Multiple aggregate functions in Java 8 Stream API

I have a class defined like

public class TimePeriodCalc {
    private double occupancy;
    private double efficiency;
    private String atDate;
}

I would like to perform the following SQL statement using Java 8 Stream API.

SELECT atDate, AVG(occupancy), AVG(efficiency)
FROM TimePeriodCalc
GROUP BY atDate

I tried :

Collection<TimePeriodCalc> collector = result.stream().collect(groupingBy(p -> p.getAtDate(), ....

What can be put into the code to select multiple attributes ? I'm thinking of using multiple Collectors but really don't know how to do so.