使用Java 8流API按多个属性对对象进行分组

Question

鉴于我们有一个银行列表，每个银行都有多个办事处，

public class Bank {
   private String name;
   private List<String> branches;
   public String getName(){
       return name;
   }
   public List<String> getBranches(){
       return branches;
   }
}

例如：

Bank "Mizuho": branches=["London", "New York"]
Bank "Goldman": branches = ["London", "Toronto"]

给定一个银行列表，我将拥有每个城市的银行代表图。在上面的示例中，我需要

Map["London"] == ["Mizuho", "Goldman"]
Map["New York"] == ["Mizuho"]
Map["Toronto"] == ["Goldman"]

如何使用Java 8 API实现该结果？使用Java8之前的版本很简单，但很繁琐。谢谢你。

Answer 1

Map<String, Set<Bank>> result = new HashMap<>();
for (Bank bank : banks) {
    for (String branch : bank.getBranches()) {
        result.computeIfAbsent(branch, b -> new HashSet<Bank>()).add(bank);
    }
}

Answer 2

 banks.flatMap(bank -> bank.getBranches()
             .stream()
             .map(branch -> new AbstractMap.SimpleEntry<>(branch, bank)))
       .collect(Collectors.groupingBy(
             Entry::getKey, 
             Collectors.mapping(Entry::getValue, Collectors.toList())));

结果将是：

{London=[Mizuho, Goldman], NewYork=[Mizuho], Toronto=[Goldman]}

Answer 3

您可以使用接受供应商、累加器函数和组合器函数的Stream.collect版本来完成此操作，如下所示：

Map<String, List<Bank>> result = banks.stream()
    .collect(
        HashMap::new,
        (map, bank) -> bank.getBranches().forEach(branch ->
            map.computeIfAbsent(branch, k -> new ArrayList<>()).add(bank)),
        (map1, map2) -> map2.forEach((k, v) -> map1.merge(k, v, (l1, l2) -> {
            l1.addAll(l2);
            return l1;
        })));