RFR: 8367499: Refactor exhaustiveness computation from Flow into a separate class

[Francesco Andreuzzi]

On Fri, 12 Sep 2025 09:49:14 GMT, Jan Lahoda <jlahoda at openjdk.org> wrote:

> Currently the switch exhaustiveness computation code is part of `Flow`. And while conceptually the check is part of the `Flow` phase, the code is >500 lines of code currently, and likely to get bigger/more complicated in the future. Among other reasons due to enhancements like https://bugs.openjdk.org/browse/JDK-8367530.
> 
> The proposal herein is to move the exhaustiveness computation to a separate class `ExhastivenessComputer`. There's no functional change, only move of the code. This is intentional, to aid the review process.
> 
> One possibility to inspect what is happening is:
> 
> git show upstream/pr/27247:src/jdk.compiler/share/classes/com/sun/tools/javac/comp/Flow.java >/tmp/Flow-original.java
> diff -d -w /tmp/Flow-original.java src/jdk.compiler/share/classes/com/sun/tools/javac/comp/ExhaustivenessComputer.java >/tmp/exhaustivenesscomputer-comparison.diff
> diff -d -w /tmp/Flow-original.java src/jdk.compiler/share/classes/com/sun/tools/javac/comp/Flow.java >/tmp/flow-comparison.diff
> 
> and inspecting `/tmp/exhaustivenesscomputer-comparison.diff`, `/tmp/flow-comparison.diff`.

src/jdk.compiler/share/classes/com/sun/tools/javac/comp/ExhaustivenessComputer.java line 204:

> 202: 
> 203:         for (PatternDescription pdOne : patterns) {
> 204:             if (pdOne instanceof BindingPattern bpOne) {

The scope of this `if` is quite wide, you could consider inverting the condition and `continue`ing if the condition does not match. This would save an indentation level.

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PR Review Comment: https://git.openjdk.org/jdk/pull/27253#discussion_r2343822773