Open/Closed Principle

1// ❌ BAD: Violates OCP - must modify code for new shapes
2class AreaCalculator {
3    public double calculateArea(Object shape) {
4        if (shape instanceof Circle) {
5            Circle circle = (Circle) shape;
6            return Math.PI * circle.radius * circle.radius;
7        } else if (shape instanceof Rectangle) {
8            Rectangle rect = (Rectangle) shape;
9            return rect.width * rect.height;
10        }
11        // Must add new else-if for every new shape!
12        // else if (shape instanceof Triangle) { ... }
13        return 0;
14    }
15}
16
17// ✅ GOOD: Follows OCP - extend without modifying
18interface Shape {
19    double calculateArea();
20}
21
22class Circle implements Shape {
23    private double radius;
24    
25    public Circle(double radius) {
26        this.radius = radius;
27    }
28    
29    @Override
30    public double calculateArea() {
31        return Math.PI * radius * radius;
32    }
33}
34
35class Rectangle implements Shape {
36    private double width, height;
37    
38    public Rectangle(double width, double height) {
39        this.width = width;
40        this.height = height;
41    }
42    
43    @Override
44    public double calculateArea() {
45        return width * height;
46    }
47}
48
49// Adding new shape - NO modification to existing code!
50class Triangle implements Shape {
51    private double base, height;
52    
53    public Triangle(double base, double height) {
54        this.base = base;
55        this.height = height;
56    }
57    
58    @Override
59    public double calculateArea() {
60        return 0.5 * base * height;
61    }
62}
63
64class AreaCalculator {
65    public double calculateTotalArea(List<Shape> shapes) {
66        double total = 0;
67        for (Shape shape : shapes) {
68            total += shape.calculateArea();  // Works for any shape!
69        }
70        return total;
71    }
72}