code: public SingleOutputStreamOperator<T> returns(TypeInformation<T> typeInfo) {
  		requireNonNull(typeInfo, "TypeInformation must not be null");
  
  		transformation.setOutputType(typeInfo);
  		return this;
  	}
docs: Adds a type information hint about the return type of this operator. This method
  can be used in cases where Flink cannot determine automatically what the produced
  type of a function is. That can be the case if the function uses generic type variables
  in the return type that cannot be inferred from the input type.
  
  <p>In most cases, the methods {@link #returns(Class)} and {@link #returns(TypeHint)}
  are preferable.
  
  @param typeInfo type information as a return type hint
  @return This operator with a given return type hint.