java.lang.NoClassDefFoundError: com.google.ads.AdView

Just create manually the "libs" folder, and add your libraries there. By default, they doesn't exist

1. Project-> Clean
2. Project settings -> Java Build Path. Open "Order and export" tab.
3. Check GoogleAdMobsAdsSDK.

It is worked for me.

I had a similar problem, but did not have to add the libs folder; I tried, with no success, then removed them afterwards, when I realized that I had just forgotten to export the included Jar archived.

Fix:
Select project settings > Java Build Path, then Order and export.

Here, check the GoogleAdMobAdsSdk-package.

This should solve the NoClassDefFoundError-issue.