Developing your own type-safe HTTP library to interface with a REST API can be a real pain: you have to handle many aspects, such as making connections, caching, retrying failed requests, threading, response parsing, error handling, and more. Retrofit, on the other hand, is a well-planned, documented and tested library that will save you a lot of precious time and headaches.
Volley v/s Retrofit see this link
http://vickychijwani.me/retrofit-vs-volley/
compile 'com.google.code.gson:gson:2.6.2'
compile 'com.squareup.retrofit2:retrofit:2.1.0'// compulsory
compile 'com.squareup.retrofit2:converter-gson:2.1.0' //for retrofit conversion