What are the differences between an abstract class and an interface in object-oriented programming?

Abstract class and interface are two ways to define contracts and behaviors in object-oriented programming, but with different features:

• Abstract class can contain both abstract methods (without implementation) and concrete methods, as well as fields (state). It serves as a base class for inheritance.
• Interface defines only method signatures (contract), contains no state, and traditionally no implementation (though default methods are allowed in some languages).

Differences:

• A class can inherit only one abstract class but implement multiple interfaces.
• Abstract class can have constructors, interface cannot.
• Abstract class is used when partial implementation is needed; interface when only the contract.

Example in Dart (Flutter):

abstract class Animal {
  void speak(); // abstract method
  void sleep() {
    print('Sleeping'); // concrete method
  }
}

// Interface in Dart is a class with only abstract methods
abstract class Flyable {
  void fly();
}

class Bird extends Animal implements Flyable {
  @override
  void speak() {
    print('Bird is speaking');
  }

  @override
  void fly() {
    print('Bird is flying');
  }
}