- Tatu Saloranta scripsit:

> > There would be a penalty for keeping a TreeMap versus a HashMap.

Actually, it isn't. Even though the javadoc claims that get and put

> > HashMap obviously performs better (constant time put & get) for when

>

> I understand this, and it's a valid point.

are constant time, a moment's reflection shows that this cannot be so.

HashMap is implemented (in OpenJDK, and almost certainly in earlier

versions) using an array of specialized objects that implement Map.Entry.

Each element of the array is one hash bucket, and the Entry objects form

a linked list. So average get/put time is approximately N/c*2, where c

is the capacity (initially 16 unless specified otherwise), which is O(N).

The hope is that N/c*2 is always small enough that the performance looks

like O(1), but as N grows, c is increased using a full rehash, which is

again O(N) and is independent of c.

So I'd say using TreeMap throughout, with its guaranteed O(log N)

performance, is a perfectly reasonable thing to do; it makes debugging

easier, it is prettier, and it guarantees getting the same output on

the same input.

--

On the Semantic Web, it's too hard to prove John Cowan cowan@...

you're not a dog. --Bill de hOra http://www.ccil.org/~cowan - --- In json@yahoogroups.com, John Cowan <cowan@...> wrote:
>

when

> Tatu Saloranta scripsit:

>

> > > There would be a penalty for keeping a TreeMap versus a HashMap.

> > > HashMap obviously performs better (constant time put & get) for

> >

Map.Entry.

> > I understand this, and it's a valid point.

>

> Actually, it isn't. Even though the javadoc claims that get and put

> are constant time, a moment's reflection shows that this cannot be so.

>

> HashMap is implemented (in OpenJDK, and almost certainly in earlier

> versions) using an array of specialized objects that implement

> Each element of the array is one hash bucket, and the Entry objects form

O(N).

> a linked list. So average get/put time is approximately N/c*2, where c

> is the capacity (initially 16 unless specified otherwise), which is

> The hope is that N/c*2 is always small enough that the performance looks

Not true. Full rehashes are indeed O(N), but they take place at a

> like O(1), but as N grows, c is increased using a full rehash, which is

> again O(N) and is independent of c.

frequency proportional to 1/c because each time a rehash takes place

the size is doubled, so the rehashes become farther apart the larger c

gets. Therefore the average rehash time for a put operation is

O(N*1/c) which averages to constant time. Both get and put only have

to iterate through their own bucket, so as long as the hash method is

reasonable, both get and put are indeed O(1), constant time operations.

Kris - --- In json@yahoogroups.com, "Arthur Blake" <arthur.blake@...> wrote:
> When I have machine generated JSON data that is stored in files, I

Excellent suggestion. I have modified JSON.Object.

> like to have the JSON data be sorted alphabetically by the keys in the

> output.

>

> It's much easier to find what you are looking for quickly this way

> when viewing or editing the data (especially when the amount of data

> is large.)

>

> Because there is no defined order of the keys in JSON, it doesn't

> matter what order you put them in. So we might as well put it in an

> order that makes it more human readable in contexts where the data is

> meant to be viewed or edited by humans.