Java 8 : Merge two list of objects by id

Question

我有两个对象列表。 Person 和 PurchaseOrder。

Person 有姓名、年龄、身份证等。
PurchaseOrder 有一个 personId(FK Person id)、时间戳、金额等。

使用 Java 8，如何获取年龄 < 18 岁的人的平均采购订单金额？

List<Person> personList = Arrays.asList(person);
List<PurchaseOrder> purchaseOrderList = Arrays.asList(purchaseOrders);

List<Person> underAgePerson = personList.stream()
        .filter(p -> p.getAge < 18)
        .collect(Collectors.toList());

// TODO:
purchaseOrderList.stream()
        .forEach(o1 -> underAgePerson.stream()
                .filter(o2 -> o1.getPersonId() == o2.getId()));

purchaseOrderList.stream().mapToDouble(o2 -> Long.valueOf(o2.getAmount())).average();

Answer 1

我会创建一组小于 18 岁的 Person id，然后计算他们的平均购买量:

List<Person> persons = Arrays.asList(
    new Person(1, 18), //(id, age)
    new Person(2, 24),
    new Person(3, 14),
    new Person(4, 10)
);

//Same person can place multiple purchases.
List<PurchaseOrder> purchaseOrders = Arrays.asList(
    new PurchaseOrder(1, 24), //(person id, purchase amount)
    new PurchaseOrder(1, 40),
    new PurchaseOrder(2, 10),
    new PurchaseOrder(2, 24),
    new PurchaseOrder(3, 23),
    new PurchaseOrder(3, 45),
    new PurchaseOrder(4, 68),
    new PurchaseOrder(4, 12),
    new PurchaseOrder(4, 23)
);

Set<Integer> personIds = persons
    .stream()
    .filter(person -> person.getAge() < 18)
    .map(Person::getId)
    .collect(Collectors.toSet());

OptionalDouble average = purchaseOrders
    .stream()
    .filter(purchaseOrder -> personIds.contains(purchaseOrder.getPersonId()))
    .mapToInt(PurchaseOrder::getAmount)
    .average();