retrofit

Retrofit is a term used in engineering and information technology to describe the addition of new features or modernization to existing systems after their initial manufacture or deployment. It can refer to physical refurbishment of buildings, equipment, or infrastructure, as well as software libraries and services that enable new functionality without replacing the whole system.

In construction and infrastructure, a retrofit involves modifying an existing structure or system to improve performance, safety, efficiency, or compliance with current standards. Common examples include energy retrofits that add insulation and efficient windows, seismic retrofits to resist earthquakes, upgrading electrical or mechanical systems, and retrofitting for accessibility or fire safety. The approach can be passive (design changes) or active (new equipment, controls, or monitoring).

In software, Retrofit is a type-safe HTTP client for Android and Java originally developed by Square. It lets developers define a web API as a Java or Kotlin interface with annotated methods corresponding to HTTP requests, and then generates implementation code at runtime. Retrofit integrates with converter libraries to translate JSON or XML payloads into language objects and can adapt results to reactive streams or coroutines. It is built on top of OkHttp and supports features such as asynchronous requests, custom call adapters, and multiple data converters (for example, Gson or Moshi).

Retrofit is widely used in Android development to simplify network communication and is available as an open-source project with contributions from the community and Square. While other HTTP clients exist, Retrofit’s design emphasizes ease of use, testability, and maintainability through declarative interfaces.