Каковы четыре столпа ООП?

abstract class Shape {            // ABSTRACTION: "what", not "how"
    abstract double area();       // each shape decides its own formula
}

class Circle extends Shape {      // INHERITANCE: Circle is a Shape
    private double r;             // ENCAPSULATION: r is private
    Circle(double r) { this.r = r; }
    double area() { return Math.PI * r * r; }   // POLYMORPHISM: own area()
}

class Square extends Shape {
    private double s;
    Square(double s) { this.s = s; }
    double area() { return s * s; }
}

Shape shape = new Circle(2);      // POLYMORPHISM in action:
System.out.println(shape.area()); // calls Circle.area() at runtime