for  ( int  count =  0 ; count <  5 ; count++) {
             long  time = System.currentTimeMillis();
             for  ( int  i =  0 ; i <  5 ; i++) {
                 for  ( int  k =  0 ; k <  500000 ; k++) {
                     byte []  a =  {( byte )('A'+i)};
                     byte []  b =  {( byte )('1'+i)};
                    String o =  new  String(a) +  new  String(b);
                }
            }
            System.out.println("time(4): " + (System.currentTimeMillis()-time));
        }

time( 4 ):  7031 
time( 4 ):  7141 
time( 4 ):  7201 
time( 4 ):  7232 
time( 4 ):  6951 

private String encode_move(int x, int y) {
        int i = (int) ((x - startx) / cell_w) +  1 ;
        int j = (int) ((y - starty) / cell_h) +  1 ;
	return (direction== 0 )?""+(char) ('A' + i) + (char) ('1' + i):""+(char) ('A' + i) + (char) ('9' - i);
}