Re: HashMaps, hashcodes, equals, and Serialization

From:

Lew <lew@nowhere.com>

Newsgroups:

comp.lang.java.programmer

Date:

Sat, 30 Dec 2006 10:46:46 -0500

Message-ID:

<I4idne4uE9x1FAvYnZ2dnUVZ_oytnZ2d@comcast.com>

Siam wrote:

Hi all,

As part of my application, I have an ArticleManager class, that
maintains a HashMap of Article objects (which have an overloaded equals
method - but not an overloaded hashcode). When my application is
closed, the ArticleManager serializes itself and all the Articles.
Questions:

1) How compulsory is it to overload the hashcode method, having
overloaded the equals method.

It is very compulsory, except when you have a distinct reason to make them
inconsistent. Even then it's compulsory, you just might have a reason to
disregard the rule.

No "Hash.+" anything will work correctly if hashCode() and equals() are
inconsistent. That is why they have "Hash" in their names!

2) The contract for hashcode, as written in the API, states "This
integer [the hashcode] need not remain consistent from one execution of
an application to another execution of the same application." Will this
cause complications for when my articles and the hashmap are
serialized, and later deserialized on another execution of my
application?

Your serialization format should not in any way depend on the hash. If it
does, then you have not written your readObject)() and writeObject() methods
correctly.

Read Joshua Bloch's _Effective Java Programming_, which has chapters covering
both these questions in detail.

- Lew