this.longitud = latlon.getLongitude().degrees;
}
Note that Point class is of com.vividsolutions.jts.geom.Point class type.