How does using abstract classes differ from interfaces in object-oriented programming?

Abstract classes and interfaces are two ways to describe contracts and behaviors in object-oriented programming, but they have key differences:

• Abstract class can contain both implemented methods and abstract methods (without implementation). It serves as a base class that can be inherited. An abstract class can have state (fields).

• Interface only describes a set of methods (in modern languages, interfaces can contain default implementations, but essentially it is a contract). An interface does not contain state.

Main differences:

1. Inheritance: A class can inherit from only one abstract class but can implement multiple interfaces.
2. State: An abstract class can hold state, an interface cannot.
3. Purpose: An abstract class is used when there is a common implementation and state, while an interface is for defining a contract without implementation.

Example in Java:

abstract class Animal {
    abstract void makeSound();
    void breathe() {
        System.out.println("Breathing...");
    }
}

interface Flyable {
    void fly();
}

class Bird extends Animal implements Flyable {
    void makeSound() {
        System.out.println("Chirp");
    }
    public void fly() {
        System.out.println("Flying");
    }
}

Here, Animal is an abstract class with partial implementation, and Flyable is an interface defining the ability to fly.