new Java对象列表,匹配另外两个列表中的条目

Question

假设我有这两个列表

List<Person> persons = Arrays.asList(
                new Person(1, "Mike", "Canada"),
                new Person(2, "Jill", "England"),
                new Person(3, "Will", "Whales"),
                new Person(4, "Mary", "Spain"));


List<Metadata> metadata= Arrays.asList(
                new metadata(1, "2000-01-01", "Naturalized", "Bachelor's of Arts"),
                new metadata(2, "2001-01-01", "ExPat", "Masters of Chemestry"),
                new metadata(3, "2017-05-01", "Citizen", "Buiness Management"),
                new metadata(4, "2018-04-16", "Work Visa", "Nursing"));

最终结果是新列表:

List<PersonWithMetadata> personsAndMEtadata = Arrays.asList(
                new PersonWithMetadata(1, "Mike", "Canada", "2000-01-01", "Naturalized", "Bachelor's of Arts"),
                new PersonWithMetadata(2, "Jill", "England", "2001-01-01", "ExPat", "Masters of Chemestry"),
                new PersonWithMetadata(3, "Will", "Whales", "2017-05-01", "Citizen", "Buiness Management"),
                new PersonWithMetadata(4, "Mary", "Spain", "2018-04-16", "Work Visa", "Nursing"));

我试图找到一种Java流方式将前两个列表组合成第三个 - 就像第一个输入上的SQL连接是一个id号.似乎应该有办法做到这一点,但我不知所措.怎么做的？另外,假设两个输入列表之间至多有一个匹配.

Answer 1

YCF_L的解决方案应该可行,但它是一个O(n ²)解决方案.通过将一个列表转换为从id到对象的映射,然后迭代到另一个并从地图获取匹配值,可以实现O(n)解决方案:

Map<Integer, Person> personMap = 
    persons.stream().collect(Collectors.toMap(Person::getId, Function.identity());

List<PersonWithMetadata> result = 
    metadata.stream()
            .map(m -> new PersonWithMetadata(personMap.get(m.getId()), m)
            .collect(Collectors.toList());

在示例数据中,列表具有匹配顺序的匹配对象.如果这个假设对于真正的问题也是如此,那么解决方案可能必须更容易 - 您可以流式传输索引并从列表中获取相应的值:

List<PersonWithMetadata> result = 
    IntStream.reange(0, persons.size())
             .map(i -> new PersonWithMetadata(persons.get(i), metadata.get(i))
             .collect(Collectors.toList());