Re: hashCode() for Custom classes

From:

Lew <lew@lewscanon.com>

Newsgroups:

comp.lang.java.programmer

Date:

Sat, 19 Apr 2008 02:52:34 -0400

Message-ID:

<v7WdnY5ZnIQlC5TVnZ2dnUVZ_hadnZ2d@comcast.com>

Peter Duniho wrote:

Either you misunderstood me, or I have no idea what you're talking about.

I'm not talking about classes that _are_ collections. I'm talking about
classes that might be contained _within_ a collection.

If you understood that, then I think I need you to explain what you mean
by "those use what one might deem a 'natural' hash code for their
contents" and "the contract is spelled out at the interface level",
among other things. I don't see how those apply to user classes that
are similar to each other (e.g. share a common base class) and all of
which might be stored in the same collection.

You are correct. I misunderstood you to mean collections being compared to
each other, in the way that Patricia Shanahan mentioned in her post with
respect to Sets and Lists, I think you will find the points cogent in that
context.

They apply as well to many examples where people wish to compare "similar"
classes. Set and List are just examples of where one might use hashCode()
between instances of different classes.

Set is an example of how a class-specific different seed for the hashCode()
would break the contract that it be consistent with equals().

--
Lew