code: public ConnectedStreams<IN1, IN2> keyBy(String[] fields1, String[] fields2) {
  		return new ConnectedStreams<>(environment, inputStream1.keyBy(fields1),
  				inputStream2.keyBy(fields2));
  	}
docs: KeyBy operation for connected data stream using key expressions.
  the elements of input1 and input2 according to fields1 and fields2. A
  field expression is either the name of a public field or a getter method
  with parentheses of the {@link DataStream}S underlying type. A dot can be
  used to drill down into objects, as in {@code "field1.getInnerField2()" }
  .
  
  @param fields1
  The grouping expressions for the first input
  @param fields2
  The grouping expressions for the second input
  @return The grouped {@link ConnectedStreams}