根据教师 ID 对学生列表进行排序

Question

我有一份学生名单，上面有他们联系的老师 ID。下面是学生豆。

public class Student {
    private Integer Id;
    private String Name;
    private Integer teacherId;

public Student(Integer empId, String empName, Integer teacherId) {
    super();
    this.empId = empId;
    this.empName = empName;
    this.teacherId = teacherId;
}
   // setters getters
}

现在在我的主要课程中，我有学生列表，其数据如下。

public static void main(String[] args) {
     int teacherId = 6;
    List<Student> list = new ArrayList<Student>();

    list.add(new Student(1002, "Kausik",4));
    list.add(new Student(1004, "Tridibendu",6));
    list.add(new Student(1007, "Ram",8));
    list.add(new Student(1003, "Neeraj",6));
    list.add(new Student(1009, "Manish",8));
    list.add(new Student(1001, "Shyam",1));

    // Sorting collection in ascending order based on teacher id
    Collections.sort(list, new StudentComparator());

}

下面是我的学生比较类。谁能帮我在比较方法中写什么，以便如果我将 2 作为教师 id 传递，列表应该按这样一种方式排序，即所有与教师 id 2 联系的学生都应该在顶部，其余的必须在底部.

或者有没有其他方法可以在java中做到这一点？

public class StudentComparator implements Comparator<Student> {

    @Override
    public int compare(Student o1, Student o2) {        
        return o1.getTeacherId().compareTo(o2.getTeacherId());
    }
}

下面是我得到的输出。

Id: 1001, Name: Shyam, TeacherId: 1
Id: 1002, Name: Kausik, TeacherId: 4
Id: 1004, Name: Tridibendu, TeacherId: 6
Id: 1003, Name: Neeraj, TeacherId: 6
Id: 1007, Name: Ram, TeacherId: 8
Id: 1009, Name: Manish, TeacherId: 8

但我希望输出基于传递的教师 ID 作为参数。如果我将 6 作为教师 ID 传递，那么我的输出应该如下：

Id: 1004, Name: Tridibendu, TeacherId: 6
 Id: 1003, Name: Neeraj, TeacherId: 6
 Id: 1001, Name: Shyam, TeacherId: 1
 Id: 1002, Name: Kausik, TeacherId: 4
 Id: 1007, Name: Ram, TeacherId: 8
 Id: 1009, Name: Manish, TeacherId: 8

所以老师的 id 因为 6 应该在列表的顶部。

Answer 1

你可以做的是让Student实现Comparable<Student>。为此，您可以这样做：

public class Student implements Comparable<Student> {

...

    @Override
    public int compareTo(Student s) {
        return this.teacherId.compareTo(s.teacherId);
    }
}

与您的其他问题有关，您可以只实现另一个比较器，它允许您根据给定的教师 ID 将列表分成两部分。

public class StudentComparator implements Comparator<Student> {
    private Integer id;

    public StudentComparator(Integer id) {
        this.id = id;
    }

    @Override
    public int compare(Student o1, Student o2) {
        int id1 = o1.getTeacherId(); // Let the unboxing do its job
        int id2 = o2.getTeacherId();
        if (id1 == id2) return 0;
        if (id1 == this.id) return 1;
        if (id2 == this.id) return -1;
        return 0;
    }
}

因此，无论何时您想根据一个教师 ID 对列表进行分区，您都可以致电 Collections.sort(studentList, new StudentComparator(teacherId))

Answer 2

您可以强制它为匹配的老师返回负数，然后正常排序其余的：

public class StudentComparator implements Comparator<Student> {
    private int contactedTeacher;
    public StudentComparator(int contactedTeacherId) {
        this.contactedTeacher = contactedTeacherId;
    }
    @Override
    public int compare(Student o1, Student o2) {
        if(this.contactedTeacher == o1.teacherId) {
            return -1; //Force negative
        } else if(this.contactedTeacher == o2.teacherId) {
            return 1; //Force positive
        }

        //If neither of the compared students contacted the teacher, compare normally by teacher ID
        return o1.getTeacherId().compareTo(o2.getTeacherId());
    }
}

那么你的排序调用变成：

Collections.sort(list, new StudentComparator(teacherId));