code: public static <T> Filter<T> isa(final Class clazz) {
          return new NonMutatingFilter<T>() {
              @Override
              public boolean accepts(T item) {
                  return clazz.isInstance(item);
              }
          };
      }
docs: Gets a filter that returns the original object unchanged if it
  is an <code>instanceof</code> the given class, <code>null</code>
  otherwise.
  
  @param clazz the class.
  @param <T> the type.
  @return the filter.