Java21 Coding Conventions Training

Key things the skill must make very clear:

• Use pattern matching for switch over sealed types; rely on exhaustiveness instead of default
• Use record patterns to destructure records; compose them with switch pattern matching
• Use virtual threads (Executors.newVirtualThreadPerTaskExecutor, Thread.ofVirtual) for blocking IO; one per task, never pooled
• Use sequenced collection methods (getFirst/getLast/reversed) instead of list.get(0) and Collections.reverse
• Pattern variables are always final
• No string templates (STR."..."), unnamed patterns, or Java 22+ features

Step 2: Validate with test cases

After regenerating, validate the skill using the test cases in evals/evals.json.

For each test case, spawn a subagent that:

1. Reads the general skill from plugins/conventions/skills/java-coding-conventions/SKILL.md
2. Reads the generated Java 21 skill from plugins/conventions/skills/java21-coding-conventions/SKILL.md
3. Reads any input files specified
4. Completes the task following both skills
5. Writes output Java files to a workspace directory

Then run the shared grading script:

cd plugins/training
python -m shared.grade <workspace-dir>

The script checks Java 21-specific assertions:

• no-post-java21-features — no string templates, unnamed patterns, flexible constructor bodies
• uses-records, uses-sealed, uses-pattern-matching, uses-switch-expressions, uses-text-blocks, modern-collections

Plus general assertions (immutable fields, small methods, streams over loops, etc.).

Test cases

1. Refactor OrderService — mutable state, loops, checked exceptions, null returns, string concatenation report
2. Refactor UserProfile — mutable JavaBean → record
3. Greenfield PaymentProcessing — sealed PaymentResult hierarchy + dispatcher using pattern matching for switch

Input files for refactoring tests are in evals/files/.

Conventions reference

The authoritative conventions are in references/conventions.md.