[CALCITE-7632] Replace java.util.Stack with java.util.ArrayDeque in Pattern and TopDownRuleDriver

[lanky228]

Summary

Replace java.util.Stack with java.util.ArrayDeque in two locations, completing a TODO left in place since 2020.

Background

In CALCITE-4314 (commit e537246), Vladimir Sitnikov enabled Error Prone checking for the Calcite codebase. As part of that effort, he identified 2 remaining java.util.Stack usages as obsolete and added @SuppressWarnings("JdkObsolete") with // TODO: replace with Deque to suppress warnings. The commit message explicitly stated "It should be replaced with Deque", but the actual refactoring was deferred.

CALCITE-4314 was a broad static-analysis enablement issue — the Stack→Deque replacement was too granular to be tracked there specifically. This PR creates a dedicated issue (CALCITE-7632) to properly track and complete that deferred work.

Changes

• core/src/main/java/org/apache/calcite/runtime/Pattern.java: Stack<Pattern> → Deque<Pattern> in PatternBuilder.stack field (used for SQL MATCH_RECOGNIZE NFA pattern matching)
• core/src/main/java/org/apache/calcite/plan/volcano/TopDownRuleDriver.java: Stack<Task> → Deque<Task> in task queue (used for Volcano optimizer top-down Cascades-style optimization)

Also removed the corresponding @SuppressWarnings("JdkObsolete") annotations and // TODO: replace with Deque comments.

Rationale

• java.util.Stack extends Vector — all methods are synchronized, adding unnecessary lock overhead in single-threaded contexts
• ArrayDeque is non-synchronized, officially recommended by the Java documentation as a replacement for Stack
• Eliminates Error Prone JdkObsolete warnings without suppression
• Better memory expansion: ArrayDeque doubles capacity on resize vs Vector's linear growth

Testing

• Compilation: BUILD SUCCESSFUL
• Tests passed: AutomatonTest (9), DeterministicAutomatonTest (4), EnumerablesTest (53) — 0 failures
• No remaining java.util.Stack references in core/src/main

Related

• JIRA: CALCITE-7632
• Origin: CALCITE-4314 / commit e537246

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
75.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[mihaibudiu]

I cannot see this issue in the JIRA, does it exist?

[mihaibudiu]

This looks fine, but I'd like to see the issue addressed before commenting.

[lanky228]

Hi @mihaibudiu, thanks for reviewing.

The JIRA issue has been created: CALCITE-7632

Sorry for the earlier confusion — the issue was not created at the time the PR was opened.