An interactive Sudoku game implemented in Java, featuring four parallel UI implementations — Swing, JavaFX, Spring Boot/Thymeleaf (server-side rendered), and Angular 21 (SPA) — all built from the same core engine. The project serves as a hands-on comparison of different UI approaches available on the Java platform and beyond.

• Puzzle generator — generates fresh puzzles at three difficulty levels (Easy / Medium / Hard) with a guaranteed unique solution
• Backtracking solver — fully solves any valid puzzle; also powers the hint system
• Hint system — reveals a single cell using naked-single and hidden-single constraint propagation (shows why the value is correct)
• Real-time conflict highlighting — instantly flags duplicate values in rows, columns, and 3×3 boxes
• Mistake counter — tracks entries that conflict with the correct solution
• Timer — records elapsed time per game; pauses on completion

sudoku/
├── pom.xml            ← parent POM (Java 26, Maven 3.9+)
├── sudoku-core/       ← game logic: model, validator, solver, generator
├── sudoku-swing/      ← interactive UI built with Java Swing
├── sudoku-javafx/     ← interactive UI built with JavaFX
├── sudoku-web/        ← Spring Boot 3.4 + Thymeleaf SSR + REST API
└── sudoku-angular/    ← Angular 21 SPA (consumes sudoku-web REST API)

sudoku-core has no UI dependencies. All UI modules depend only on sudoku-core.

Java download: Adoptium Temurin or OpenJDK

Maven download: maven.apache.org

# Build all modules (includes Angular production build; Angular unit tests not run via Maven)
mvn clean install

# Run core unit tests only
mvn -pl sudoku-core test

# Run tests with HTML report
mvn -pl sudoku-core surefire-report:report
# Report: sudoku-core/target/site/surefire-report.html

# Swing
mvn -pl sudoku-swing exec:java

# JavaFX
mvn -pl sudoku-javafx javafx:run

# Start the Spring Boot server (serves Thymeleaf + Angular SPA)
mvn -pl sudoku-web spring-boot:run

# Thymeleaf SSR:  http://localhost:8080/thymeleaf
# Angular SPA:    http://localhost:8080/

# Requires sudoku-web running on :8080 for the REST API
cd sudoku-angular
npm install     # first time only
npm start       # dev server on http://localhost:4200

1. Select a difficulty and click New Game
2. Click any empty cell and type a digit (1–9)
3. Use arrow keys to move between cells
4. Conflicting cells are highlighted in red immediately
5. Use Hint to reveal one logically deducible cell
6. Use Solve to auto-complete the puzzle
7. Use Reset to clear all your entries and start over
8. The game ends when all cells are filled correctly — your time and mistake count are shown

The project follows a strict separation of concerns:

• Model (sudoku-core) — SudokuBoard, GameState, SudokuValidator, SudokuSolver, SudokuGenerator
• Desktop UIs (sudoku-swing, sudoku-javafx) — implement GameEventListener to react to state changes
• Web backend (sudoku-web) — Spring Boot REST API + Thymeleaf; GameState is stored in HTTP session scope
• Web frontend (sudoku-angular) — Angular 21 SPA calling the REST API

GameState fires GameEvents to registered listeners. Desktop UIs dispatch updates to their UI threads (SwingUtilities.invokeLater / Platform.runLater). The web UI manages its own client-side timer via setInterval.

The solver uses a backtracking + MRV (Minimum Remaining Values) heuristic: always fill the empty cell with the fewest legal candidates first.

All endpoints return a BoardDto with values[][], given[][], conflicts[][], complete, mistakeCount, difficulty.

Other Java UI options: AWT (rarely used directly), SWT (Eclipse's native-widget toolkit). For most modern Java desktop apps, JavaFX is preferred over Swing. For web, Spring Boot + a JS frontend is the dominant pattern.

Migrated from a legacy Eclipse/Java stub project (BasicSwingComponents, SudokuTableModel) with no working game logic. The salvaged piece is the field2 puzzle from the original prototype (included as a built-in sample after fixing a duplicate-digit bug in row 9).