Java8?stream流分組groupingBy的使用方法代碼

更新時間：2024年01月29日 15:14:45 作者：蘇格拉帝

對于java8的新特性groupingBy方法,相信有很多人都在工作中用過,這篇文章主要給大家介紹了關(guān)于Java8?stream流分組groupingBy的使用方法,需要的朋友可以參考下

眾所周知，使用stream流可以讓我們的代碼看上去很簡潔，現(xiàn)在我們實戰(zhàn)使用一下stream的分組與分區(qū)。

準備用到的數(shù)據(jù)類
public class Student{
        //年級
        private String grade;
        //班級
        private String classNumber;
        //姓名
        private String name;
        //年齡
        private int age;
        //地址
        private String address;
        //數(shù)學成績
        private int mathScores;
        //語文成績
        private int chainessScores;
    }
添加數(shù)據(jù)
        Student student1 = new Student("701","張三",16,"北京",78,90);
        Student student2 = new Student("700","李四",17,"北京",78,90);
        Student student3 = new Student("703","王五",16,"上海",78,90);
        Student student4 = new Student("701","趙六",16,"上海",78,90);
        Student student5 = new Student("700","錢七",18,"",78,90);
        Student student6 = new Student("701","老八",17,"",78,90);
//這是一個高二年級的成績單
        List<Student> students = Stream.of(student1,student2,student3,student4,student5,student6).collect(Collectors.toList());

按照班級分組

Map<String, List<Student>> collect = students.stream().collect(Collectors.groupingBy(Student::getClassNumber));
System.out.println(JSON.toJSONString(collect));
//{"700":[{"age":17,"chainessScores":90,"classNumber":"700","mathScores":78,"name":"李四"},{"age":18,"chainessScores":90,"classNumber":"700","mathScores":78,"name":"錢七"}],
//"701":[{"age":16,"chainessScores":90,"classNumber":"701","mathScores":78,"name":"張三"},{"age":16,"chainessScores":90,"classNumber":"701","mathScores":78,"name":"趙六"},{"age":17,"chainessScores":90,"classNumber":"701","mathScores":78,"name":"老八"}],
//"703":[{"age":16,"chainessScores":90,"classNumber":"703","mathScores":78,"name":"王五"}]}

按照班級分組得到每個班級的同學姓名

Map<String, List<String>> collect = students.stream().collect(Collectors.groupingBy(Student::getClassNumber, Collectors.mapping(Student::getName, Collectors.toList())));
System.out.println(JSON.toJSONString(collect));
//{"700":["李四","錢七"],"701":["張三","趙六","老八"],"703":["王五"]}

統(tǒng)計每個班級人數(shù)

Map<String, Long> collect = students.stream().collect(Collectors.groupingBy(Student::getClassNumber, Collectors.counting()));
System.out.println(JSON.toJSONString(collect));
//{"700":2,"701":3,"703":1}

求每個班級的數(shù)學平均成績

Map<String, Double> collect = students.stream().collect(Collectors.groupingBy(Student::getClassNumber, Collectors.averagingDouble(Student::getMathScores)));
System.out.println(JSON.toJSONString(collect));
//{"700":65.0,"701":61.0,"703":82.0}

按班級分組求每個同學的總成績

Map<String, Map<String, Integer>> collect = students.stream().collect(Collectors.groupingBy(Student::getClassNumber, Collectors.toMap(Student::getName, student -> student.getMathScores() + student.getChainessScores())));
System.out.println(JSON.toJSONString(collect));
//{"700":{"錢七":150,"李四":160},"701":{"張三":168,"老八":148,"趙六":137},"703":{"王五":172}}

嵌套分組，先按班級分組，再按年齡分組

Map<String, Map<Integer, List<Student>>> collect = students.stream().collect(Collectors.groupingBy(Student::getClassNumber, Collectors.groupingBy(Student::getAge)));

分組后得到一個線程安全的ConcurrentMap

ConcurrentMap<String, List<Student>> collect = students.stream().collect(Collectors.groupingByConcurrent(Student::getClassNumber));

加上排序來一波

根據(jù)年齡分組并小到大排序,TreeMap默認為按照key升序

TreeMap<Integer, List<String>> collect = students.stream().collect(Collectors.groupingBy(Student::getAge, TreeMap::new, Collectors.mapping(Student::getName, Collectors.toList())));
System.out.println(JSON.toJSONString(collect));
 //{16:["張三","王五","趙六"],17:["李四","老八"],18:["錢七"]}

根據(jù)年齡分組并大到小排序,因為TreeMap默認為按照key升序，所以排完順序再反轉(zhuǎn)一下就OK了

TreeMap<Integer, List<String>> collect = students.stream().collect(Collectors.groupingBy(Student::getAge, TreeMap::new, Collectors.mapping(Student::getName, Collectors.toList())));
Map<Integer, List<String>> collect2 = collect.descendingMap();
System.out.println(JSON.toJSONString(collect2));
//{18:["錢七"],17:["李四","老八"],16:["張三","王五","趙六"]}

