RFR: 8343418: Unnecessary Hashtable usage in CSS.htmlAttrToCssAttrMap [v3]

[Andrey Turbanov]

> If a thread-safe implementation is not needed, it is recommended to use HashMap instead of legacy synchronized Hashtable. 
> Map `CSS.htmlAttrToCssAttrMap` is modified only within static initialization block and then only `get` method is called.
> 
> The only subtle difference is that Hashtable doesn't allow `null` keys and throws NPE from `get` method.
> I've checked all possible keys which are passed to `htmlAttrToCssAttrMap.get`.
> Currently 3 different execution paths are possible:
> 
> **1,2**
> When `HTML.Attribute.BORDER` is passed as a key. It's definitely non-null.
> 
> javax.swing.text.html.CSS#translateHTMLToCSS
>   translateAttribute(HTML.Attribute.BORDER, "1", cssAttrSet);
>     CSS.Attribute[] cssAttrList = getCssAttribute(key);
> 
> 
> javax.swing.text.html.CSS#translateAttributes
>    translateAttribute(HTML.Attribute.BORDER, Integer.toString(borderWidth), cssAttrSet);
>      CSS.Attribute[] cssAttrList = getCssAttribute(key);`
> 
> 
> **3**
> When non-null `key` is passed as a key.
> 
> javax.swing.text.html.CSS#translateAttributes
> 
>     if (name instanceof HTML.Attribute) { // from this `instanceof` we know that it's non-null
>          HTML.Attribute key = (HTML.Attribute)name;
> 
>       translateAttribute(key, (String) htmlAttrSet.getAttribute(key), cssAttrSet);
>         CSS.Attribute[] cssAttrList = getCssAttribute(key);
> 
> 
> ![idea_analyze_dataflow_to_here](https://github.com/user-attachments/assets/48ace4de-6d0a-468e-bb14-b579a193254b)
> 
> 
> I've used HashMap.newHashMap method instead of constructor to avoid resizing of internal HashMap array.

Andrey Turbanov has updated the pull request incrementally with one additional commit since the last revision:

  Use immutable map instead of HashMap.
  It fits even better: has the same null handling as Hashtable

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/21785/files
  - new: https://git.openjdk.org/jdk/pull/21785/files/b653278f..a0860021

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21785&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21785&range=01-02

  Stats: 54 lines in 1 file changed: 5 ins; 1 del; 48 mod
  Patch: https://git.openjdk.org/jdk/pull/21785.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21785/head:pull/21785

PR: https://git.openjdk.org/jdk/pull/21785