why should i need to override hashcode when i override equals method

[saikrishna cinux]

hi, why should i override hashcode method when i override equals method?

and can any one provide me a program how it really needs ?
thanks in advance

[Christophe Verré]

Because two instances returning different hashcodes won't be equal.
A foolish example. equals() always returns true, but "c" is not found.

import java.util.*;
 
public class HashCode {
int val;

public HashCode(int v) {
val = v;
}

public boolean equals(Object o) {
return true;
}

public int hashCode() {
return val;
}

static public void main(String[] args) {

HashCode a  = new HashCode(1);
HashCode b  = new HashCode(1);
HashCode c  = new HashCode(2);
Map myList = new HashMap();
myList.put(a, "A" );

System.out.println("Contains a ? " + myList.containsKey(a));
System.out.println("Contains b ? " + myList.containsKey(b));
System.out.println("Contains c ? " + myList.containsKey(c));
}
}

[Carl Trusiak]

There are three methods that are closely related, each may be used by different Collection type to determine equality.
equals
hashCode
compareTo (For Sorted Collection, you must implement the Comparable innerface)

For your object to function in the various Collections as documented, the following rule should be followed.

hashCode will return the same value and compareTo will return 0 when equals returns true

If your objects will never participate in hash based collections or sorted collections, it's not necessary to follow that rule however, it will either limit what you can do or result in unexpected results.

I've seen this in one project. TreeSet is documented to contain one and one one of any object which equas is true. However, following the rule above, it actually uses compareTo returning 0 to do this. On this job someone wrote compareTo in a way inconsistant with the rule and as a result, doubles where going in and some objects where apparently randomly being excluded from the TreeSet.

Unless you have a large complelling reason to not follow the rules, I recommend you make it a habit. And any time you diviate prepare for some unexpected results.