java & fullscreen over multiple monitors

You could try:

int width = 0;
int height = 0;
GraphicsEnvironment ge = GraphicsEnvironment.getLocalGraphicsEnvironment();
GraphicsDevice[] gs = ge.getScreenDevices();
for (GraphicsDevice curGs : gs)
{
  DisplayMode mode = curGs.getDisplayMode();
  width += mode.getWidth();
  height = mode.getHeight();
}

This should calculate the total width of multiple screens. Obviously it only supports horizontally aligned screens in the form above - you'd have to analyse the bounds of the graphics configurations to handle other monitor alignments (depends how bulletproof you want to make it).

Edit: And then set the size of your frame: f.setSize(width, height);

This is not what "setFullScreenWindow" is for. It's really for applications that want more direct access to the framebuffer (better performance) - like a 3D game does in DirectX, for instance. This kind of implies ONE monitor.

See this other answer I did: JDialog Not Displaying When in Fullscreen Mode

That is the normal behavior when you maximize a window in Windows when you have two monitors. In order two get the full resolution size, you will need to look at GraphicsConfiguration to check each GraphicsDevice.

A more general solution to Ash's code is to union the bounds of all the graphics configurations

Rectangle2D result = new Rectangle2D.Double();
GraphicsEnvironment localGE = GraphicsEnvironment.getLocalGraphicsEnvironment();
for (GraphicsDevice gd : localGE.getScreenDevices()) {
  for (GraphicsConfiguration graphicsConfiguration : gd.getConfigurations()) {
    result.union(result, graphicsConfiguration.getBounds(), result);
  }
}
f.setSize(result.getWidth(), result.getHeight());

This will work for vertically aligned monitors as well as horizontal.