I've been struggling with the same thing for many hours, trying to get the Gson jar to work no less. I finally cracked it – here are the steps I took:

1. Put the Gson jar (in my case, gson-2.2.4.jar) into the libs folder
2. Right click it and hit 'Add as library'

3. Ensure that compile files('libs/gson-2.2.4.jar') is in your build.gradle file (or compile fileTree(dir: 'libs', include: '*.jar') if you are using many jar files)

   Edit : Use implementation files('libs/gson-2.2.4.jar') (or implementation fileTree(dir: 'libs', include: '*.jar')) in Android Studio 3.0+

4. Do a clean build (you can probably do this fine in Android Studio, but to make sure I navigated in a terminal to the root folder of my app and typed gradlew clean. I'm on Mac OS X, the command might be different on your system

After I did the above four, it started working fine. I think the 'Add as library' step was the one I'd previously missed, and it didn't work until I cleaned it either.

[Edit - added the build.gradle step which is also necessary as others have pointed out]

Step 1 (Use default gradle wrapper)

File→Settings→Build, Execution, Deployment→Build Tools→Gradle→Use default Gradle wrapper (recommended)

Changing to Gradle Wrapper in the new version of Android Studio:

Step 2 (Select desired gradle version)

File→Project Structure→Project

The following table shows compatibility between Android plugin for Gradle and Gradle:

Latest stable versions you can use with Android Studio 4.1.2 (March 2021):

Android Gradle Plugin version: 4.1.2
Gradle version: 6.5

Official links

• Version Compatibility
• Migrate to Android Plugin for Gradle 3.0.0
• Find out the latest version of Gradle
• Info about the Gradle Wrapper