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Designing for Change: Boundaries, Contracts, and Dependency Inversion in Java

Fri, 03 Jul 2026 00:00:00 +0000

Introduction

Most Java systems don’t fail because a single class was written badly. They fail because the boundaries between classes were drawn in the wrong place — a database library leaks into business logic, a payment provider’s SDK shows up in twelve unrelated files, and changing one third-party dependency means touching half the codebase.

Designing for change is not about predicting the future correctly. It’s about isolating what varies so that when it inevitably does vary — a new payment provider, a swapped database, a different message broker — the blast radius is one adapter, not the whole system.

Designing for Change: Boundaries, Contracts, and Dependency Inversion in Python

Fri, 03 Jul 2026 00:00:00 +0000

Introduction

Python’s flexibility makes it easy to wire everything together directly — call the requests library from inside your business logic, import the ORM model straight into your pricing rules, instantiate a third-party SDK client wherever it’s needed. It works, right up until the payment provider changes, or you want a fast unit test that doesn’t need a live database.

Designing for change means drawing boundaries around the parts of the system likely to move — third-party services, storage, transport — so that a swap or a test double touches one small adapter instead of rippling through the codebase.