Open/Closed Principle (OCP)

1. What Is It?

The Open/Closed Principle states that software entities (classes, modules, functions) should be open for extension, but closed for modification. Originally formulated by Bertrand Meyer (1988) and popularized by Robert C. Martin, it means you should be able to add new behavior to a system without changing the existing, tested code.

Without OCP, adding a new feature requires editing existing classes. Every edit is a risk: you may introduce bugs, break existing tests, or require a new round of regression testing. OCP drives you toward a design where new behavior is added by writing new code — new subclasses, new strategy implementations, new decorators — rather than cracking open existing files. The existing code becomes a stable foundation, not a target for modification.

2. How It Works

The core mechanic: identify what varies in your system and encapsulate that variation behind an abstraction. Callers depend on the abstraction; new variants are added by implementing the abstraction, not by modifying the caller.

The classic example is a shape-area calculator. A naive implementation uses a series of if/elif checks on shape type — adding a new shape requires modifying the calculator. The OCP version defines a Shape abstraction; the calculator delegates to it, and new shapes are added without touching the calculator.

Mermaid Class Diagram

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Python Example

Copy
from abc import ABC, abstractmethod
from typing import List


# BEFORE — OCP violation: calculator must be modified for every new shape
class AreaCalculatorBad:
    def total_area(self, shapes: list) -> float:
        total = 0.0
        for shape in shapes:
            if shape["type"] == "circle":
                import math
                total += math.pi * shape["radius"] ** 2
            elif shape["type"] == "rectangle":
                total += shape["width"] * shape["height"]
            # Adding "triangle" requires modifying this method
        return total


# AFTER — OCP applied: calculator is closed to modification, open to extension

class Shape(ABC):
    """Abstraction that makes the calculator closed for modification."""

    @abstractmethod
    def area(self) -> float:
        ...


class Circle(Shape):
    def __init__(self, radius: float) -> None:
        self.radius = radius

    def area(self) -> float:
        import math
        return math.pi * self.radius ** 2


class Rectangle(Shape):
    def __init__(self, width: float, height: float) -> None:
        self.width = width
        self.height = height

    def area(self) -> float:
        return self.width * self.height


class Triangle(Shape):
    """New shape added WITHOUT touching AreaCalculator."""

    def __init__(self, base: float, height: float) -> None:
        self.base = base
        self.height = height

    def area(self) -> float:
        return 0.5 * self.base * self.height


class AreaCalculator:
    """Closed for modification: never needs to change when new shapes are added."""

    def total_area(self, shapes: List[Shape]) -> float:
        return sum(shape.area() for shape in shapes)


# Usage
shapes: List[Shape] = [Circle(5), Rectangle(4, 3), Triangle(6, 8)]
calculator = AreaCalculator()
print(calculator.total_area(shapes))  # Works with any Shape subclass

Java Example

Copy
import java.util.List;

// BEFORE — OCP violation
public class AreaCalculatorBad {
    public double totalArea(List<Object> shapes) {
        double total = 0;
        for (Object shape : shapes) {
            if (shape instanceof Circle c) {
                total += Math.PI * c.radius * c.radius;
            } else if (shape instanceof Rectangle r) {
                total += r.width * r.height;
            }
            // Must modify here to add Triangle
        }
        return total;
    }
}

// AFTER — OCP applied

public interface Shape {
    double area();
}

public class Circle implements Shape {
    private final double radius;

    public Circle(double radius) {
        this.radius = radius;
    }

    @Override
    public double area() {
        return Math.PI * radius * radius;
    }
}

public class Rectangle implements Shape {
    private final double width;
    private final double height;

    public Rectangle(double width, double height) {
        this.width = width;
        this.height = height;
    }

    @Override
    public double area() {
        return width * height;
    }
}

// New shape added WITHOUT modifying AreaCalculator
public class Triangle implements Shape {
    private final double base;
    private final double height;

    public Triangle(double base, double height) {
        this.base = base;
        this.height = height;
    }

    @Override
    public double area() {
        return 0.5 * base * height;
    }
}

// Closed for modification
public class AreaCalculator {
    public double totalArea(List<Shape> shapes) {
        return shapes.stream()
                     .mapToDouble(Shape::area)
                     .sum();
    }
}

3. Variants & Comparisons

OCP is achieved through different mechanisms depending on language and context:

4. When to Use It (and When NOT To)

Use OCP when:

• A class has an if/elif chain or switch that must grow every time a new variant is added
• You have a core algorithm that different use cases must customize (pricing, validation, export format)
• You're building a library or framework where users must extend behavior without forking your code

Anti-patterns:

• Premature abstraction: Extracting an interface for behavior that only ever has one implementation. Wait for the second variant before abstracting.
• Violating OCP in the abstraction: Making the base class area() method contain default logic with conditionals — the abstraction itself is now closed to extension.
• Over-engineering with plugins: Building a plugin registry for behavior that changes once a year. OCP is a trade-off — the extension infrastructure has a cost.

Decision triggers:

• "If you see a growing if shape_type == 'X' chain — reach for OCP via an abstraction."
• "If adding a new feature means editing 5 existing files — your design is not closed for modification."

5. Real-World Usage

Java's Comparator interface: Collections.sort() is closed for modification — it never changes. New sort orders are added by implementing Comparator<T>. The entire Java sort infrastructure follows OCP.

Django's middleware: Django's request/response pipeline is a chain of middleware objects. Adding new behavior (logging, authentication, rate limiting) means adding a new middleware class — the core WSGIHandler is never modified.

JUnit's @Extension / TestRule: JUnit's test runner is closed for modification. New test lifecycle behaviors (test retries, custom logging, database setup) are added by implementing TestRule or @Extension, not by modifying the runner.

6. Interview Cheat Sheet

Key sentences to say:

1. "OCP means once a class is tested and shipped, I shouldn't need to crack it open to add new behavior — I extend it instead."
2. "The mechanism is always the same: identify what varies, put it behind an abstraction, and add new variants by implementing the abstraction."
3. "The cost of OCP is the upfront abstraction. I apply it when I see the second use case — not the first."
4. "OCP and the Strategy pattern are closely related — Strategy is the runtime mechanism that makes a class open for extension."

Common follow-up questions:

"Is OCP always achievable?"

No — some changes are inherently cross-cutting and require touching existing code. OCP is a design goal, not a law. The goal is to minimize modifications to stable, tested code.

"How do you decide when to abstract?"

The 'Rule of Three': on the first use case, implement it directly. On the second, consider abstracting. On the third, definitely abstract. Don't prematurely design for extensibility that may never be needed.

"What's the difference between OCP and just using interfaces?"

Interfaces are the mechanism; OCP is the principle. You can have interfaces everywhere and still violate OCP if you're constantly modifying the classes that depend on those interfaces.

Connections to other concepts:

• OCP is achieved through polymorphism — without polymorphism, there's no way to extend behavior without modifying callers
• Strategy Pattern is the most direct runtime implementation of OCP
• Decorator Pattern extends behavior by wrapping, which is OCP without inheritance
• Dependency Inversion Principle enables OCP — when high-level modules depend on abstractions, adding new low-level variants doesn't require touching the high-level code