Why Open/Closed Principle in LLD? - Purpose & Use Cases

The Big Idea

What if you could add new features without ever breaking your existing app?

The Scenario

Imagine you built a simple app that calculates discounts for products. Every time a new discount rule comes up, you open the existing code and change it directly.

At first, it works fine. But soon, the code becomes a tangled mess of if-else checks, and every change risks breaking something else.

The Problem

Changing the same code repeatedly is slow and risky. You might accidentally break existing features. Testing becomes harder because changes affect many parts. It's like fixing a leaky pipe by patching holes everywhere instead of replacing the pipe.

The Solution

The Open/Closed Principle says: design your system so it is open for extension but closed for modification. This means you add new features by adding new code, not by changing old code. Your original code stays safe and stable.

This keeps your system flexible and easier to maintain as it grows.

Before vs After

✗ Before

if (discountType == 'seasonal') { applySeasonalDiscount(); } else if (discountType == 'clearance') { applyClearanceDiscount(); }

✓ After

class Discount { apply() {} } class SeasonalDiscount extends Discount { apply() { /* seasonal logic */ } } class ClearanceDiscount extends Discount { apply() { /* clearance logic */ } }

What It Enables

You can add new features without fear of breaking existing code, making your system more reliable and easier to grow.

Real Life Example

Think of a music player app that supports new audio formats. Instead of rewriting the player each time, it adds new format handlers separately, keeping the core player stable.

Key Takeaways

Changing existing code often leads to bugs and slow progress.

Open/Closed Principle encourages adding new code instead of changing old code.

This approach makes software easier to maintain and extend.

Practice

1. What does the Open/Closed Principle (OCP) primarily encourage in software design?

easy

A. Software entities should be open for extension but closed for modification.

B. Software entities should be open for modification but closed for extension.

C. Software entities should be neither open nor closed for changes.

D. Software entities should always be rewritten for new features.

Solution

1. Step 1: Understand the definition of OCP

   The Open/Closed Principle states that software components should allow new functionality to be added without changing existing code.

2. Step 2: Match the principle to the options

   Software entities should be open for extension but closed for modification correctly states that entities should be open for extension (new features) but closed for modification (existing code remains unchanged).

3. Final Answer:

   Software entities should be open for extension but closed for modification. -> Option A

4. Quick Check:

   OCP = Open for extension, closed for modification [OK]

Hint: Remember: extend without changing existing code [OK]

Common Mistakes:

• Confusing 'open for extension' with 'open for modification'
• Thinking OCP means rewriting code for new features
• Ignoring the 'closed for modification' part

2. Which of the following code structures best supports the Open/Closed Principle?

easy

A. Using inheritance or interfaces to add new behavior without changing existing code.

B. Hardcoding all features inside one large class.

C. Copying and pasting code to create new features.

D. Modifying existing classes directly to add new features.

Solution

1. Step 1: Identify structures that allow extension without modification

   Inheritance and interfaces allow new classes to add behavior without changing existing classes.

2. Step 2: Evaluate options against OCP

   Using inheritance or interfaces to add new behavior without changing existing code uses inheritance/interfaces, which aligns with OCP. Other options involve modifying or duplicating code, violating OCP.

3. Final Answer:

   Using inheritance or interfaces to add new behavior without changing existing code. -> Option A

4. Quick Check:

   Inheritance/interfaces = OCP support [OK]

Hint: Choose inheritance or interfaces for extension [OK]

Common Mistakes:

• Thinking direct modification supports OCP
• Believing code duplication is acceptable
• Ignoring design patterns that enable extension

3. Consider this code snippet:

class Shape {
  area() { return 0; }
}

class Rectangle extends Shape {
  constructor(width, height) {
    super();
    this.width = width;
    this.height = height;
  }
  area() { return this.width * this.height; }
}

class Circle extends Shape {
  constructor(radius) {
    super();
    this.radius = radius;
  }
  area() { return Math.PI * this.radius * this.radius; }
}

function totalArea(shapes) {
  return shapes.reduce((sum, shape) => sum + shape.area(), 0);
}

What principle does this design demonstrate?

medium

A. Liskov Substitution Principle by replacing shapes with unrelated classes.

B. Single Responsibility Principle by separating area calculation.

C. Dependency Inversion Principle by depending on concrete classes.

D. Open/Closed Principle by allowing new shapes without modifying totalArea.

Solution

1. Step 1: Analyze how new shapes are added

   New shape classes (Rectangle, Circle) extend Shape and implement area(). totalArea works without changes.

2. Step 2: Match design to OCP

   Since totalArea does not change when new shapes are added, the design follows Open/Closed Principle.

3. Final Answer:

   Open/Closed Principle by allowing new shapes without modifying totalArea. -> Option D

4. Quick Check:

   Adding shapes without changing totalArea = OCP [OK]

Hint: Look for unchanged code when adding new features [OK]

Common Mistakes:

• Confusing OCP with other SOLID principles
• Thinking totalArea must change for new shapes
• Ignoring inheritance usage

4. Given this code violates the Open/Closed Principle:

class Report {
  generate(type) {
    if (type === 'pdf') {
      // generate PDF
    } else if (type === 'html') {
      // generate HTML
    }
  }
}

What is the best fix to comply with OCP?

medium

A. Add more if-else cases inside generate() for new report types.

B. Create separate classes for each report type implementing a common interface.

C. Remove the generate() method and write report code inline.

D. Use global variables to control report type.

Solution

1. Step 1: Identify why current code violates OCP

   Adding new report types requires modifying generate() method, breaking OCP.

2. Step 2: Choose design that allows extension without modification

   Separate classes for each report type implementing a common interface allow adding new types without changing existing code.

3. Final Answer:

   Create separate classes for each report type implementing a common interface. -> Option B

4. Quick Check:

   Separate classes = extend without modify [OK]

Hint: Use separate classes, not if-else chains [OK]

Common Mistakes:

• Adding more if-else blocks inside methods
• Ignoring interface or inheritance use
• Using global variables for behavior control

5. You are designing a payment system that must support new payment methods without changing existing code. Which approach best follows the Open/Closed Principle?

hard

A. Rewrite the payment processor each time a new method is added.

B. Add new payment method code inside a large switch statement in the payment processor.

C. Implement a Payment interface and create new classes for each payment method.

D. Use global flags to select payment methods inside one class.

Solution

1. Step 1: Understand the requirement for extensibility without modification

   The system must allow adding new payment methods without changing existing code.

2. Step 2: Evaluate design options for OCP compliance

   Implementing a Payment interface and creating new classes for each method allows extension by adding classes, not modifying existing ones.

3. Final Answer:

   Implement a Payment interface and create new classes for each payment method. -> Option C

4. Quick Check:

   Interface + new classes = OCP compliance [OK]

Hint: Use interfaces and new classes for new features [OK]

Common Mistakes:

• Using switch statements that require modification
• Rewriting existing code repeatedly
• Using global flags for behavior control