nameof operator allows you to get the name of a variable, type or member in string form without hard-coding it as a literal.
The operation is evaluated at compile-time, which means that you can rename a referenced identifier, using an IDE's rename feature, and the name string will update with it.
The official Retrofit page describes itself as
A type-safe REST client for Android and Java.
Retrofit turns your REST API into a Java interface. It uses annotations to describe HTTP requests, URL parameter replacement and query parameter support is integrated by default. Additionally, it provides functionality for multipart request body and file uploads.
null. The examples provided expose various cases where this error arises, together with some recommendations on how to mitigate the error.