public void setPrimaryDisplay(int displayPos) {
        if (displayPos < displays.size()) {
            DisplayInfo info = displays.get(displayPos);
            if (info != null) info.setPrimary(true);
        }
    }
This function will mark a certain display as the primary display.

@param displayPos the position in the ArrayList of which display is the primary display
jme3-core/src/main/java/com/jme3/system/Displays.java