Step 1 - extends (single superclass)

class Animal {
  void breathe() => print('breathing');
}

class Dog extends Animal {
  void bark() => print('woof');
}

void main() {
  Dog d = Dog();
  d.breathe();
}

• You get superclass methods and fields. Override with @override when you replace behavior.

Step 2 - implements (no inherited body)

class Console {
  void write(String s) => print(s);
}

class FileSink implements Console {
  @override
  void write(String s) {
    print('[file] $s');
  }
}

void main() {
  Console c = FileSink();
  c.write('hi');
}

• FileSink must provide every member of Console even if Console had defaults (those defaults are not copied via implements).

Step 3 - Rules of thumb

• extends: true subtype, reuse and specialize behavior.
• implements: adapt an existing class shape, or satisfy multiple interfaces.
• Dart allows only one extends; you can implements many types.

Practice tasks

• Make an abstract class Storage with void save(String key, String value) and String? load(String key). Implement it twice: MemoryStorage and LoggingStorage that delegates to another Storage.
• Compare: extends a class with a non-empty method body vs implements the same class; note what you must reimplement.