refactor: migrate all Xtend files to Java 21

.gitignore

@@ -4,6 +4,3 @@
 *.java._trace
 *.smap
 *.checkbin
-*.xtendbin
-/*/xtend-gen/*
-!/*/xtend-gen/.gitignore

AGENTS.md

@@ -9,7 +9,7 @@ This document helps AI coding agents work effectively with the DSL DevKit codeba
 - **Java**: 21+
 - **Maven**
 - **Tycho**
-- **Xtext/Xtend**
+- **Xtext**
 
 ## Key Directories
 
@@ -50,7 +50,7 @@ export WORKSPACE=$(pwd)
 
 ### PMD
 - **Ruleset**: `ddk-configuration/pmd/ruleset.xml`
-- Excludes: `src-gen/`, `src-model/`, `xtend-gen/`
+- Excludes: `src-gen/`, `src-model/`
 
 ### Checkstyle
 - **Config**: `ddk-configuration/checkstyle/avaloq.xml`
@@ -79,10 +79,6 @@ Tests are disabled by default and activated only in test bundles.
 These directories contain generated code - do not edit manually:
 - `src-gen/` - Xtext generated sources
 - `src-model/` - EMF model generated sources
-- `xtend-gen/` - Xtend transpiled Java sources
-
-### Xtend
-- `.xtend` files in `src/` compile to Java in `xtend-gen/`
 
 ## Common Tasks
 
@@ -95,7 +91,7 @@ These directories contain generated code - do not edit manually:
 
 ### Fixing PMD Violations
 1. Check ruleset at `ddk-configuration/pmd/ruleset.xml`
-2. Violations in generated code (`src-gen/`, `xtend-gen/`) are excluded
+2. Violations in generated code (`src-gen/`) are excluded
 3. Run `mvn pmd:check -f ./ddk-parent/pom.xml` to verify fixes
 
 ### Fixing Checkstyle Violations
@@ -116,3 +112,4 @@ xvfb-run mvn verify -f ./ddk-parent/pom.xml -pl :com.avaloq.tools.ddk.xtext.test
 - **Platform**: GitHub Actions
 - **Workflow**: `.github/workflows/verify.yml`
 - Triggers on: push to master, pull requests
+

