Collections.binarySearch() problem

[Prasun Howlader]

import java.util.*;
public class GenericDemo{
        public static void main(String[]args){
                List<Super> list = new ArrayList<Super>();
                list.add(new Super("adf"));
                list.add(new Super("adf"));
                Collections.binarySearch(list,new Super());
                                                                                
                                                                                
        }
                                                                                
}
class Super implements Comparable{
        String name;
        Super(String name){this.name = name;}
        public int compareTo(Object o){
                return 1;
        }
        public String toString(){
                return name;
        }
        Super(){}
}

Why this program fails to compile?

[Deepak Chopra]

Can you paste your error here..!! Well i have java 1.4 install on my system..and after changing :

List<Super> list = new ArrayList<Super>();
 
to
List list = new ArrayList();

I compiled and run it successfully..

[Kaydell Leavitt]

I believe the problem is in using generics which came out in Java 5.0.

The binarySearch() method of the Collections class uses generics in an advanced way that I'm not familiar with.

Java 6 Tutorial from Sun on Generics:
http://java.sun.com/docs/books/tutorial/java/generics/index.html

[Vierda Mila]

try to add <Super> after class Super implements Comparable and change parameter in compareTo method from (Object o) into (Super s)

thanks & regards,
-mila-

[Prasun Howlader]

But why not this way program not compile?

[Jesper de Jong]

According to the documentation of class Collections, the binarySearch method you are calling looks like this:

public static <T> int binarySearch(List<? extends Comparable<? super T>> list, T key)

Especially note the type of the list. It must be a list of objects that extend Comparable, but the Comparable must also be generic. You must change your class Super so that it extends Comparable<Super>, instead of just Comparable:

class Super implements Comparable<Super>{
        String name;
        Super(String name){this.name = name;}
        public int compareTo(Super o){
                return 1;
        }
        public String toString(){
                return name;
        }
        Super(){}
}

[Prasun Howlader]

But sort method (except comparator sort) not required this.