RFR: 8358533: Improve performance of java.io.Reader.readAllLines

[Stuart Marks]

On Thu, 19 Jun 2025 11:08:32 GMT, Markus KARG <duke at openjdk.org> wrote:

>> Replaces the implementation `readAllCharsAsString().lines().toList()` with reading into a temporary `char` array which is then processed to detect line terminators and copy non-terminating characters into strings which are added to the list.
>
> src/java.base/share/classes/java/io/Reader.java line 508:
> 
>> 506:         }
>> 507: 
>> 508:         return lines;
> 
> Do we really want to return a mutable `ArrayList` here? In earlier discussions about this very API I was told that it deliberately returns `String` instead of `CharSequence` due to *intended* immutability, even if that potentially implied slower performance. Following this logic, it would be just straightforward to `return Collections.unmodifiableList(lines);` here. 🤔

Right, the specification here requires an unmodifiable List, so an unmodifiable wrapper or a list from `List.copyOf()` is appropriate.

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PR Review Comment: https://git.openjdk.org/jdk/pull/25863#discussion_r2162555958