code: public List<VirtualFile> getChildren(VirtualFileFilter filter) throws IOException {
          // isDirectory does the read security check
          if (!isDirectory()) { return Collections.emptyList(); }
          if (filter == null) { filter = MatchAllVirtualFileFilter.INSTANCE; }
          FilterVirtualFileVisitor visitor = new FilterVirtualFileVisitor(filter, null);
          visit(visitor);
          return visitor.getMatched();
      }
docs: Get the children
  
  @param filter to filter the children
  @return the children
  @throws IOException           for any problem accessing the virtual file system
  @throws IllegalStateException if the file is closed or it is a leaf node