code: public SingleOutputStreamOperator<T> filter(FilterFunction<T> filter) {
  		return transform("Filter", getType(), new StreamFilter<>(clean(filter)));
  
  	}
docs: Applies a Filter transformation on a {@link DataStream}. The
  transformation calls a {@link FilterFunction} for each element of the
  DataStream and retains only those element for which the function returns
  true. Elements for which the function returns false are filtered. The
  user can also extend {@link RichFilterFunction} to gain access to other
  features provided by the
  {@link org.apache.flink.api.common.functions.RichFunction} interface.
  
  @param filter
  The FilterFunction that is called for each element of the
  DataStream.
  @return The filtered DataStream.