Step 1 - Interface

abstract class Logger {
  void info(String message);
  void error(String message);
}

Step 2 - Console implementation

class ConsoleLogger implements Logger {
  @override
  void info(String message) => print('[INFO] $message');

  @override
  void error(String message) => print('[ERROR] $message');
}

Step 3 - Fake file logger (no real I/O)

class FileLogger implements Logger {
  final String path;
  final List<String> lines = [];

  FileLogger(this.path);

  @override
  void info(String message) {
    lines.add('$path [INFO] $message');
  }

  @override
  void error(String message) {
    lines.add('$path [ERROR] $message');
  }
}

Step 4 - Service uses Logger

class Greeter {
  final Logger log;
  Greeter(this.log);

  void hello(String name) {
    log.info('hello $name');
  }
}

void main() {
  Greeter(ConsoleLogger()).hello('Asha');
  FileLogger f = FileLogger('/tmp/app.log');
  Greeter(f).hello('Rafi');
  print(f.lines.join('\n'));
}

Practice tasks

• Add void warn(String message) to Logger and update both implementations.
• Inject Logger into a second service (for example UserService that logs sign-in).
• Add MultiLogger(List<Logger> sinks) that forwards each method to every sink.