在collectors.groupingby（student:：getgrade，…）；使用getgrade（）方法时没有新的。getgrade不是静态方法。如果没有新的

class TestClass {
    public static void main(String[] args) {

        var ls = Arrays.asList(new Student("S1", Student.Grade.A),
                new Student("S2", Student.Grade.A),
                new Student("S3", Student.Grade.B),
                new Student("S4", Student.Grade.C),
                new Student("S5", Student.Grade.F));

        var group = ls.stream().filter(student -> student.getGrade() != Student.Grade.F).collect(
                Collectors.groupingBy(Student::getGrade, Collectors.mapping(Student::getName, Collectors.toList())));
        System.out.println(group);

    }
}

lamda格式

var group1 = ls.stream().filter(student -> student.getGrade() != Student.Grade.F).collect(
            Collectors.groupingBy(student -> student.getGrade(), Collectors.mapping(student -> student.getName(), Collectors.toList())));

在学生课堂上，getgrade方法不是静态方法。

class Student {
    public static enum Grade {
        A, B, C, D, F
    }

    private String name;
    private Grade grade;

    public Student(String name, Grade grade) {
        this.name = name;
        this.grade = grade;
    }

    public String toString() {
        return name + ":" + grade;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Grade getGrade() {
        return grade;
    }

    public void setGrade(Grade grade) {
        this.grade = grade;
    }
}