在此示例中，对实例方法的方法引用在没有new（）的情况下如何工作？

Question

在Collectors.groupingBy（学生：：getGrade，…）；使用getGrade（）方法时不使用新关键字。方法getGrade不是静态方法。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())));

在学生类中，getLevel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;
    }
}

Answer 1

方法引用匹配函数

(element) -> element.getGrade()