How can it be demonstrated that the methods of an abstract class are mandatory to override?

In Android (Java/Kotlin), an abstract class can contain abstract methods — methods without implementation that must be overridden in subclasses. If a subclass does not implement these methods, the code will not compile.

Example in Java:

abstract class Animal {
    // Abstract method, must be overridden
    abstract void makeSound();
}

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

// If you try to create a class without implementing makeSound(), it will result in a compilation error

Thus, having an abstract method requires all subclasses to implement this method, ensuring polymorphism and adherence to SOLID principles.