use comparator in jdk1.4

[FY Hsieh]

I have class like

   

class Student implements Comparator {
  private int id;
  ...
   public int compare (Object o1, Object o2) {
       Student s1 = (Student) o1;
       Student s2 = (Student) o2;
       if(s1.getId() < s2.getId()) return -1;
       ....


   }

}


public class Test {
    ArrayList students = new ArrayList();
    // add several student object to the list...
    ...
    Collections.sort(students);

}

 

It compiles but gives run time ClassCastException error "Student can not be casted to Comparable". I am not using Comparable. I am using Comparator.
Why do I get that error ? what's the best and easy way to fix assuming I still use Comparator and Arraylist ?

[Christophe Verré]

If you want to make Students comparable, you should implement the Comparable interface instead. If you insist on making the Student class to be a Comparator as well, you have to use the sort method the following way :

Collections.sort(students, new Student());

This will tell the Collections class to use a new comparator to compare the Students. That looks awkward though. You should use Comparable instead.