I retrieve from a JPA Repository query an Entity which refers to these table

I'd like to group by my List of Entities by City and obtain this response:

{"New York":["John","Josh"],"Atlanta":["Paul", "Charles"],"Los Angeles":["Mark","Susan"]}

I tried to follow This link but I only can obtain a Map<String, List< RegistryEntity >. I didn't want these, I want Map<String, List< String >> so I did in these way.

public Map<String, List<String>> findAllUsers() {
    List<RegistryEntity> items = registryRepository.findAll();
    Map<String, List<String>> itemsGrouped = new HashMap<>();
    for (RegistryEntity s: items) {
        if (itemsGrouped.get(s.getCity()) != null) {
            itemsGrouped.get(s.getCity()).add(s.getName());
        }
        else {
            List<String> tempResults = new ArrayList<>();
            tempResults.add(s.getCity());
            itemsGrouped.put(s.getName(), tempResults);
        }
    }
    return itemsGrouped;
}

The code works, but I'd like to know if there's something more efficient and more elegant to group by my entity.

CodePudding user response：

You can use Stream then you need also collectors.groupingBy and collectors.mapping, i tried to code something like down below.

Map<String, List<String>> itemsGrouped = 
    items.stream()
           .collect(Collectors.groupingBy(
               RegistryEntity::getCity,
               Collectors.mapping(RegistryEntity::getName, Collectors.toList()))
           );