Java17 Coding Conventions Training

Key things the skill must make very clear:

• Prefer records for pure data carriers; use the compact constructor for validation and List.copyOf defensive copies
• Use sealed interfaces/classes for closed hierarchies; permitted subtypes must be final, sealed, or non-sealed
• Use instanceof pattern matching with a final pattern variable instead of cast-after-check
• Use switch expressions with ->; never use the old case X: + break form
• Pattern matching for switch is NOT in Java 17 — it's preview there, stable in 21
• Use text blocks (""") for multi-line strings instead of concatenation with \n
• Prefer Stream.toList() over Collectors.toUnmodifiableList() / Collectors.toList()
• Use final var for locals when the type is obvious
• No pattern matching for switch, record patterns, virtual threads, or sequenced collections (all Java 21)

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 17 skill from plugins/conventions/skills/java17-coding-conventions/SKILL.md
3. Reads any input files specified in the test case
4. Completes the task described in the prompt, following both skills' conventions
5. Writes output Java files to a workspace directory

Then run the shared grading script against the outputs:

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

The script checks Java 17-specific assertions:

• no-post-java17-features — no pattern matching for switch, record patterns, virtual threads, sequenced collections
• uses-records — data carriers should be records
• uses-pattern-matching — instanceof should use pattern matching with final pattern variables
• uses-switch-expressions — switch uses arrow form
• uses-text-blocks — multi-line strings use """
• modern-collections — Stream.toList() / List.of / List.copyOf

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 for report
2. Refactor UserProfile — mutable JavaBean with getters/setters; should become a record
3. Greenfield PaymentProcessing — write a sealed PaymentResult hierarchy with a dispatcher that uses pattern matching

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

Conventions reference

The authoritative conventions are in references/conventions.md.