RFR: 8341566: Add Reader.of(CharSequence) [v15]

[Markus KARG]

On Wed, 23 Oct 2024 08:58:30 GMT, Markus KARG <duke at openjdk.org> wrote:

>> This Pull Requests proposes an implementation for [JDK-8341566](https://bugs.openjdk.org/browse/JDK-8341566): Adding the new method `public static Reader Reader.of(CharSequence)` will return an anonymous, non-synchronized implementation of a `Reader` for each kind of `CharSequence` implementation. It is optimized for `String`, `StringBuilder`, `StringBuffer` and `CharBuffer`.
>> 
>> In addition, this Pull Request proposes to replace the implementation of `StringReader` to become a simple synchronized wrapper around `Reader.of(CharSequence)` for the case of `String` sources. To ensure correctness, this PR...
>> * ...simply moved the **original code** of `StringBuilder` to become the de-facto implementation of `Reader.of()`, then stripped synchronized from it on the left hand, but kept just a synchronized wrapper on the right hand. Then added a `switch` for optimizations within the original code, at the exact location where previously just an optimization for `String` lived in.
>> * ...added tests for all methods (`Of.java`), and applied that test upon the modified `StringBuilder`.
>> 
>> Wherever new JavaDocs were added, existing phrases from other code locations have been copied and adapted, to best match the same wording.
>
> Markus KARG has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - Fixed wording as proposed by Alan Bateman: 'The returned reader supports the mark and reset ... could move up to become the second paragraph.'
>  - Fixed wording as proposed by Alan Bateman: 'make The reader is initially .. the second sentence of the first paragraph rather than a new paragraph.'

I have drafted a release note for [JDK-8341566](https://bugs.openjdk.org/browse/JDK-8341566) in [JDK-8344910](https://bugs.openjdk.org/browse/JDK-8344910). Kindly asking for reviews! :-)

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PR Comment: https://git.openjdk.org/jdk/pull/21371#issuecomment-2495508992