In which cases is it recommended to use an abstract class in object-oriented programming?

An abstract class in object-oriented programming is recommended to be used when:

• You need to define a common interface and a basic implementation for a group of related classes.
• The class itself should not be instantiated directly, but only serve as a template for subclasses.
• It is important that all subclasses implement certain methods (which are declared as abstract).

For example, if you have a class Animal with an abstract method makeSound(), then specific animals (dogs, cats) will inherit this class and implement the method in their own way.

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("Woof");
    }
}

Thus, an abstract class helps organize code by ensuring the implementation of key methods in subclasses and allowing the use of common implementation parts.