GroupingBy with multiple columns

Below is learnt from my colleague. Next time i wish i can solve it independently.

 

import static java.util.stream.Collectors.groupingBy;
import static java.util.stream.Collectors.reducing;
import static java.util.stream.Collectors.toList;

import java.util.List;
import java.util.Map;
import java.util.Optional;
import java.util.stream.Collectors;
...

list.stream().collect(groupingBy(item -> item.getType() + item.getLevel(),
                reducing(Item::summarize))) // summarize: static function to aggregate items
        .values().stream().map(Optional::get)
        .collect(toList());