HANDOVER.md

@@ -0,0 +1,118 @@
+# Handover — Fix All PMD & Checkstyle Violations for Xtend-to-Java Migration
+
+*Generated: Sun Mar 1 23:26 CET 2026*
+*Branch: feature/xtend-to-java-migration*
+*Last commit: 645407dd5 — fix: resolve BasicEList compilation error in XbaseGeneratorFragmentTest*
+
+## What We Were Working On
+
+Resolving all PMD and Checkstyle violations introduced by the Xtend-to-Java migration on the `feature/xtend-to-java-migration` branch. The migration converted ~90 Xtend files to Java 21 across the entire dsl-devkit project, and the auto-generated Java code had widespread style violations that failed CI.
+
+The initial plan estimated ~542 violations across 12 files in 4 modules. In practice, violations were discovered iteratively across **10+ modules and 36 files**, totaling roughly 600+ violations.
+
+## What Got Done
+
+- [x] Fixed all PMD and Checkstyle violations — CI is fully green (all 3 jobs: maven-verify, pmd, checkstyle)
+- [x] 4 commits covering the fixes:
+  - `18fb5a34e` — bulk fix across 33 files in 10 modules
+  - `8d4d826d7` — StringToString, UnnecessaryCast, MissingOverride, LooseCoupling
+  - `ab0d6df10` — UseCollectionIsEmpty in PropertiesInferenceHelper
+  - `645407dd5` — BasicEList compilation error in XbaseGeneratorFragmentTest
+- [x] All pushed and CI confirmed green (run 22551555855)
+
+### Modules touched:
+- `check.core` (8 files) — largest module, ~510 violations
+- `check.core.test` (10 files) — ~150 violations
+- `check.ui` (2 files)
+- `check.ui.test` (1 file)
+- `checkcfg.core` (4 files)
+- `checkcfg.core.test` (4 files)
+- `xtext.format.generator` (1 file)
+- `xtext.export.generator` (1 file)
+- `xtext.generator.test` (1 file)
+- `xtext.generator` (1 file)
+
+### Fix categories applied:
+- **FinalParams** (~354): Added `final` to method parameters
+- **UnnecessaryReturn** (~53): Removed trailing `return;` in void dispatch methods
+- **MethodName** (~27): Class-level `@SuppressWarnings({"checkstyle:MethodName"})` for dispatch methods (`_format()`, `_scope()`, etc.)
+- **AppendCharacterWithChar** (~20): `.append("x")` → `.append('x')`
+- **MultipleStringLiterals** (~32): `CHECKSTYLE:CONSTANTS-OFF/ON` wrappers
+- **MemberName** (~6): Renamed `_fieldName` → `fieldName`
+- **MissingOverride**: Added `@Override` annotations
+- **LooseCoupling**: Changed implementation types to interfaces (`BasicEList` → `EList`, `TreeMap` → `Map`, `ArrayList` → `List`)
+- **InsufficientStringBufferDeclaration**: Increased `StringBuilder` capacities
+- **Various others**: IllegalCatch suppression, JavadocMethod tags, UseIndexOfChar, ExhaustiveSwitchHasDefault, etc.
+
+## What Worked
+
+- **Parallel team agents** for the initial bulk work — two agents handled `check.core` (8 files) and the smaller modules (4 files) simultaneously, getting the majority of mechanical fixes done quickly.
+- **Iterative verification loop** — running `mvn checkstyle:check pmd:check` after each round of fixes, then fixing what remained. This was essential because new modules kept appearing as CI checks expanded beyond the initial plan.
+- **Suppression patterns** for rules that couldn't be fixed (dispatch method names, intentional exception catching):
+  - `// CHECKSTYLE:CONSTANTS-OFF` / `// CHECKSTYLE:CONSTANTS-ON`
+  - `// CHECKSTYLE:CHECK-OFF <Rule>` / `// CHECKSTYLE:CHECK-ON <Rule>`
+  - `@SuppressWarnings({"checkstyle:MethodName", "PMD.UnusedFormalParameter"})`
+
+## What Didn't Work
+
+- **Local PMD checks without `compile`** — Running `mvn checkstyle:check pmd:check` without a prior `compile` phase misses PMD rules that require type resolution (MissingOverride, UnnecessaryCast, LooseCoupling, UseCollectionIsEmpty). This caused violations to slip through local verification and only appear in CI.
+- **Grepping build output with `head -30`** — When verifying with `--fail-at-end`, the first 30 lines were all `0 Checkstyle violations` messages, hiding the actual `BUILD FAILURE` at the bottom. A compilation error was missed because of this.
+- **Initial scope estimation** — The plan identified 4 modules but violations existed in 10+. Each CI run surfaced new modules we hadn't checked locally.
+- **Agent-generated fixes sometimes incomplete** — Agents fixed the obvious cases but missed edge cases (e.g., expanding a star import but not checking all usages, changing an import without updating all references).
+
+## Key Decisions & Rationale
+
