Re: hash two keys to one index

From:

Patricia Shanahan <pats@acm.org>

Newsgroups:

comp.lang.java.programmer

Date:

Thu, 16 Nov 2006 02:22:42 GMT

Message-ID:

<SXP6h.7850$ig4.4648@newsread2.news.pas.earthlink.net>

Mark wrote:

Eric Sosman wrote:

Mark wrote:

Hi,

I need to figure out how to map two different keys (a string and an
integer) to the same index. I don't even know where to start... but I
don't want two copies of the the hash table.

Any ideas?

My only "idea" is that you should take a few steps back
from the immediate tactical issue and explain your overall
strategic goal. *Why* do you want "ABC" and new Integer(42)
to produce the same hashCode(), and what good do you think
it will do you? On first sight, at least, it seems daft.
Pray explain the well-concealed sanity that lies beneath.

--
Eric Sosman
esosman@acm-dot-org.invalid

Well, if I want to pull up all the data pertaining to a student, but I
only know his last name or his student number, not both...

That is a very good reason for having a HashMap with names as keys, and
a HashMap with student numbers as keys...

Note that every non-null non-primitive variable or expression in Java is
a pointer to some object, not the value of an object. That means that
the two HashMap instances can point to the same Student instances.

For the specific case of name and number, you could get away with a
single table. It might be slightly slower than two tables, but probably
only slightly. However, mixed key tables cannot take advantage of
generics, and you must be very sure that you never need e.g. to index by
two different numbers, where there could be a risk of hitting on the
wrong key.

Generally, two tables is much cleaner.

Patricia