Add docs/JAVA.md and update AGENTS.md to reference it

Co-authored-by: afranken <763000+afranken@users.noreply.github.com>

Author: Copilot

AGENTS.md

@@ -66,6 +66,8 @@ docker/              # Docker image build
 
 See **[docs/KOTLIN.md](docs/KOTLIN.md)** for Kotlin idioms, naming conventions, common anti-patterns, and KDoc guidelines.
 
+See **[docs/JAVA.md](docs/JAVA.md)** for Java idioms, naming conventions, common anti-patterns, and Javadoc guidelines.
+
 **Spring**: `@RestController`, `@Service`, `@Component`, constructor injection over field injection
 
 ## XML Serialization

docs/JAVA.md

@@ -0,0 +1,74 @@
+# Java Guidelines — S3Mock
+
+Canonical reference for Java idioms, naming conventions, and code quality standards used across this project.
+
+## Style
+
+Java code follows the **[Google Java Style Guide](https://google.github.io/styleguide/javaguide.html)** enforced by Checkstyle (`etc/checkstyle.xml`).
+
+Key rules:
+- **Indentation**: 2 spaces (no tabs)
+- **Line length**: 120 characters maximum
+- **Braces**: Always use braces for `if`, `for`, `while`, `do` blocks
+- **Imports**: Static imports first, then third-party packages; alphabetical within groups; no wildcard imports
+
+## Modern Java Idioms
+
+### Local Type Inference
+- Use `var` for local variables when the type is clear from context:
+  ```java
+  var uploadFile = new File(UPLOAD_FILE_NAME);
+  var response = s3Client.getObject(...);
+  ```
+- Avoid `var` when the inferred type would be ambiguous or unclear
+
+### Collections
+- `list.size() == 0` / `list.size() > 0` → `list.isEmpty()` / `!list.isEmpty()`
+- Use `List.of(...)`, `Map.of(...)` for immutable collections instead of `Collections.unmodifiableList(...)`
+- Prefer streams over explicit loops for transformations:
+  ```java
+  buckets.stream().map(Bucket::name).collect(Collectors.toSet())
+  ```
+
+### Switch Expressions
+- Prefer switch expressions over `if-else` chains with 3+ branches
+
+### Text Blocks
+- Use text blocks for multi-line strings
+
+## Common Anti-Patterns
+
+| Anti-Pattern | Refactor To |
+|---|---|
+| `list.size() == 0` | `list.isEmpty()` |
+| `Collections.emptyList()` | `List.of()` |
+| `Collections.unmodifiableList(new ArrayList<>(...))` | `List.copyOf(...)` |
+| `"" + value` | `String.valueOf(value)` or `String.format(...)` |
+| Empty catch blocks | At minimum, log the exception |
+| Magic numbers/strings | Named constants |
+
+## Naming Conventions
+
+Follows [Google Java Style](https://google.github.io/styleguide/javaguide.html#s5-naming):
+- **Classes/Interfaces/Enums**: `UpperCamelCase`
+- **Methods/Variables**: `lowerCamelCase`
+- **Constants** (`static final`): `UPPER_SNAKE_CASE`
+- **Booleans**: `is-`/`has-`/`should-`/`can-` prefixes
+- **Collections**: plural nouns
+- **Avoid abbreviations**: `bucketMetadata` not `bktMd`
+- **Abbreviations** in type names: at most 4 consecutive uppercase letters (e.g., `KmsKeyRef` not `KMSKeyRef`)
+
+## Test Naming
+
+- **Method names**: Use descriptive verb phrases — `shouldUploadAndDownloadObject`, `defaultBucketsGotCreated`
+- **Avoid**: generic names like `testSomething` or `test1`
+- **Pattern**: Arrange-Act-Assert
+
+## Javadoc
+
+- Use `/** */` for public APIs; `//` inline comments for rationale
+- Comments explain **why**, never **what** — remove comments that restate the code
+- Javadoc tag order: `@param`, `@return`, `@throws`, `@deprecated`
+- Add comments for edge cases, non-obvious S3 semantics, or workarounds
+- Link to AWS API docs or GitHub issues where relevant
+- Single-line Javadoc is allowed: `/** Short description. */`