Java 8 按字符串分组 [英] Java 8 group by String

问题描述

这是我的代码:

public class StudentData {

    public static List<Student> getData() {

        return Arrays.asList(
            new Student(1, "a1", 1, Arrays.asList("cricket", "football", "basketball")),
            new Student(2, "a2", 1, Arrays.asList("chess", "football")),
            new Student(3, "a3", 2, Arrays.asList("running")),
            new Student(4, "a4", 2, Arrays.asList("throwball", "football")),
            new Student(5, "a5", 3, Arrays.asList("cricket", "basketball")),
            new Student(6, "a6", 4, Arrays.asList("cricket")), new Student(7, "a7", 5, Arrays.asList("basketball")),
            new Student(8, "a8", 6, Arrays.asList("football")),
            new Student(9, "a9", 8, Arrays.asList("tennis", "swimming")),
            new Student(10, "a10", 8, Arrays.asList("boxing", "running")),
            new Student(11, "a11", 9, Arrays.asList("cricket", "football")),
            new Student(12, "a12", 11, Arrays.asList("tennis", "shuttle")),
            new Student(13, "a13", 12, Arrays.asList("swimming"))
        );
    }

}

如何根据兴趣对学生进行分组.我尝试了以下代码:

How to group the student based on hobbies. I tried this below code:

List<Student> data = StudentData.getData();
data.stream().collect(Collectors.groupingBy(s -> s.getHobbies().stream()));

它没有给出正确的答案.

It is not giving the right answer.

推荐答案

你基本上需要一个由 Pair 组成的 Stream(我选择 AbstractMap.SimpleEntry 这里)，左边部分是爱好，右边是学生(可能反过来，没关系).

You basically need a Stream that is made out of a Pair (I choose AbstractMap.SimpleEntry here) that has the left part as a Hobby and right as the Student (could be the other way around, does not matter).

稍后只需根据 Hobby (在您的情况下为字符串)对那些进行分组.

Later just group those based on Hobby (that is a String in your case).

data.stream()
    .flatMap(student -> student.getHobbies().stream().map(hobby -> new SimpleEntry<>(hobby, student)))
    .collect(Collectors.groupingBy(
            Entry::getKey,
            Collectors.mapping(Entry::getValue, Collectors.toList())
));

Entry::getKey 是获取密钥的方法引用，如果对您更有意义，您也可以将其编写为 lambda 表达式:

Entry::getKey being a method reference that gets the key, you could write it as a lambda expression too, if it makes more sense for you:

Collectors.groupingBy(entry -> entry.getKey())

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