Can you explain what the concept of an abstract class means in object-oriented programming?

An abstract class in object-oriented programming is a class that is not intended to be instantiated directly. It serves as a template for other classes and can contain both implemented methods and abstract methods (without implementation) that must be implemented in subclasses.

For example, in Dart (the language used in Flutter), an abstract class is declared using the abstract keyword:

abstract class Animal {
  void makeSound(); // abstract method

  void breathe() {
    print('Breathing'); // implemented method
  }
}

class Dog extends Animal {
  @override
  void makeSound() {
    print('Bark');
  }
}

void main() {
  // Animal animal = Animal(); // Error: cannot instantiate an abstract class
  Dog dog = Dog();
  dog.makeSound(); // Bark
  dog.breathe();   // Breathing
}

Thus, an abstract class defines a common interface and basic functionality, while concrete classes implement the details.