# SOLID Applied Briefly — High-Level & Low-Level Design Interview Problems

Source: https://www.skillbyai.com/en/design-interviews/lld-solid

> Five principles in interview terms.

## What each letter means in practice

**S**ingle responsibility: a class has one reason to change (pricing is separate from ticket storage). **O**pen/closed: add behaviour by adding classes, not by editing a growing `if/else` (a new vehicle type adds a class). **L**iskov substitution: subtypes must work wherever the base type is expected without surprises. **I**nterface segregation: prefer small interfaces (`Payable`, `Refundable`) over one fat interface. **D**ependency inversion: high-level code depends on abstractions, and concrete implementations are injected. In interviews, mention a principle when it drives a decision, not as a list.

## Open/closed and dependency inversion together

Java sketch.

```java
interface PricingStrategy {
    long priceInCents(Ticket ticket, java.time.Instant exitTime);
}

class HourlyPricing implements PricingStrategy {
    public long priceInCents(Ticket t, java.time.Instant exit) {
        long hours = Math.max(1, java.time.Duration.between(t.entryTime(), exit).toHours());
        return hours * 200;   // assumed rate: 2.00 per hour
    }
}

class CheckoutService {
    private final PricingStrategy pricing;      // depends on the abstraction
    CheckoutService(PricingStrategy pricing) { this.pricing = pricing; }
    long checkout(Ticket t, java.time.Instant exit) { return pricing.priceInCents(t, exit); }
}
```

## Do not recite the acronym

Interviewers credit applying a principle to a concrete choice far more than listing all five definitions.

**Quiz:** Adding a new pricing rule by writing a new class rather than editing existing ones follows which principle?

- [ ] Liskov substitution
- [x] Open/closed
- [ ] Interface segregation
- [ ] Single table inheritance

*Answer:* Open/closed. Open for extension, closed for modification.