Re: HashTable
George wrote:
That is not really the case in my program - I will briefly try to explain
the situation.
Please do not top-post - it makes it harder to understand to what you are
responding.
/What/ is really not the case? That you need to override equals()? But you
most certainly do need to.
I have a custom data structure, say custom1, made up of two string fields
and another custom data type, say custom2. So, there is a series of
custom2 objects creation, then used for a series of custom1 object
creation. The custom1 class also has a static HashTable field, indexed by
Again, it's "Hashtable", not "HashTable".
a HashSet. The HashSet is constructed in a separate method which has
statements of the type hashsetname.add(new custom3(...)).
Thus guaranteeing that the inserted objects do not == objects in another Set.
When this method finishes, before insertion into the main HashTable I run
a check with containsKey - the problem is that it fails even for custom3
objects with exactly the same fields.
Because you did not override equals(), as has been repeatedly explained.
Please offer some advice if you can, as I am stuck at this point for about
a week now and it brought all my work to a halt.
My advice is to follow the advice you've already been given.
From: Patricia Shanahan <pats@acm.org>
|These paragraphs strongly imply that you consider clones to be equal.
|That means two distinct instances of your class can be equal, so the
|Object equals and hashCode behavior is completely wrong for that class.
|You do need to override.
....
|"something more closer" suggests that you are not there yet. If so, I
|suggest doing the following:
|
|1. Write a trivial hashCode() that always returns zero.
|
|2. Concentrate on equals. Think carefully about what determines whether
|an instance of your class is equal to another object, and implement that
|test as your equals method.
|
|The hash-based collections work correctly, but inefficiently, with a
|trivial hashCode().
|
|Once you are sure equals is correct, go back and implement hashCode so
|that all equal instances have the same hashCode while minimizing
|collisions between unequal objects.
--
Lew