code: public <R> MapOperator<T, R> map(MapFunction<T, R> mapper) {
  		if (mapper == null) {
  			throw new NullPointerException("Map function must not be null.");
  		}
  		return new MapOperator<T, R>(this, mapper);
  	}
docs: Applies a Map transformation on a {@link DataSet}.<br/>
  The transformation calls a {@link 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 MapFunction
  @see MapOperator
  @see DataSet