Open/Closed Principle

Also Known As

TL;DR

Explanation

The O in SOLID, OCP states that you should be able to extend a class's behaviour without modifying its source code. In practice: use interfaces and polymorphism so new behaviour is added by writing a new class, not editing an existing one. A switch statement on a type field that grows every time a new type is added is a classic OCP violation — replace it with a polymorphic hierarchy. OCP reduces the risk that adding new features breaks existing tested behaviour.

Common Misconception

Why It Matters

Common Mistakes

• Adding a new case to a switch statement every time a new type is introduced — add a new class instead.
• Modifying a method's internals to handle a new special case rather than using polymorphism.
• Closing too early — abstracting before you know the variation points leads to wrong abstractions.
• Not using interfaces as the seam for extension — concrete classes are harder to extend without modification.

Code Examples

// Must edit this class every time a new payment provider is added
class PaymentService {
    public function charge(string \$provider, int \$amount): void {
        if (\$provider === 'stripe')  { /* Stripe SDK */ }
        elseif (\$provider === 'paypal') { /* PayPal SDK */ }
        // Adding Braintree means editing this method
    }
}

interface PaymentGateway {
    public function charge(int \$amount): ChargeResult;
}
class StripeGateway    implements PaymentGateway { public function charge(int \$a): ChargeResult {} }
class PayPalGateway    implements PaymentGateway { public function charge(int \$a): ChargeResult {} }
class BraintreeGateway implements PaymentGateway { public function charge(int \$a): ChargeResult {} }

class PaymentService {
    public function __construct(private PaymentGateway \$gateway) {}
    public function charge(int \$amount): ChargeResult { return \$this->gateway->charge(\$amount); }
    // Never changes when a new provider is added
}

References

• ↗ https://en.wikipedia.org/wiki/Open%E2%80%93closed_principle

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