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+# CLAUDE.md - AI Assistant Guide for CodeLibs CoreLib
+
+Java 21 utility library. Maven build system, Apache License 2.0.
+
+## Repository Structure
+
+```
+src/main/java/org/codelibs/core/
+├── beans/       # Bean manipulation & introspection (converter/, factory/, impl/, util/)
+├── collection/  # Enhanced collections (LruHashMap, CaseInsensitiveMap)
+├── convert/     # Type conversion (*ConversionUtil)
+├── exception/   # Runtime exception wrappers
+├── io/          # I/O & resource utilities
+├── lang/        # Reflection & language utilities
+├── misc/        # AssertionUtil, Base64Util, etc.
+├── text/        # Text processing (JSON, Tokenizer)
+└── ...          # Others (crypto, log, net, xml, zip, etc.)
+src/test/java/   # Test classes (mirrors main structure)
+```
+
+## Development Commands
+
+```bash
+mvn test                              # Run tests
+mvn test -Dtest=ClassName#methodName  # Run specific test
+mvn clean package                     # Build
+mvn formatter:format                  # Format code
+mvn license:format                    # Apply license headers
+mvn verify                            # Generate coverage report
+```
+
+## Code Conventions
+
+### Class Structure
+
+- **Utility classes**: `abstract` class + `protected` constructor
+- **Constants classes**: Same pattern
+
+### Argument Validation
+
+Use `AssertionUtil` at method entry:
+- `AssertionUtil.assertArgumentNotNull("argName", value)`
+- `AssertionUtil.assertArgumentNotEmpty("argName", value)`
+
+### Exception Handling
+
+- Wrap checked exceptions in runtime exceptions from `org.codelibs.core.exception`
+- Use error codes (e.g., "ECL0008" = null argument)
+
+### Test Structure
+
+- JUnit 4 + Hamcrest matchers
+- Test class: `{ClassName}Test.java`
+- Test method: `test{MethodName}`
+
+## Naming Conventions
+
+| Type | Pattern |
+|------|---------|
+| Utility | `{Feature}Util.java` |
+| Conversion | `{Type}ConversionUtil.java` |
+| Exception | `{Name}RuntimeException.java` / `Cl{Name}Exception.java` |
+| Implementation | `{Interface}Impl.java` |
+
+## Important Notes
+
+1. Always run `mvn test` after changes
+2. Run `mvn formatter:format` before committing
+3. Add license headers to new files with `mvn license:format`
+4. Maintain backwards compatibility - deprecate before removing
+5. Follow existing patterns - study similar classes first