collections.binarySearch() is not working with comparable

[Nakul P. Patel]

After sorting List in ascending and descending order i want to invoke binarySearch method. But it not working as i have implemented comparable.

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("\n2 is at:"+Collections.binary);    //facing exception!!
}

}

I could have done this by implementing comparator but i have such requirement in my project Please guide me.

[K. Tsang]

Comparable returns a int -1, 0 or 1. Zero is equals, -1 if input1 is bigger, and 1 if input2 is bigger.

In your code, you should do a < or > comparison instead of the difference.

If you want to use the student name as comparison, you can use the string's compareTo() method.

[Winston Gutkowski]

Nakul P. Patel wrote:After sorting List in ascending and descending order i want to invoke binarySearch method. But it not working as i have implemented comparable.

Are you sure about that? It looks to me as though you're just not calling it properly.

It's probably also worth mentioning that if you're using Comparators, you have to invoke it with the same Comparator that was used to sort.

PS: Please UseCodeTags (←click) when you post code. I've added them for you this time - See how much better it looks?

Winston

[Rob Spoor]

K. Tsang wrote:Comparable returns a int -1, 0 or 1. Zero is equals, -1 if input1 is bigger, and 1 if input2 is bigger.

In your code, you should do a < or > comparison instead of the difference.

Not true. The return value is < 0, 0 or > 0. The < 0 doesn't have to be -1, and the > 0 doesn't have to be 1. Unless the subtraction leads to overflow, it's a valid implementation.

[Seetharaman Venkatasamy]

Rob Spoor wrote:

K. Tsang wrote:Comparable returns a int -1, 0 or 1. Zero is equals, -1 if input1 is bigger, and 1 if input2 is bigger.

In your code, you should do a < or > comparison instead of the difference.

Not true.

yes , until i write -4 or 4

[Rob Spoor]

The "not true" was meant for the -1, 0 or 1. I shouldn't have quoted that last sentence, because you should use < and > -- not inside compareTo but for the result of it.