code: public void addProperty(final String propertyName, final String value) {
          Preconditions.checkNotNull(propertyName);
          Preconditions.checkNotNull(value);
          List<String> values = properties.get(propertyName);
          if (values == null) {
              values = new ArrayList<>();
              values.add(value);
              properties.put(propertyName, values);
          } else {
              values.add(value);
          }
      }
docs: Add a value to a property on this bean.
  
  @param propertyName name of the property as defined by the bean's schema.
  @param value final String representations of the property that conforms to
  its type as defined by the bean's schema.