code: function(bounds) {
          if (!this.url) {
              return;
          }
          var xyz = this.getXYZ(bounds), x = xyz.x, y = xyz.y, z = xyz.z;
          var quadDigits = [];
          for (var i = z; i > 0; --i) {
              var digit = '0';
              var mask = 1 << (i - 1);
              if ((x & mask) != 0) {
                  digit++;
              }
              if ((y & mask) != 0) {
                  digit++;
                  digit++;
              }
              quadDigits.push(digit);
          }
          var quadKey = quadDigits.join("");
          var url = this.selectUrl('' + x + y + z, this.url);
  
          return OpenLayers.String.format(url, {'quadkey': quadKey});
      }
docs: Method: getURL
  
  Paramters:
  bounds - {<OpenLayers.Bounds>}