code: private Optional<String> get(String key, Set<String> processingKeys) {
  
          for (ConfigurationSource configurationSource : config.getConfigurationSources()) {
  
              Optional<String> value = configurationSource.get(key);
  
              if (value.isPresent()) {
  
                  return Optional.of(interpolateString(
                          key, ConfigurationDecoderUtils.decodeConfigValueIfEncoded(key, value.get()), processingKeys));
              }
          }
  
          return Optional.empty();
      }
docs: // Private methods