在对象列表中查找元素的最大大小

Question

@Data

class Person {

    private String fname;

    private String lname;

    private List<String> friends;

    private List<BigDecimal> ratings;

    ...

}

@Data

class People {

    private List<Person> people;

    ...

}

假设我有上述课程，并且需要弄清楚一个人拥有的最多朋友数量是多少。我知道 pre-streams 的做法......通过循环列表中的每个元素并检查friends.size() 是否大于当前存储的最长列表的数量。有没有办法用流简化代码？像这个答案？

Answer 1

根据依赖于 的大小的比较器计算Person包含的最大值：PeoplePerson.getFriends()Optional<Person> p = people.getPeople()&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;.stream()&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;.max(Comparator.comparingInt(p -> p.getFriends()&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;.size()));请注意，您还可以仅检索不与 Person 关联的朋友的最大数量：OptionalInt maxFriends = people.getPeople()&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;.stream()&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;.mapToInt(p -> p.getFriends()&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;.size())&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;.max();

Answer 2

您可以在Person类中声明以下方法：public int getNumberOfFriends(){&nbsp; &nbsp;return friends.size();}然后在这样的流中使用它：Optional <Person> personWithMostFriends = people.stream().max(Comparator.comparingInt(Person::getNumberOfFriends));通过这种方法，您将获得Person最多朋友的对象，而不仅仅是某人建议的最大朋友数。

Answer 3

你的问题已经回答了。但我添加这个是为了一件事。如果您的人员列表很大，并且您有多核电脑并且您想有效地使用它，那么请使用parallelstream()。得到人：Person person = people.getPeople().parallelStream().max(Comparator.comparingInt(p-> p.getFriends().size())).orElse(null);获取尺寸：int size = people.getPeople().parallelStream().mapToInt(p -> p.getFriends().size()).max().orElse(0);