Re: Unique Long for Map
caultonpos@gmail.com wrote:
Hi,
I have a common need to keep a small cache of values which are often
identified by two or more long values.
e.g. (1,2) ---- > "Abc"
(1,9) -----> "Def"
For performance I like using a hashtable but to use it I do this:
public void addToCache(Long nbr1, Long nbr2, String value) {
StringBuffer sb = new StringBuffer(32);
sb.append(nbr1);
sb.append(":");
sb.append(nbr2);
hashtable.put (sb.toString(), value);
}
or something similar to that. The point is to make the key unique I
create a String. I would imagine lookups would be more efficient if
the key was not a String but a number. But how to make a unique
number from these two. Perhaps if I knew both numbers would be less
than a billion then (nbr1 * 1,000,000,00 + nbr2) but that seems a
little clunky.
Is there a well defined method to create a single unique number based
upon two values?
Yes: Concatenate them, as you have done above. You
could also build a BigInteger or a BitSet or some other
object; uniqueness is preserved if the bits of nbr1 do
not interact with those of nbr2. Clearly, though, nothing
you do can guarantee uniqueness when mapping 128 bits of
input to a 32-bit hashCode() value.
Absent other information about the joint distributions
of nbr1 and nbr2, all I can suggest is that you build a
LongPair class that implements equals() and hashCode().
And if you're so avid for performance, you should probably
be trafficking in longs rather than Longs. (But since you
describe your cache as "small," one wonders whether this is
the optimization you should be spending your time on ...)
--
Eric Sosman
esosman@ieee-dot-org.invalid