stream 分组大全

对象准备：

public class Student {
    //学生年级
    private String grade;
    //参加的科目
    private String subject;
    //名字
    private String name;
    //科目成绩
    private Double mark;
 
    public Student(String grade, String subject, String name, Double mark) {
        this.grade = grade;
        this.subject = subject;
        this.name = name;
        this.mark = mark;
    }
 
    public String getGrade() {
        return grade;
    }
 
    public void setGrade(String grade) {
        this.grade = grade;
    }
 
    public String getSubject() {
        return subject;
    }
 
    public void setSubject(String subject) {
        this.subject = subject;
    }
 
    public String getName() {
        return name;
    }
 
    public void setName(String name) {
        this.name = name;
    }
 
    public Double getMark() {
        return mark;
    }
 
    public void setMark(Double rank) {
        this.mark = rank;
    }
 
    @Override
    public String toString() {
        return "Student{" +
                "grade='" + grade + '\'' +
                ", subject='" + subject + '\'' +
                ", name='" + name + '\'' +
                ", mark='" + mark + '\'' +
                '}';
    }
}

构建数据

List<Student> students = new ArrayList<>();
        Student student1 = new Student("六年级","Math","小张",86D);
        Student student2 = new Student("六年级","English","小张",84D);
        Student student3 = new Student("六年级","Chinese","小张",83D);
        Student student1s = new Student("六年级","Math","小宋",84D);
        Student student2s = new Student("六年级","English","小宋",95D);
        Student student3s = new Student("六年级","Chinese","小宋",83D);
        students.add(student1);
        students.add(student1s);
        students.add(student2);
        students.add(student2s);
        students.add(student3);
        students.add(student3s);
 
        Student student4 = new Student("五年级","Math","小杨",78D);
        Student student5 = new Student("五年级","English","小杨",97D);
        Student student6 = new Student("五年级","Chinese","小杨",68D);
        Student student4s = new Student("五年级","Math","小赵",67D);
        Student student5s = new Student("五年级","English","小赵",93D);
        Student student6s = new Student("五年级","Chinese","小赵",92D);
        students.add(student4);
        students.add(student4s);
        students.add(student5);
        students.add(student5s);
        students.add(student6);
        students.add(student6s);

数据分组操作

1.单条件分组

根据姓名进行分组：

 /*按名字分组*/
        Map<String,List<Student>> map = students.stream().collect(Collectors.groupingBy(Student::getName));
        System.out.println(map);
        Set ketSet = map.keySet();
        System.out.println(ketSet);
        for (Object key:ketSet){
            System.out.println(key + " 同学：");
            System.out.println(map.get(key));
        }

2.多条件分组

先按照分数进行分组，再按照科目进行分组

/*先根据成绩分组，再根据名字分组*/

Map<Double,Map<String,List<Student>>> map2 =
                students.stream().collect(Collectors.groupingBy(Student::getMark,Collectors.groupingBy(Student::getSubject)));
        System.out.println(map2);
        Set ketSet2 = map2.keySet();
        System.out.println(ketSet2);
        for (Object key:ketSet2){
            System.out.println(map2.get(key));
            System.out.println("==============");
        }

3.单条件分组，按照自定义条件分组

//找出所有成绩在85之上的学生(包括85)分为合格，85之下的为不合格

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public static void getMarkByStandard(){
    Map<String,List<Student>> map = students.stream().collect(Collectors.groupingBy(student ->{
        if (student.getMark().intValue() >= 85){
            return "合格";
        }else{
            return "不合格";
        }
    }));

    System.out.println(map.get("合格"));
    System.out.println(map.get("不合格"));
    }

4.多条件分组，按照自定义条件分组

上面两个例子的分组产生的Map的key值都是直接取的对象里的值，其实也可以自己规定分组的规则和key的名字，我们可以在Lambda的右边使用大括号，在大括号里面写逻辑代码，只要保证返回的值是符合格式的就可以，比如现在就是以85分为界限进行分组，Map的key值也也就是我们返回的两个值。

Map<String,Map<String,List<Student>>> map = students.stream().collect(Collectors.groupingBy(Student::getGrade,
                Collectors.groupingBy(student -> {
                    if (student.getMark().intValue() >= 85){
                        return "合格";
                    }else{
                        return "不合格";
                    }
                })));
        System.out.println(map);
        Set ketSet = map.keySet();
        System.out.println(ketSet);
        System.out.println(map.get("五年级"));
        System.out.println(map.get("六年级"));

5.分组数量统计

5.1.单条件分组数量统计

students.stream().collect(Collectors.groupingBy(EnergyLog::getMark,Collectors.counting()));

5.2.多条件分组数量统计

有时候我们只是想要统计每个分组的数量，而不是获得每个分组里面元素的详细信息，这个也是可以实现的。只是在上述的二级分组基础上调用了Collectors.counting()方法来统计每个分组里面的数量

Map<String,Map<String,Long>> map = students.stream().collect(Collectors.groupingBy(Student::getGrade,
                Collectors.groupingBy(student -> {
                    if (student.getMark().intValue() >= 85){
                        return "合格";
                    }else{
                        return "不合格";
                    }
                },Collectors.counting())
                )
        );
        System.out.println(map);


输出：{五年级={合格=3, 不合格=3}, 六年级={不合格=4, 合格=2}}

6. 取最大值

在分组的基础上获得分数最高的学生的信息

Map<String, Optional<Student>> map = students.stream().collect(Collectors.groupingBy(Student::getGrade,
                Collectors.maxBy(Comparator.comparing(Student::getMark))));
        System.out.println(map);


输出：{五年级=Optional[Student{grade='五年级', age=11, subject='English', name='小杨', mark='97.0'}], 六年级=Optional[Student{grade='六年级', age=12, subject='English', name='小宋', mark='95.0'}]}

7.分区

分区其实就是特殊的分组，它只是固定将流中的数据分为两组，大致用法其实和分组一样。

public static void partitionByStandard(){
    Map<String,Map<Boolean,Long>> map = students.stream().collect(Collectors.groupingBy(Student::getGrade,
        Collectors.partitioningBy(student -> {
            if (student.getMark().intValue() >= 85){
                return true;
            }else{
                return false;
            }
        },Collectors.counting())
        )
    );
    System.out.println(map);
}

输出：{六年级={false=4, true=2}, 五年级={false=3, true=3}}

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