AI BriefingTurning a Single NPE into a Team Defense System
Key point
Analyzed an NPE incident in production and proposed design principles using Optional and domain objects to structurally prevent null errors.
Details
The root cause of the coupon restoration failure incident that occurred on a day off was a NullPointerException (NPE). During refactoring, the if statement checking for budget-limited coupons was removed, causing null values for general coupons to be passed to utility functions and triggering errors.
This bug bypassed compiler limitations, test coverage gaps, and code review pitfalls. It was not merely a lack of attention, but a structural vulnerability.
The solution involves using the Optional type and domain objects to explicitly declare nullability within the type system. By returning budget information as Optional<Budget> in the Policy class, the design fundamentally prevents developers from omitting null checks.
Additionally, null-checking logic was moved from generic utilities into the Value Object (IssuePeriod), which owns the data, preventing domain knowledge from contaminating utility code. This leads to safe system design that does not tolerate developer mistakes.
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