+1. **Suppress vs fix for MethodName violations**: Dispatch methods like `_format()`, `_scope()`, `_computeTypes()` must keep underscore prefixes (Xtext dispatch pattern). Used class-level `@SuppressWarnings` rather than renaming.
+
+2. **CHECKSTYLE:CONSTANTS-OFF for MultipleStringLiterals**: Template-generating methods that build Java source code via StringBuilder inherently have repeated string literals. Extracting constants would hurt readability.
+
+3. **CHECKSTYLE:CHECK-OFF for IllegalCatch**: Some methods intentionally catch `Exception` (e.g., in code generation utilities). Suppressed rather than changed.
+
+4. **Switch to `mvn clean compile pmd:check` for local verification**: After discovering that PMD needs compiled classes for type-resolution rules, this became the correct local verification command.
+
+## Lessons Learned & Gotchas
+
+1. **PMD type-resolution rules need compiled code**: `MissingOverride`, `UnnecessaryCast`, `LooseCoupling`, `UseCollectionIsEmpty` all need class files. Always run `mvn clean compile pmd:check` locally, not just `pmd:check`.
+
+2. **Checkstyle does NOT need compilation** — it works purely on source files.
+
+3. **`--fail-at-end` hides early failures** — When grepping output, always check the final `BUILD SUCCESS/FAILURE` line, not just intermediate results.
+
+4. **Xtend dispatch methods produce underscore-prefixed Java methods** — These are a known pattern (`_methodName`) that Xtext's dispatch resolution depends on. They cannot be renamed.
+
+5. **CI modules are discovered incrementally** — The Maven reactor with `--fail-at-end` skips downstream modules when an upstream module fails. Fixing one module can unblock compilation of others, revealing their violations.
+
+6. **`ByteArrayInputStream.close()` is a no-op** — When simplifying try-finally around BAIS, safe to just remove the close call entirely.
+
+7. **PMD's InsufficientStringBufferDeclaration calculates actual string sizes** — A StringBuilder(512) may still trigger if the appended content totals >512 bytes. Some methods needed 2048.
+
+## Next Steps
+
+1. **Consider squashing the 4 style-fix commits** before merging to master — they're all part of the same logical change. The commits are `18fb5a34e`, `8d4d826d7`, `ab0d6df10`, `645407dd5`.
+
+2. **Clean up untracked `xtend-gen/` directories** — 25 untracked `xtend-gen/` directories remain from the migration. These should either be `.gitignore`d or removed.
+
+3. **Consider adding `mvn clean compile pmd:check` to the local dev workflow** — Document that `pmd:check` alone misses type-resolution rules.
+
+4. **PR is ready for review** — CI is green. The PR is #1274 on the dsl-devkit repo.
+
+## Key Files & Locations
+
+| File | Purpose |
+|------|---------|
+| `check.core/.../formatting2/CheckFormatter.java` | Largest single file fix (~143 violations). Dispatch-based formatter. |
+| `check.core/.../jvmmodel/CheckJvmModelInferrer.java` | JVM model inference with dispatch methods. Had LooseCoupling, UnnecessaryCast, MissingOverride. |
+| `check.core/.../generator/CheckGeneratorExtensions.java` | Code generation utilities. Star import expansion, JavadocMethod, IllegalCatch. |
+| `check.core/.../generator/CheckGenerator.java` | Main code generator. AppendCharWithChar, InsufficientStringBufferDeclaration. |
+| `check.core/.../generator/CheckGeneratorNaming.java` | Naming conventions. StringToString fix (getLastSegment().toString() redundant). |
+| `check.core/.../scoping/CheckScopeProvider.java` | Scoping with dispatch methods. UnusedFormalParameter, UseIndexOfChar. |
+| `xtext.generator.test/.../XbaseGeneratorFragmentTest.java` | Test file. LooseCoupling (BasicEList→EList), ConstantName renames, ImmutableField. |
+| `xtext.generator/.../AnnotationAwareAntlrContentAssistGrammarGenerator.java` | Grammar generator. MissingOverride, UnnecessaryBoxing fixes. |
+| `checkcfg.core/.../PropertiesInferenceHelper.java` | Properties inference. ExhaustiveSwitchHasDefault (switch expression), UseCollectionIsEmpty. |
+| `check.core.test/.../util/CheckModelUtil.java` | Test model utilities. JavadocMethod, VariableDeclarationUsageDistance, AppendCharWithChar. |
+
+## Additional Notes
+
+- The `xtend-gen/` directories in the untracked files list are leftover from the Xtend build infrastructure that was removed in commit `41edd59ab`. They should be added to `.gitignore` or deleted.
+- The worktree branches (`worktree-agent-*`) visible in `git branch` are leftovers from agent execution. They can be cleaned up with `git branch -D`.
+- You may want to add `HANDOVER.md` to `.gitignore`.