附：高級用法

只是個人感覺比上面的高級一些，可能在大佬看來本質(zhì)上都一樣，只是更復雜了一點點。

1. 改變分組后的key和value

在上面的分組中，key還好，在4和5自定義條件分組中，改變了key。但是value呢？好像都是定死了，普通的分組就是該對象的集合，求和、統(tǒng)計就是一個數(shù)字，能不能我自己來規(guī)定返回的value類型呢？比如我要按照門派分組，得到每個門派里面每個人的名字，不需要其他的性別、年齡信息

代碼

// 按照門派分組，分組后 key 為Group對象， value變成String名字
Map<Group, List<String>> collect2 = users.stream()
                .collect(Collectors.groupingBy(x -> new Group(x.getGroup(), x.getGroup() + "的描述"),
                        Collectors.mapping(User::getUsername, Collectors.toCollection(ArrayList::new))));

結(jié)果

{BlogMain1.Group(name=明教, desc=明教的描述)=[張無忌, 金毛獅王--謝遜, 紫衫龍王--黛綺絲, 白眉鷹王--殷天正, 青翼蝠王--韋一笑], BlogMain1.Group(name=武當, desc=武當?shù)拿枋?=[張三豐, 宋遠橋, 張翠山, 殷梨亭], BlogMain1.Group(name=少林, desc=少林的描述)=[圓真, 空智], BlogMain1.Group(name=峨眉, desc=峨眉的描述)=[滅絕師太, 周芷若, 宋青書]}

2. 多級分組

先按照一個條件分組，在分組的基礎上再按照另一個條件分組

代碼

// 多級分組，先按照門派分組，門派里面再按照名字長度分組
Map<Group, Map<Integer, List<User>>> collect3 = users.stream().collect(
        Collectors.groupingBy(x -> new Group(x.getGroup(), x.getGroup() + "的描述"),
                Collectors.groupingBy(x -> x.username.length())));

結(jié)果

{BlogMain1.Group(name=明教, desc=明教的描述)={3=[BlogMain1.User(username=張無忌, gender=男, group=明教, age=23)], 8=[BlogMain1.User(username=金毛獅王--謝遜, gender=男, group=明教, age=47)], 9=[BlogMain1.User(username=紫衫龍王--黛綺絲, gender=女, group=明教, age=32), BlogMain1.User(username=白眉鷹王--殷天正, gender=男, group=明教, age=38), BlogMain1.User(username=青翼蝠王--韋一笑, gender=男, group=明教, age=35)]}, BlogMain1.Group(name=武當, desc=武當?shù)拿枋?={3=[BlogMain1.User(username=張三豐, gender=男, group=武當, age=56), BlogMain1.User(username=宋遠橋, gender=男, group=武當, age=42), BlogMain1.User(username=張翠山, gender=男, group=武當, age=41), BlogMain1.User(username=殷梨亭, gender=男, group=武當, age=37)]}, BlogMain1.Group(name=少林, desc=少林的描述)={2=[BlogMain1.User(username=圓真, gender=男, group=少林, age=35), BlogMain1.User(username=空智, gender=男, group=少林, age=47)]}, BlogMain1.Group(name=峨眉, desc=峨眉的描述)={3=[BlogMain1.User(username=周芷若, gender=女, group=峨眉, age=21), BlogMain1.User(username=宋青書, gender=男, group=峨眉, age=22)], 4=[BlogMain1.User(username=滅絕師太, gender=女, group=峨眉, age=51)]}}

3. 分組排序

就是先分組, 再每組分別排序, 經(jīng)常寫SQL的小伙伴可能經(jīng)常碰到這個問題, steam api 就非常簡單

代碼

Map<String, List<User>> userMap = users.stream().collect(Collectors.groupingBy(User::getGender,
        Collectors.mapping(u -> u, Collectors.collectingAndThen(Collectors.toList(),
                sortedStudents -> sortedStudents.stream().sorted((Comparator.comparing(User::getAge)))
                        .collect(Collectors.toList())))));

userMap.forEach((k, v) -> {
    System.err.println("k: " + k);
    v.forEach(System.err::println);
});

結(jié)果

k: 女
BlogMain1.User(username=周芷若, gender=女, group=峨眉, age=21)
BlogMain1.User(username=紫衫龍王--黛綺絲, gender=女, group=明教, age=32)
BlogMain1.User(username=滅絕師太, gender=女, group=峨眉, age=51)
k: 男
BlogMain1.User(username=宋青書, gender=男, group=峨眉, age=22)
BlogMain1.User(username=張無忌, gender=男, group=明教, age=23)
BlogMain1.User(username=青翼蝠王--韋一笑, gender=男, group=明教, age=35)
BlogMain1.User(username=圓真, gender=男, group=少林, age=35)
BlogMain1.User(username=殷梨亭, gender=男, group=武當, age=37)
BlogMain1.User(username=白眉鷹王--殷天正, gender=男, group=明教, age=38)
BlogMain1.User(username=張翠山, gender=男, group=武當, age=41)
BlogMain1.User(username=宋遠橋, gender=男, group=武當, age=42)
BlogMain1.User(username=金毛獅王--謝遜, gender=男, group=明教, age=47)
BlogMain1.User(username=空智, gender=男, group=少林, age=47)
BlogMain1.User(username=張三豐, gender=男, group=武當, age=56)

4. 我在項目中的使用例子