983Re: Sorting the keys of formatted (pretty printed) JSON output
- Apr 10, 2008--- In firstname.lastname@example.org, John Cowan <cowan@...> wrote:
> Tatu Saloranta scripsit:
> > > There would be a penalty for keeping a TreeMap versus a HashMap.
> > > HashMap obviously performs better (constant time put & get) for
> > 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 formO(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 looksNot 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.
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