code: public <R> MapOperator<T, R> map(MapFunction<T, R> mapper) {
  		if (mapper == null) {
  			throw new NullPointerException("Map function must not be null.");
  		}
  
  		String callLocation = Utils.getCallLocationName();
  		TypeInformation<R> resultType = TypeExtractor.getMapReturnTypes(mapper, getType(), callLocation, true);
  		return new MapOperator<>(this, resultType, clean(mapper), callLocation);
  	}
docs: Applies a Map transformation on this DataSet.
  
  <p>The transformation calls a {@link org.apache.flink.api.common.functions.MapFunction} for each element of the DataSet.
  Each MapFunction call returns exactly one element.
  
  @param mapper The MapFunction that is called for each element of the DataSet.
  @return A MapOperator that represents the transformed DataSet.
  
  @see org.apache.flink.api.common.functions.MapFunction
  @see org.apache.flink.api.common.functions.RichMapFunction
  @see MapOperator