code: public String getString(String key) {
  		if (key == null || key.isEmpty()) {
  			return null;
  		}
  		String property = null;
  		for (ConfigurationSource configurationSource : configurationSources) {
  			property = configurationSource.getValue(key);
  			if (property != null) {
  				break;
  			}
  		}
  		return property;
  	}
docs: Gets the value of a configuration key as string
  
  @param key the configuration key
  @return the value of this configuration key