What are the main functions and applications of abstract classes in object-oriented programming?

Abstract classes in object-oriented programming are classes that are not intended to be instantiated directly, but serve as base templates for other classes.

Main functions of abstract classes:

• Defining a common interface and basic behavior for a group of related classes.
• The ability to declare abstract methods (without implementation) that must be implemented by subclasses.
• Allowing partial implementation of functionality that is inherited by child classes.

Applicability:

• When you need to set a common contract for different implementations.
• When part of the logic is common, and part needs to be implemented in subclasses.

Example in Java:

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

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

Here, Animal cannot be created directly, but you can create a Dog, which implements the makeSound method.