Get Longitude Laltitude of a point in my Worldmap in Mollweide projection

I searched about a day now, but didnt find any example for my problem in Javacode.

I have a worldmap with a size of 2000*1400 Pixels with a 'Mollweide' projection. How can I find out what is the longitude and laltitude of the point (500,300) in my map ? I would like to code this in Java.

I tried to do this with the 'Java Map Projection Library' :

Point2D.Double pointonmap = null;
Point2D.Double latlon = null;
MolleweideProjection molproj=new MolleweideProjection();

pointonmap = new Point2D.Double (1400,1000);  

latlon=molproj.inverseTransform(pointonmap,new Point2D.Double ());

System.out.println("latlon: " + latlon.getX() + ", " + latlon.getY());

Could anyone help me with that ? Codeexample or hint.

thanks and regards