Okhttp or HTTPClient : Which offers better functionality and more efficiency?

I used both. Okhttp is easier but it has limitations. So you can not change timeout like configuration after creating a singleton connection. It is a serious limitation.

I suggest to use okhttp. Here is the reason: https://github.com/square/okhttp/issues/3472

OkHttp has HTTP/2, a built-in response cache, web sockets, and a simpler API. It’s got better defaults and is easier to use efficiently. It’s got a better URL model, a better cookie model, a better headers model and a better call model. OkHttp makes canceling calls easy. OkHttp has carefully managed TLS defaults that are secure and widely compatible. Okhttp works with Retrofit, which is a brilliant API for REST. It also works with Okio, which is a great library for data streams. OkHttp is a small library with one small dependency (Okio) and is less code to learn. OkHttp is more widely deployed, with a billion Android 4.4+ devices using it internally.