Step 1 - Abstract method

abstract class Shape {
  double area();
}

class Square extends Shape {
  final double side;
  Square(this.side);

  @override
  double area() => side * side;
}

void main() {
  Shape s = Square(3);
  print(s.area());
}

• Shape promises area() but does not implement it. Subtypes must.

Step 2 - Abstract class with concrete members

abstract class Logger {
  void log(String message);

  void logError(String message) {
    log('ERROR: $message');
  }
}

class ConsoleLogger extends Logger {
  @override
  void log(String message) {
    print(message);
  }
}

void main() {
  Logger l = ConsoleLogger();
  l.logError('oops');
}

• Shared default behavior can live on the abstract class; hooks stay abstract.

Good habit

• Use abstract class when you want both interface-like contracts and shared implementation.

Practice tasks

• Add Circle extends Shape with radius and area() => pi * radius * radius (import dart:math for pi if you want).
• Try Shape s = Shape(); and read the compile error.