In this recipe, you performed two tasks; the configuration of the app to enable Google Sign-in, and the actual code for using Google to log in your users.
The first step involved activating Google Sign-in from the Firebase authentication page. All sign-in methods are disabled by default, so when you want to use a new method, you need to activate it explicitly.
Some services, including Google Sign-in, require the SHA-1 fingerprint of your app’s signing certificate. One of the possible ways to get the SHA-1 fingerprint is by using Keytool. This is a command-line tool that generates public and private keys and stores them in a Java KeyStore.
The tool is included in the Java SDK, so you should already have it if you have configured your Android environment or the Java SDK. In order to use it, you need to open your terminal and reach the bin directory of your Java SDK installation (or add it to your environment paths).
Once you get your SHA1 fingerprint, you need...