你如何组装地图 <Key, Value> 从使用Java8流的嵌套对象？

Question

Suppose I have 3 cars, each car having 3 engines, each engine having 3 parts. How can I create a map of <engine_model, List<parts>> out of this using Java8 streams?

In regular java I would do the following:

Map<String, List<Parts>> map = new HashMap<String, List<Parts>>();
for (Car car: cars){
  for (Engine engine: car.getEngines()){
    if (!map.contains(engine.getModel())){
      map.put(engine.getModel(), new ArrayList<Part>());
    }
    for (Part  part: engine.getParts()){
      map.get(engine.getModel()).add(part);
    }
  }
}

here map is a map that maps the engine's model ( type String ) to the list of all the parts pertaining to that model. different cars can have same engine model.

I tried to do the following:

carList.stream().flatMap(p -> p.getEngines().stream())
  .collect(Collectors.groupingBy(p -> p.getModel(),
               Collectors.mapping(Engine::getParts, Collectors.toList())))

the problem is that using above I am getting Map<String, List<List<Part>>> and I want Map<String, List<Part>>

I guess an equivalent question would be: how do I change the List of Lists to just List in the context of the above example?

Answer 1

First of all, you can even simplify your imperative variant:

Map<String, List<Part>> map = new HashMap<>();
for(Car car: cars) {
    for(Engine engine: car.getEngines()) {
        map.computeIfAbsent(engine.getModel(), key -> new ArrayList<>())
           .addAll(engine.getParts());
    }
}

That's important when deciding whether the functional variant is really an improvement. The flatMap + groupingBy collector is the right approach, but the fact that you have a list (of Part s) to flatten is an additional obstacle. In Java 9, you could write

Map<String, List<Part>> map = cars.stream()
    .flatMap(car -> car.getEngines().stream())
    .collect(Collectors.groupingBy(Engine::getModel,
                 Collectors.flatMapping(e -> e.getParts().stream(), Collectors.toList())));

But until then, we have to manually assemble an equivalent collector:

Map<String, List<Part>> map = cars.stream()
    .flatMap(car -> car.getEngines().stream())
    .collect(Collectors.groupingBy(Engine::getModel,
                 Collector.of(ArrayList::new, (l,e) -> l.addAll(e.getParts()),
                              (l1,l2) -> { l1.addAll(l2); return l1; })));

So I would consider staying with the loop for now.