I have a GPS-coordinate in WGS84 that I would like to transform to a map-projection coordinate in SWEREF99 TM using PROJ.4 in Java or proj4js in JavaScript.

Its hard to find documentation for PROJ.4 and how to us it. If you have a good link, please post it as a comment.

The PROJ.4 parameters for SWEREF99 TM is +proj=utm +zone=33 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs

I have tried to use a PROJ.4 Java library and tried this code and values:

String[] proj4_w = new String[] {
    "+proj=utm",
    "+zone=33",
    "+ellps=GRS80",
    "+towgs84=0,0,0,0,0,0,0",
    "+units=m",
    "+no_defs"
};

Projection proj = ProjectionFactory.fromPROJ4Specification(proj4_w);        

Point2D.Double testLatLng = new Point2D.Double(55.0000, 12.7500);
Point2D.Double testProjec = proj.transform(testLatLng, new Point2D.Double());

This give me the point Point2D.Double[5197915.86288144, 1822635.9083898761] but I should be N: 6097106.672, E: 356083.438 What am I doing wrong? and what method and parameters should I use instead?

The correct values is taken from Lantmäteriet.

I am not sure if proj.transform(testLatLng, new Point2D.Double()); is the right method to use.