code: public ConnectedStreams<IN1, IN2> keyBy(String field1, String field2) {
  		return new ConnectedStreams<>(environment, inputStream1.keyBy(field1),
  				inputStream2.keyBy(field2));
  	}
docs: KeyBy operation for connected data stream using key expressions. Assigns keys to
  the elements of input1 and input2 according to field1 and field2. 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 field1
  The grouping expression for the first input
  @param field2
  The grouping expression for the second input
  @return The grouped {@link ConnectedStreams}