code: @Override
      public Integer getInteger(final String key) {
          return retrieve(new Callable<Integer>() {
              @Override
              public Integer call() throws Exception {
                  return configuration.getInt(key);
              }
          }, null);
      }
docs: Get a property as Integer or null if not there / or property no integer.
  
  @param key the key
  @return the property or {@literal null} if not there or property no integer