XML : Using XPath to search with, and retrieve attribute in XML

I'm trying to use XPath to retrieve an attribute value in an XML file containing a lot of data. I want to search the XML file for a particular attribute, and return a corresponding attribute.

My code at the moment gives an Expression Error which I guess means I've gotten the expression wrong.

  private String getPrivateID(String platformNo)  {        String platformTag = null;        try      {          InputStream is = getResources().openRawResource(R.raw.platform);          InputSource inputSrc = new InputSource(is);            XPath xpath = XPathFactory.newInstance().newXPath();            String expression = String.format("//Platform[@PlatformNo=%s]/@PlatformTag", platformNo);          NodeList nodes = (NodeList) xpath.evaluate(expression, inputSrc, XPathConstants.NODESET);            platformTag = xpath.evaluate(expression, nodes);            return platformTag;        }      catch (Exception e)      {          return null;      }  }    

My XML is like this:

  <JPPlatforms>    <Platform PlatformTag="2980" PlatformNo="47280" Name="AGHS" BearingToRoad="2.6606268e+002" RoadName="Avonside Dr">      <Position Lat="-4.352447905000000e+001" Long="1.726611665000000e+002"/>    </Platform>    <Platform PlatformTag="1219" PlatformNo="28142" Name="Addington Village" BearingToRoad="3.2193924e+002" RoadName="Lincoln Rd">      <Position Lat="-4.354269524000000e+001" Long="1.726134222000000e+002"/>    </Platform>    <Platform PlatformTag="1220" PlatformNo="44108" Name="Addington Village" BearingToRoad="1.4198888e+002" RoadName="Lincoln Rd">      <Position Lat="-4.354386705000000e+001" Long="1.726111654000000e+002"/>    </Platform>    <Platform PlatformTag="2940" PlatformNo="45477" Name="Aidanfield Dr near Bibiana St" BearingToRoad="2.1811232e+002" RoadName="Aidanfield Dr">      <Position Lat="-4.356581921000000e+001" Long="1.725743414000000e+002"/>    </Platform>    <Platform PlatformTag="2941" PlatformNo="47192" Name="Aidanfield Dr near Bibiana St" BearingToRoad="3.8112324e+001" RoadName="Aidanfield Dr">      <Position Lat="-4.356595693000000e+001" Long="1.725742336000000e+002"/>    </Platform>  <JPPlatforms>    

Where have I gone wrong? Thanks in advance!