Instruction file imported from jesse31/Work-Eversun-EngineeringDepartment-ERSA_Powerflow_ZH (.github/instructions/Java 指南.instructions.md). Copyright stays with the author.

Java Development

General Instructions

• First, prompt the user if they want to integrate static analysis tools (SonarQube, PMD, Checkstyle) into their project setup.
  • If yes, document a recommended static-analysis setup.
    • Prefer SonarQube/SonarCloud (SonarLint in IDE + sonar-scanner in CI).
    • Create a Sonar project key.
    • Store the scanner token in CI secrets.
    • Provide a sample CI job that runs the scanner.
    • If the team declines Sonar, note this in the project README and continue.
  • If Sonar is bound to the project:
    • Use Sonar as the primary source of actionable issues.
    • Reference Sonar rule keys in remediation guidance.
  • If Sonar is unavailable:
    • Perform up to 3 troubleshooting checks:
      1. Verify project binding and token.
      2. Ensure SonarScanner runs in CI.
      3. Confirm SonarLint is installed and configured.
    • If still failing after 3 attempts:
      • Enable SpotBugs, PMD, or Checkstyle as CI fallbacks.
      • Open a short tracker issue documenting the blocker and next steps.
• If the user declines static analysis tools or wants to proceed without them, continue with implementing the Best practices, bug patterns and code smell prevention guidelines outlined below.
• Address code smells proactively during development rather than accumulating technical debt.
• Focus on readability, maintainability, and performance when refactoring identified issues.
• Use IDE / Code editor reported warnings and suggestions to catch common patterns early in development.

Best practices

• Records: For classes primarily intended to store data (e.g., DTOs, immutable data structures), Java Records should be used instead of traditional classes.
• Pattern Matching: Utilize pattern matching for instanceof and switch expression to simplify conditional logic and type casting.
• Type Inference: Use var for local variable declarations to improve readability, but only when the type is explicitly clear from the right-hand side of the expression.
• Immutability: Favor immutable objects. Make classes and fields final where possible. Use collections from List.of()/Map.of() for fixed data. Use Stream.toList() to create immutable lists.
• Streams and Lambdas: Use the Streams API and lambda expressions for collection processing. Employ method references (e.g., stream.map(Foo::toBar)).
• Null Handling: Avoid returning or accepting null. Use Optional<T> for possibly-absent values and Objects utility methods like equals() and requireNonNull().

Naming Conventions

• Follow Google's Java style guide:
  • UpperCamelCase for class and interface names.
  • lowerCamelCase for method and variable names.
  • UPPER_SNAKE_CASE for constants.
  • lowercase for package names.
• Use nouns for classes (UserService) and verbs for methods (getUserById).
• Avoid abbreviations and Hungarian notation.

Common Bug Patterns

Below are concise, human-readable rules you can apply regardless of which static analysis tool you use. If you run Sonar/SonarLint, the IDE will show the matching rule and location — direct Sonar connections are preferred and should override this ruleset.

• Resource management — Always close resources (files, sockets, streams). Use try-with-resources where possible so resources are closed automatically.
• Equality checks — Compare object equality with .equals() or Objects.equals(...) rather than == for non-primitives; this avoids reference-equality bugs.
• Redundant casts — Remove unnecessary casts; prefer correct generic typing and let the compiler infer types where possible.
• Reachable conditions — Avoid conditional expressions that are always true or false; they indicate bugs or dead code and should be corrected.

For contributors who do use Sonar or SonarLint: the IDE/scan will show the specific rule key (for example, S2095 for resource leaks) and the affected file/line. Use that information to navigate to the exact location, then apply the recommended remediation.

Common Code Smells

These patterns are phrased for humans; they map cleanly to checks in Sonar, SpotBugs, PMD, or Checkstyle but do not require those tools to be useful.

• Parameter count — Keep method parameter lists short. If a method needs many params, consider grouping into a value object or using the builder pattern.
• Method size — Keep methods focused and small. Extract helper methods to improve readability and testability.
• Cognitive complexity — Reduce nested conditionals and heavy branching by extracting methods, using polymorphism, or applying the Strategy pattern.
• Duplicated literals — Extract repeated strings and numbers into named constants or enums to reduce errors and ease changes.
• Dead code — Remove unused variables and assignments. They confuse readers and can hide bugs.
• Magic numbers — Replace numeric literals with named constants that explain intent (e.g., MAX_RETRIES).

If you run a static analyzer like Sonar or SonarLint — direct Sonar connections are preferred and should override this ruleset. Sonar rule keys are useful for automation and suppression, but they are not required in day-to-day developer guidance.

Build and Verification

• After adding or modifying code, verify the project continues to build successfully.
• If the project uses Maven, run mvn clean install.
• If the project uses Gradle, run ./gradlew build (or gradlew.bat build on Windows).
• Ensure all tests pass as part of the build.