----- Original Message -----
From: "Jonathan K. Weedon" <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Sent: Tuesday, September 12, 2000 17:18
Subject: Writing a uniform hashCode method
> Consider the seemingly trivial example of a primary key
> containing two integers:
>
> public class MyKey implements java.io.Serializable {
>
> public int a;
> public int b;
>
> public boolean equals(Object object) {
> if(object == null || !(object instanceof MyKey)) {
> return false;
> }
> MyKey that = (MyKey) object;
> return this.a == that.a && this.b == that.b;
> }
>
> public int hashCode() {
> return a ^ b;
> }
>
> }
>
> This was not very hard, was it? Unfortunately, it was
> wrong.
>
> (You might now stop reading, and try to figure out the
> bug in the above code. I will explain it in what follows.
> Kudos if you do see the bug. Hint: the bug is not in the
> equals method, but in the hashCode method.)
>
Well actually the implementation of the equals method could be improved
as it can erroneously return true when comparing instances of MyKey to
instances of sub classes of MyKey
see http://www.javaworld.com/javaworld/jw-01-1999/jw-01-object_p.html
for an interesting discussion on this issue
Of course this just lends credence to the fact that it is hard to correctly
implement equals and hashCode
...David
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