code: public String getRequiredString(String key) throws IllegalStateException {
          Optional<String> property = getString(key);
          if (property.isPresent()) {
              return property.get();
          } else {
              throw new IllegalStateException(String.format("required key [%s] not found", key));
          }
      }
docs: Return the String property value associated with the given key (never {@code null}).
  
  @return The String property
  @throws IllegalStateException if the key cannot be resolved