OPEN - CLOSED PRINCIPLE

#ocp #openclosedprinciple #solidprinciples #programming

OPEN - CLOSED PRINCIPLE

The integral parts of a code should be open for extension and closed for modification.

The idea is to avoid relying on conditional checks using if-else, ternary operators, and switch.

When a new requirement arrives, you add the behavior by creating something new, without modifying the old code.

TO BE FAIR

The idea of not using conditional operators is a result of correctly applying OCP, not the focus itself.

This is because in order to not depend on these operators, excellence in applying polymorphism and abstraction is necessary.

With this, it becomes possible to couple new implementations without rewriting the already existing code.

BAD EXAMPLE

Every new payment method will cause an edit here. It makes new implementations difficult and puts the old ones at risk.

Code Example:

// BAD: Tight coupling. The class needs to be modified for each new rule.
class PaymentProcessor {
  public process(amount: number, method: 'CREDIT_CARD' | 'PIX' | 'PAYPAL'): void {
    if (method === 'CREDIT_CARD') {
      console.log(`Processing $${amount} via Credit Card...`);
      // Complex logic for acquirer integration...
    }
    else if (method === 'PIX') {
      console.log(`Processing $${amount} via Pix key...`);
      // Complex logic for QRCode generation...
    }
    else if (method === 'PAYPAL') {
      console.log(`Processing $${amount} via PayPal...`);
      // Complex redirection logic...
    }
    else {
      throw new Error('Payment method not supported.');
    }
  }
}

GOOD EXAMPLE

Just fulfill the “IPaymentMethod” contract and the method is already valid.

We can then add a “DebitCardPayment”, without affecting the other methods.

Code Example:

// 1. The Abstraction (The Contract)
interface IPaymentMethod {
  processPayment(amount: number): void;
}

// 2. The Extensions (New isolated behaviors)
class CreditCardPayment implements IPaymentMethod {
  public processPayment(amount: number): void {
    console.log(`Processing $${amount} via Credit Card...`);
  }
}

class PixPayment implements IPaymentMethod {
  public processPayment(amount: number): void {
    console.log(`Processing $${amount} via Pix key...`);
  }
}

// 3. The Closed Application (The Consumer)
class PaymentProcessor {
  public process(amount: number, paymentMethod: IPaymentMethod): void {
    paymentMethod.processPayment(amount);
  }
}

// Practical use:
const processor = new PaymentProcessor();
processor.process(150.0, new PixPayment());