Additional libraries

We configured our build types and flavors, now we will need some third-party libraries. We will use and add support for Retrofit, OkHttp, and Gson. This is an explanation for each of them:

• Retrofit is a type-safe HTTP client for Android and Java by Square, Inc. Retrofit is one of the most popular HTTP client library for Android as a result of its simplicity and its great performance compared to the others.
• OkHttp is an HTTP client that's efficient by default--HTTP/2 support allows all requests to the same host to share a socket.
• Gson is a Java library that can be used to convert Java objects into their JSON representation. It can also be used to convert a JSON string to an equivalent Java object. Gson can work with arbitrary Java objects including preexisting objects that you do not have a source code for.

There are a few open source projects that can convert Java objects to JSON. Later in this book, we will add Kotson to provide Gson bindings for Kotlin.

Let's extend build.gradle with dependencies for Retrofit and Gson:

    dependencies { 
      ... 
      compile 'com.google.code.gson:gson:2.8.0' 
      compile 'com.squareup.retrofit2:retrofit:2.2.0' 
      compile 'com.squareup.retrofit2:converter-gson:2.0.2' 
      compile 'com.squareup.okhttp3:okhttp:3.6.0' 
      compile 'com.squareup.okhttp3:logging-interceptor:3.6.0' 
      ... 
    } 

After you updated your Gradle configuration, sync it again when asked!