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

[XenoAmess]

On Sun, 20 Feb 2022 18:20:27 GMT, liach <duke at openjdk.java.net> wrote:

>> XenoAmess has updated the pull request incrementally with three additional commits since the last revision:
>> 
>>  - refine test
>>  - 1. optimize IdentityHashMap that when calling ::new(Map), do not call map.size() twice but once.
>>    2. delete the this((int) ((1 + m.size()) * 1.1)); as it makes the table over-allocate when size = 19.
>>  - refine test
>
> 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 because I don't like to call m.size() twice.
the original codes did so: one call at the `public IdentityHashMap(Map<? extends K, ? extends V> map)` , and the second inside of `putAll`.
well in most Map implementations `size()` seems O1, and returns a single int number field, but it actually defers in some Map implementations.
But java grammar do not allow me to make a local variable before calling constructor in a constructor.
In other words, `this()` must be the first line in a constructor.

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