根据对象 JAVA 11 中值的自定义优先级对对象列表进行排序

Question

我有一个Student(name, result, rollNo)对象列表。

List.of(
                new Student("Sam", "Pass", "100"),
                new Student("Gill", "Not available", "101"),
                new Student("Joe", "Fail", "111"),
                new Student("Matt", "Fail", "115"),
                new Student("Ann", "Pass", "121"),
                new Student("Moss", "Pass", "133"),
        );

我需要根据以下要求对上面的列表进行排序（我使用的是 Java 11）

1. 按学生成绩的顺序对列表进行排序，顺序为“不及格”、“通过”、“不可用”（不按字母顺序排列）成绩不及格的学生应排在列表的顶部。然后“通过”，然后“不可用”
2. 如果多个学生的结果相同，则按rollNo升序排序。

最终结果应如下所示。

{"Joe", "Fail", "111"}
{"Matt", "Fail", "115"}
{"Sam", "Pass", "100"}
{"Ann", "Pass", "121"}
{"Moss", "Pass", "133"}
{"Gill", "Not available", "101"}

Answer 1

从 Java 8 开始，我们可以对多个字段进行流排序

当您想按优先级“失败”、“通过”、“不可用”排序时，一种替代方法是在学生中添加额外的字段。当没有自然顺序时，这将有助于确定排序的优先级。在此示例中，学生按“通过”、“失败”、“不可用”排序，然后按姓名排序，只是为了显示更多选项。

排序代码片段：

    Comparator<Student> compareByPriorityThenName =
            Comparator.comparing(Student::getPriority)
                    .thenComparing(Student::getName);

    List<Student> sortedStudents = students.stream()
            .sorted(compareByPriorityThenName)
            .collect(Collectors.toList());

上下文中的代码片段：

public static void main(String []args){

    List<Student> students = List.of(
            new Student("Sam", "Pass", "100"),
            new Student("Gill", "Not available", "101"),
            new Student("Joe", "Fail", "111"),
            new Student("Matt", "Fail", "115"),
            new Student("Ann", "Pass", "121"),
            new Student("Moss", "Pass", "133")
    );

    Comparator<Student> compareByPriorityThenName =
            Comparator.comparing(Student::getPriority)
                    .thenComparing(Student::getName);

    List<Student> sortedStudents = students.stream()
            .sorted(compareByPriorityThenName)
            .collect(Collectors.toList());

    sortedStudents.forEach(System.out::println);
}

输出：

Student{name='Ann', result='Pass', rollNo='121', priority='1'}
Student{name='Moss', result='Pass', rollNo='133', priority='1'}
Student{name='Sam', result='Pass', rollNo='100', priority='1'}
Student{name='Joe', result='Fail', rollNo='111', priority='2'}
Student{name='Matt', result='Fail', rollNo='115', priority='2'}
Student{name='Gill', result='Not available', rollNo='101', priority='3'}

在学生中添加字段：

private int priority;

添加了在 Student 类中排序优先级的方法：

public int getPriority() {
    setPriority();
    return priority;
}

private int setPriority() {
    if(priority > 0) {
        return priority;
    }

    switch(result) {
    case "Pass":
        priority = 1;
        break;
    case "Fail":
        priority = 2;
        break;
    default:
        priority = 3;
    }
    return priority;
}

上下文中的 getPriority() 方法：

public class Student {

    private final String name;
    private final String result;
    private final String rollNo;
    private int priority;

    public Student(String name, String result, String rollNo) {
        this.name = name;
        this.result = result;
        this.rollNo = rollNo;
        this.priority = 0;
    }

    public String getName() {
        return name;
    }

    public String getResult() {
        return result;
    }

    public String getRollNo() {
        return rollNo;
    }

    public int getPriority() {
        setPriority();
        return priority;
    }

    private int setPriority() {
        if(priority > 0) {
            return priority;
        }

        switch(result) {
        case "Pass":
            priority = 1;
            break;
        case "Fail":
            priority = 2;
            break;
        default:
            priority = 3;
        }
        return priority;
    }

    @Override
    public String toString() {
        return "Student{" +
                "name='" + name + '\'' +
                ", result='" + result + '\'' +
                ", rollNo='" + rollNo + '\'' +
                ", priority='" + priority + '\'' +
                '}';
    }
}