java Collections.binarySearch（）返回-2

Question

I have a text file which contains student information. I sort this information using the comparator class which implements the comparator interface.This sorts fine fine when calling Collection.Sort();

I then use the comparator in the binarySearch() method. I have an int index that returns the index of the binary search. after debugging it seems that the binarySearch() isnt sorting the data properly because I get an index of 0 for 501 which is correct, an index of 1 for 503 which is incorrect and an index of -2 for 502. Is the binarySearch not using the comparator apppropriately and thus not sorting the data appropriately as its returning the data in its original form.

 =================================================
    First Name: Mark
    Last Name: Evans
    Registration: 501

    Subject: Maths
    Assignment: 1
    Homewok Mark: 70
    Exam Mark: 80

    Assignment: 2
    Homewok Mark: 70
    Exam Mark: 40

    Subject: English
    Assignment: 1
    Homewok Mark: 40
    Exam Mark: 50

    Assignment: 2
    Homewok Mark: 60
    Exam Mark: 70

    Subject: Science
    Assignment: 1
    Homewok Mark: 50
    Exam Mark: 60

    Assignment: 2
    Homewok Mark: 80
    Exam Mark: 45

    Assignment: 3
    Homewok Mark: 67
    Exam Mark: 47

    Assignment: 4
    Homewok Mark: 38
    Exam Mark: 57

    =================================================
    First Name: Linda
    Last Name: Evans
    Registration: 503

    Subject: Maths
    Assignment: 1
    Homewok Mark: 50
    Exam Mark: 60

    Assignment: 2
    Homewok Mark: 70
    Exam Mark: 50

    =================================================
    First Name: Joseph
    Last Name: Evanw
    Registration: 502

    Subject: English
    Assignment: 1
    Homewok Mark: 40
    Exam Mark: 50

I have a comparator class that works fine and sorts the data appropriately.

public class StudentComparator implements Comparator<Student> {

    @Override
    public int compare(Student s1, Student s2) {

        int registrationNumber1 = s1.getRegistrationNumber();
        int registrationNumber12 = s2.getRegistrationNumber();

        return registrationNumber1 - registrationNumber12;
    }

}


public void searchStudentByName() {

        int number = 503;
        int subjectNumber = 1;

        int index = Collections.binarySearch(studentList, new Student(number),
                new StudentComparator());
        for (int i = 0; studentList.size() > i; i++) {
            if (i == index) {

                System.out.print(studentList.get(i));

            }
        }

    }

Answer 1

To perform a binary search, the collection must already be sorted. You are expecting binarySearch to sort it for you, but it doesn't sort it. It takes advantage of the assumption that it's already sorted. The Javadocs for Collections.binarySearch state:

Searches the specified list for the specified object using the binary search algorithm. The list must be sorted into ascending order according to the specified comparator (as by the sort(List, Comparator) method), prior to making this call. If it is not sorted, the results are undefined.

Add a call to Collections.sort before attempting a binary search.

StudentComparator sc = new StudentComparator();
Collections.sort(studentList, sc);

Then you can call Collections.binarySearch .