How to over ride the List.contains() method.

[Rajkamal Pillai]

I would like to override the list.contains() to suit my requirements for checking if a particular value object is contained in a list. Right now I implement the Comparable interface and code the compareTo() method in the value object. Is it the right way to go about it?

[David O'Meara]

No, you'll want equals() and only need other methods like HashMap or compareTo if the actual List implementation is something like a Hash* or Tree*

[Rob Spoor]

If you use compareTo for checking instead of equals, and a.compareTo(b) == 0 does not imply a.equals(b) for all a and b, then you are breaking the contract for List.contains. That clearly specifies that it should use equals.

However, TreeMap and TreeSet possibly violate the contracts for Map and Set since these also use compareTo (or a Comparator).

You do run a risk though when comparing: contains can take any Object, and that might not be comparable to your elements. In fact, TreeMap (and TreeSet) have the same problem. Consider the following:

import java.util.*;
 
public class Test
{
    public static void main(String[] args) throws Exception
    {
        Map<String,Object> map = new TreeMap<String,Object>();
        map.put("1", new Object());
        map.put("2", new Object());
        map.put("3", new Object());
        map.put("4", new Object());
        System.out.println(map.get("1"));
        System.out.println(map.get(1));
 
        TreeSet<String> set = new TreeSet<String>(map.keySet());
        System.out.println(set.contains("1"));
        System.out.println(set.contains(1));
    }
}

Now both System.out.println(map.get(1)); and System.out.println(set.contains(1)); will throw a ClassCastException:

Exception in thread "main" java.lang.ClassCastException: java.lang.String cannot
 be cast to java.lang.Integer
        at java.lang.Integer.compareTo(Unknown Source)
        at java.util.TreeMap.getEntry(Unknown Source)
        at java.util.TreeMap.get(Unknown Source)
        at Test.main(Test.java:13)

[Rajkamal Pillai]

Thanks Rob,

I used an overridden equals() and hashCode() and it works fine. I guess I got a little confused. I am using an instance of a LinkedList and it works fine for the contains() method.

Cheers,
Raj.