code: private static String getProperty(Properties properties, String key) {
  
          // Check if the properties do not exist.
          if (properties == null || properties.getProperty(key) == null) {
              throw new IllegalArgumentException("The property " + key + " does not exist.");
          }
  
          return properties.getProperty(key);
      }
docs: Get the property.
  
  @param  properties  the properties.
  @param  key         the property key.
  
  @return  the property.
  
  @throws  IllegalArgumentException  if the property does not exist.