Android studio used Gradle as build tool. Gradle used Groovy language which is similar to Java. gRPC require certain configurations but since we are not familiar with Gradle and Groovy, in a beginning, we had a couple of build errors. Hence, we have to spend time studying Gradle and finally fixed build errors. For instance, we set API level to 25 but app could not run on Moon's phone. We thought we only need to change the API level to 23 but it causes build failures. After research, it turns out we also have to change Gradle version from 2.3.3 to 2.2.3, as 2.3.3 is too high to support API level 23.