public static String getFileExtension( File file ) 
	throws NullPointerException {
	
	int index = file.getName().lastIndexOf(".");
	if( index == -1 || index == 0 )
	    return null;
	else if( index == file.getName().length()-1 )
	    return "";
	else
	    return file.getName().substring( index+1, file.getName().length() );
	
    }