Explain what an abstract class is and how it differs from a regular class.

An abstract class is a class that is not intended to be instantiated directly. Its main purpose is to serve as a base for other classes. An abstract class can contain abstract methods — methods without implementation that must be implemented in subclasses.

Differences from a regular class:

• You cannot create an object of an abstract class directly.
• It can contain abstract methods.
• It is used to define a common interface and behavior for a group of subclasses.

Example in Kotlin (Android):

abstract class Animal {
    abstract fun makeSound()

    fun sleep() {
        println("Sleeps")
    }
}

class Dog : Animal() {
    override fun makeSound() {
        println("Bark")
    }
}

fun main() {
    val dog = Dog()
    dog.makeSound()  // Outputs: Bark
    dog.sleep()      // Outputs: Sleeps
}