code: public <R> SingleOutputStreamOperator<R> map(MapFunction<T, R> mapper) {
  
  		TypeInformation<R> outType = TypeExtractor.getMapReturnTypes(clean(mapper), getType(),
  				Utils.getCallLocationName(), true);
  
  		return transform("Map", outType, new StreamMap<>(clean(mapper)));
  	}
docs: Applies a Map transformation on a {@link DataStream}. The transformation
  calls a {@link MapFunction} for each element of the DataStream. Each
  MapFunction call returns exactly one element. The user can also extend
  {@link RichMapFunction} to gain access to other features provided by the
  {@link org.apache.flink.api.common.functions.RichFunction} interface.
  
  @param mapper
  The MapFunction that is called for each element of the
  DataStream.
  @param <R>
  output type
  @return The transformed {@link DataStream}.