Re: Using java.util.map

From:

Patricia Shanahan <pats@acm.org>

Newsgroups:

comp.lang.java.programmer

Date:

Wed, 16 Aug 2006 15:20:55 GMT

Message-ID:

<rJGEg.6672$Qf.4699@newsread2.news.pas.earthlink.net>

Robert Klemme wrote:

On 16.08.2006 15:48, Christopher Benson-Manica wrote:

Rogan Dawes <discard@dawes.za.net> wrote:

Map.Entry<K,V> getEntry(Object key)

might be a useful method. Obviously I am not considering the
additional complexity that this adds to all implementations, etc.

What just occurred to me is that implementing getEntry() for a HashMap
would be non-trivial, if I understand how HashMap works correctly. It
seems like it would have been a fairly simple addition to TreeMap
however...

HashMap.java:

    Entry getEntry(Object key) {
        Object k = maskNull(key);
        int hash = hash(k);
        int i = indexFor(hash, table.length);
        Entry e = table[i];
        while (e != null && !(e.hash == hash && eq(k, e.key)))
            e = e.next;
        return e;
    }

IOW, the method exists already (JDK 1.4 and later) - it's just not public.

Kind regards

    robert

I don't really understand the attraction of using Entry.

The cost of the second lookup is likely to be very low. The HashMap code
is in the instruction cache. The relevant portions of the hash table are
in the data cache. The same branches will be taken and not-taken as in
the original lookup.

The cost of creating a new Integer object each time a count increments,
and garbage collecting the old one, may be greater than the cost of the
second lookup. Using Entry does nothing about that.

The mutable counter solution seems simple, clean, and obvious to me.

Patricia