facing exception in implementation of binarySearch method.

[Nakul P. Patel]

 class Student implements Comparable<Student>{
private int id;
private String name;

public Student(int id, String name){
    this.id = id;
    this.name = name;
}

public int getId(){
    return id;
}
public String getName(){
    return name;
}

@Override
public int compareTo(Student o) {

    int j = o.getId();
    int result = this.id - j;
    return result;
}


}
public class CollectionSearchDemo {


public static void main(String[] args) {

    List<Student> list = new ArrayList<Student>();
    list.add(new Student(3, "ouier"));
    list.add(new Student(2, "fdgds"));
    list.add(new Student(7, "kiluf"));
    list.add(new Student(1, "6trfd"));
    list.add(new Student(8, "hjgas"));
    list.add(new Student(5, "ewwew"));

    Collections.sort(list, new Comparator<Student>() {

        @Override
        public int compare(Student arg0, Student arg1) {

            return arg0.getId() - arg1.getId();
        }
    });

    Iterator iterator = list.iterator();
    while(iterator.hasNext()){
        Student student = (Student) iterator.next();
        System.out.print(student.getId()+":"+student.getName()+" ");
    }

    System.out.println("\nSorting in reverse order:");

//  Collections.reverse(list);
    Comparator<Student> collections = Collections.reverseOrder();
    Collections.sort(list, collections);

    Iterator iterator1 = list.iterator();
    while(iterator1.hasNext()){
        Student student = (Student) iterator1.next();
        System.out.print(student.getId()+":"+student.getName()+" ");
    }

    System.out.println("I want to do searching ");
    System.out.println("\n2 is at:"+Collections.binarySearch(list, 2, new Student()));
            // facing exception at this line.I don't know what to use as argument of binarySearch() method. 
}

}

I want to perform search operation on List of objects.The sorting of ascending and descending is working fine facing exception in implementation of binarySearch()
I am preparing for OCPJP. Please let me know briefly(if possible) where i am making mistake.

Thanks in advance

[K. Tsang]

Did you consult the javadoc? There are 2 binarySearch() methods. One with 2 without comparator and one with 3 with comparator.

I think you can use the 2 parm method to get what you want.

And when you calling binary search, which list are you using? The natural order one or the reverse order one?

[Nakul P. Patel]

[quote=

I think you can use the 2 parm method to get what you want.



I am doing this for learning purpose.I intentionally want to use method with three param.
[quote=

And when you calling binary search, which list are you using? The natural order one or the reverse order one?

Before doing search on list, the list must be sorted.Does it matter which one list i am using to search?

[K. Tsang]

Nakul P. Patel wrote:Before doing search on list, the list must be sorted.Does it matter which one list i am using to search?

I think so. Because the returned index will be different. Look for 2 in natural order list gives the index 1 assuming it's 0-based. Same search in reverse order list gives second to last index, if 10 items indexed 0 to 9, it will return 8.

If you plan to use the 3 parm method, then pass in the a "comparator" not a Student.