RFR: 8357286: (bf) Remove obsolete instanceof checks in CharBuffer.append

[Brian Burkhalter]

On Wed, 11 Jun 2025 08:40:00 GMT, Alan Bateman <alanb at openjdk.org> wrote:

>> Remove some instanceof checks which are vestigial now that `CharSequence` itself defines `getChars(int,int,char[],int)`.
>
> src/java.base/share/classes/java/nio/Heap-X-Buffer.java.template line 297:
> 
>> 295:         checkSession();
>> 296: 
>> 297:         Objects.checkFromToIndex(start, end, csq.length());
> 
> The append(CharSequence) methods are specified to append "null" when the CharSequence is null so append(null, 0, 0) will need to be handled here.

Based on code inspection, it looks like that error might have already been possible but did not turn up before.

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PR Review Comment: https://git.openjdk.org/jdk/pull/25739#discussion_r2140494453