It's just a case of converting your map coordinates to screen coordinates.
It is actually very common to convert from screen coordinates and move to map coordinates when you listen into click events. I've shown this in the code below with an extra line of code to convert to back again:
// display the screen coordinate
Point2D screenPoint = new Point2D(e.getX(), e.getY());
System.out.println("screen " + screenPoint.getX() +"," + screenPoint.getY() );
// convert the screen coordinate to map coordinate
Point actualPoint = mapView.screenToLocation(screenPoint);
System.out.println("actual point " + actualPoint.getX() +"," + actualPoint.getY());
// convert the map coordinate back to the screen coordinate
Point2D calcScreenPoint = mapView.locationToScreen(actualPoint);
System.out.println("calc screen point " + calcScreenPoint.getX() + "," + calcScreenPoint.getY());
So if you have 2 map coordinates and convert these to screen coordinates it's just simple maths to work out the number of pixels between the 2 points.
Note the import statements so you get the types correct:
Does this help?