Rethinking Expense Validation: From Hardcoded If Statements to Pluggable Engines in Java

I spent years arguing the opposite, that a pluggable engine was over-engineering for what looked like a few branches. Every new reimbursement rule forced a re-read of the whole conditional, because the branches changed shape independently of each other. The interface has to be narrow enough that adding a rule never means touching the one beside it. Otherwise the branching just relocates into the strategies.