Although most of the good Android libraries are open source, occasionally you get stuck with a third party JAR or two that doesn't have source available, or good documentation, or both.
Music streaming services in particular seem to prefer the closed source route. Many of them provide SDKs for Android such as Spotify, Deezer and Rdio but they're all in various pre-compiled formats.
Fortunately, there is a fantastic IDEA plugin to solve this problem called JD-IntelliJ. Android Studio appears to be fully compatible with IDEA plugins and this one works very well. It's not faultless and a small number of methods are output as "described byte code" but this itself isn't too tricky to follow.
Here is a quick example taken from Deezer's library: