Amazon SDE-2 LLD round . Could not answer follow-up question. Inputs appreciated. !!

Problem statement given

Design a simple rule engine that:

1. Takes a list of credit card expenses
2. Applies a set of rules
3. Flags the expenses that needs review
4. More rules can be added in future.
5. Design needs to be maintainable.

Example Rules

• Restaurant expenses should not exceed 70 dollar.
• Airfare expenses should not be allowed.
• Any expense above 250 dollar should be flagged.

My Approach (Simple Version)

I modeled each rule as an independent unit and created a rule engine to execute them.

Each rule implements a common interface:

interface Rule {
    RuleResult evaluate(Expense expense);
}

The engine iterates through all expenses and applies all rules:

for each expense:
    for each rule:
        evaluate rule
        if rule says SKIP → stop processing this expense
        if rule says FLAG → mark it
finally, return all flagged expenses.

Followup question by interviewer was

If we keep adding more rules in the future, we’ll end up creating a new class for every rule.

• Won’t this lead to too many classes (class explosion)?
• How would you redesign this to keep it scalable and maintainable?

Would love to hear how others would approach this - especially in real‑world systems.