RFR: 8281631: HashMap copy constructor and putAll can over-allocate table [v21]

[liach]

On Sun, 20 Feb 2022 18:44:09 GMT, XenoAmess <duke at openjdk.java.net> wrote:

>> src/java.base/share/classes/java/util/IdentityHashMap.java line 281:
>> 
>>> 279:      * @throws NullPointerException if the specified map is null
>>> 280:      */
>>> 281:     private IdentityHashMap(Map<? extends K, ? extends V> map, int expectedSize) {
>> 
>> Why are you writing a new constructor when you can just change the old call to `this(m.size());`?
>
>> @liach implementations `size()` seems O1, and returns a single int number field, but it actually defers in some Map implementations. 
> 
> @liach for example, in ConcurrentSkipListMap and ConcurrentHashMap, `size()` function is far complicated than reading a field, thus calling it twice meaninglessly is not a wise choice.

Imo you should just remove the `if (expectedSize == 0)` check than using this somewhat ugly trick to avoid calling `size()` twice (the second call is only used for this relatively useless fast-path, especially for the concurrent collections you refer to)

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PR: https://git.openjdk.java.net/jdk/pull/7431