Re: Can arrays be parameters to generics

=?ISO-8859-1?Q?Arne_Vajh=F8j?= <>
Sun, 03 Aug 2008 22:24:30 -0400
Christian wrote:

Tom Anderson schrieb:

On Sat, 2 Aug 2008, Christian wrote:

Daniel Pitts schrieb:

puzzlecracker wrote:

Item 25 of Effective Java 2nd Edition (Bloch): Prefer lists to

Lists are a higher level abstraction, and therefor "easier" to
work with.

and slower to work with...

Premature optimization is the root of all evil.

Lists are marginally slower to work with than arrays, but who cares
if your program is fast when it doesn't work correctly?

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

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.

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

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

And how exactly do you search or insert into this array?

Searching with binary search ... as implemented in Arrays class..

inserting creating a new array using data of the binarys search
shifting all entrys afterwards..

Yes it is slower. Bit if the arrays/sets come en masse and are rather
small , may be less than 500 entrys one won't notice it that much.

Saving space by going from O(1)/O(1) to O(logn)/O(n) is not
a general solution.

This example I made is really only important to show that even if
Collections are superior usually. My experience till now was that the
reason to choose arrays over Collection (no matter if set or list or
map) were always due to storage. There has never ever been a problem
with speed for me due to collections.

If you only have 500 entries as described above then I can not
see the space matter either.

As others pointed out ArrayList has less overhead than map or set.
Yes thats true .. but sometimes you need a Set or Map and usually I use
HashSet and HashMap for this. But sometimes an array can replace those.
Especially if the key for the map is part of the Object mapped it can
easier be put into an array as one doesn't need any special Entry
objects holding key and mapped object.

In which cases can array replace Map/Set where ArrayList can not ?


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