Re: HashMaps, hashcodes, equals, and Serialization

"Daniel Pitts" <>
30 Dec 2006 10:39:01 -0800
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.

1) How compulsory is it to overload the hashcode method, having
overloaded the equals method. If I don't, will this mean my HashMap
won't function correctly - i.e. if "Article ar1" exists in the hashmap,
and I call get(ar2) on the hashmap, such that ar1.equals(ar2), would it
not return me ar1, since their hashcodes are difference? Does the
HashMap code ever test for equality itself when putting and getting
objects, or does it simply rely on the contracted relationship between
hashcode() and equals()? How would I go about writing an effective
custom hashcode function?

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? In that, surely if the hashcode is used in determining
where in the table a certain Article object is placed, then if the
hashcode for the Article (when deserialized) changes on another
execution of the application, would the hashmap still be able to find
the Article in its previous place? Or does
serialization/deserialization maintain the hashcode of the object?

Cheers :)


It is always best practice that you override equals and hashCode
together, or not at all.

Basically, its a contract. You, as the implementor of your class, are
required to make hashCode and equals work consistently with eachother.

Anything that you use to calculate the hashCode must be represented in
the equals comparison. Note that it doesn't need to be the other way
around, however.

Hope this helps.

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