firebase-kotlin-sdk
· The Firebase Kotlin SDK is a Kotlin-first SDK for Firebase. It's API is similar to the Firebase Android SDK Kotlin Extensions but also supports multiplatform projects, enabling you to use Firebase directly from your common source targeting iOS, Android or JS.
Download JD-GUI to open JAR file and explore Java source code file (.class .java)
Click menu "File → Open File..." or just drag-and-drop the JAR file in the JD-GUI window firebase-common-android-1.8.1.jar file.
Once you open a JAR file, all the java classes in the JAR file will be displayed.
