code: private static boolean validateReturnType(Method method, Class<?> expected) {
      Class<?> returnType = method.getReturnType();
      if (returnType == void.class) {
        return false;
      }
      if (returnType != expected) {
        String expectedType = "'" + expected.getName() + "'";
        if (expected != void.class) {
          expectedType = "'void' or " + expectedType;
        }
        throw new IllegalStateException(method.getDeclaringClass().getName()
            + "."
            + method.getName()
            + " must have return type of "
            + expectedType);
      }
      return true;
    }
docs: Returns true when the return value should be propagated. Use a default otherwise.