# Python (хороший пример)
from abc import ABC, abstractmethod

class Shape(ABC):
    @abstractmethod
    def area(self):
        pass

class Rectangle(Shape):
    def __init__(self, width, height):
        self._width = width
        self._height = height

    @property
    def width(self):
        return self._width

    @property
    def height(self):
        return self._height

    def area(self):
        return self._width * self._height

class Square(Shape):
    def __init__(self, side):
        self._side = side

    @property
    def side(self):
        return self._side

    def area(self):
        return self._side ** 2