Java Coding Question – Sneaky Enum Bug

Enum bug, concept, coding and solution

Problem

Backend APIs often rely on enums to represent filter, sorting, or expansion parameters. Sounds clean and safe, right? Until it isn’t. Backend system expected enum values in lower-case snake_case, while real-world API clients were sending UPPER_CASE enum names… and everything silently broke. No errors. No warnings.
Just quietly failing behavior — the worst kind.

We have an enum:

Expected client input

"created_at"

What real clients actually send

"CREATED_AT"

or sometimes…

"Created_At"

Result in production

• Enum is not mapped

• Expansion logic doesn’t execute

• Sorting / filtering silently fails

To fix this, we want to accept all of these:

CREATED_AT
created_at
Created_At

But we must reject the following:

createdAt
created-at
createdAt123
anything_unknown

Behavior requirements:

• Normalize input

• Map correctly to enum

• Throw IllegalArgumentException for invalid values

• Don’t break existing behavior

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Solution

We will add the fromString() method to our SortField Enum. fromString() converts a string input from the client into the correct SortField enum constant.

First, we check if the input is null or blank; if so, we raise IllegalArgumentException.