return "Predicates.isNull()";
      }
    },
    /** @see Predicates#notNull() */
    NOT_NULL {
      @Override
      public boolean apply(@CheckForNull Object o) {
        return o != null;
      }

      @Override
      public String toString() {
        return "Predicates.notNull()";
      }
    };

    @SuppressWarnings("unchecked") // safe contravariant cast

        return "Predicates.isNull()";
      }
    },
    /** @see Predicates#notNull() */
    NOT_NULL {
      @Override
      public boolean apply(@CheckForNull Object o) {
        return o != null;
      }

      @Override
      public String toString() {
        return "Predicates.notNull()";
      }
    };

    @SuppressWarnings("unchecked") // safe contravariant cast