Re: How can i implements a class like an HashMap but with 2 keys
?
Gordon Beaton wrote:
On Wed, 29 Aug 2007 06:10:46 -0700, Patricia Shanahan wrote:
Roedy Green wrote:
3. concatenate your two String keys into one.
I'd avoid this one unless there is at least one char bit pattern
that cannot appear in the first input, to use as separator.
Why is it necessary to separate them? The cost of not doing so is only
an increased risk of collisions.
In fact that cost might be zero or so negligible that adding the
separator at each operation costs more than the potential collisions
it attempts to avoid.
I'm assuming that the concatenation is only used internally in the
HashMap, and that the separate components are used elsewhere in their
original, separate state.
If it were only for purposes of calculating hashCode(), I would
completely agree. However, my understanding is that this is to be used
as the key in a single key HashMap substituting for a map keyed by the
pair.
In the portion I snipped, the first two items in the list were:
1. use two HashMaps on the same set of objects. Then you can look up
by either key.
2. create a Pair class that contains key1 and key2 with an ^ (xor)
combined hashCode. Then Pair is a single key for both building and
lookup. You need both keys to lookup, like a safety deposit box.
Both of those solutions would treat <"A_B",C> and <"A","B_C"> as
different keys. Both could be present as keys, with different values, at
the same time.
Patricia