@J2ObjCIncompatible // ClassValue
    enum ClassValueValidator implements GetCheckedTypeValidator {
      INSTANCE;

      /*
       * Static final fields are presumed to be fastest, based on our experience with
       * UnsignedBytesBenchmark. TODO(cpovirk): benchmark this
       */
      private static final ClassValue<Boolean> isValidClass =
          new ClassValue<Boolean>() {
            @Override

          validClasses.clear();
        }

        validClasses.add(new WeakReference<Class<? extends Exception>>(exceptionClass));
      }
    }

    /**
     * Returns the ClassValue-using validator, or falls back to the "weak Set" implementation if
     * unable to do so.
     */
    static GetCheckedTypeValidator getBestValidator() {
      return weakSetValidator();
    }
  }