To add an external jar as library in Android Studio, follow the below given steps:
Step 1: Open your project in Android Studio. By default it open in Android explorer mode as shown in below given image.
Step 2: Click on small button showing right side of Android and select Project
Step 3: Navigate to YourProject -> Build ->libs and paste your jar in lib folder as shown in below given image.
Step 4: Select your jar and right click, that will open a window as shown in image. Click on Add As Library ...
Step 5: Previous step open a popup, select your app module and click Ok button.
Step 6: Ensure that your compiled file added in build.gradle(Module: app). Clean and Build your project to finish the process.