private String BoardPiece(int i, int j) {
        assert(i>0 && i<rows && j>0 && j< cols);
        String str = new String();
        
        if(this.cell[i][j] == CellEntry.inValid){
            str = "     ";
        }
        else if(this.cell[i][j] == CellEntry.empty){
            str = "  _  ";
        }
        else if(this.cell[i][j] == CellEntry.white){
            str = "  W  ";
        }        
        else if(this.cell[i][j] == CellEntry.black){
            str = "  B  ";
        }
        else if(this.cell[i][j] == CellEntry.whiteKing){
            str = "  W+ ";
        }
        else if(this.cell[i][j] == CellEntry.blackKing){
            str = "  B+ ";
        }
        
        return str;
    }