public CanUpload canUpload() {
   final CanUpload canUpload;
   if (treeComponent.getMultiSelection().isEmpty()) {
     canUpload = CanUpload.cannotUpload("No files selected");
   } else {
     canUpload = CanUpload.canUpload();
   }
   return canUpload;
 }
 public int getNumSelectedFiles() {
   return treeComponent.getMultiSelection().size();
 }
 private Collection<TreeItem> getTreeItems() {
   return treeComponent.getMultiSelection();
 }