What is the role of using abstract classes in object-oriented programming?

Abstract classes in object-oriented programming serve to define a common interface and behavior for a group of related classes, while themselves cannot be instantiated directly. They allow:

• To set a base implementation of methods that can be overridden in subclasses.
• To declare abstract methods without implementation, which must be implemented by subclasses.
• To ensure uniformity and structure for class hierarchies.

Example in Java:

abstract class Animal {
    abstract void makeSound();

    void breathe() {
        System.out.println("Breathing");
    }
}

class Dog extends Animal {
    @Override
    void makeSound() {
        System.out.println("Bark");
    }
}

Here, Animal sets a general contract for all animals, and specific classes implement the particular behavior.