re-instante contract of resourceCompare by throwing ClassCastException for non-FileProviders

git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@718482 13f79535-47bb-0310-9956-ffa450edef68

src/main/org/apache/tools/ant/types/resources/comparators/FileSystem.java

@@ -38,8 +38,18 @@ public class FileSystem extends ResourceComparator {
      * @throws ClassCastException if either resource is not an instance of FileResource.
      */
     protected int resourceCompare(Resource foo, Resource bar) {
-        File foofile = ((FileProvider) foo.as(FileProvider.class)).getFile();
-        File barfile = ((FileProvider) bar.as(FileProvider.class)).getFile();
+        FileProvider fooFP = (FileProvider) foo.as(FileProvider.class);
+        if (fooFP == null) {
+            throw new ClassCastException(foo.getClass()
+                                         + " doesn't provide files");
+        }
+        File foofile = fooFP.getFile();
+        FileProvider barFP = (FileProvider) bar.as(FileProvider.class);
+        if (barFP == null) {
+            throw new ClassCastException(bar.getClass()
+                                         + " doesn't provide files");
+        }
+        File barfile = barFP.getFile();
         return foofile.equals(barfile) ? 0
             : FILE_UTILS.isLeadingPath(foofile, barfile) ? -1
             : FILE_UTILS.normalize(foofile.getAbsolutePath()).compareTo(