App Engine supports most, but not all, of the types in the Java 1.7 JRE. The plugin will automatically alert you if you use an unsupported type by flagging the offending usage as an error in the Problems view.
Note: You can change the default severity of any type of problem with Preferences > Google > Errors/Warnings.
If you use unsupported classes within a part of your application that will not be deployed to App Engine, you can exclude them from validation. Right-click the project and select Google > App Engine > Validation.
You can exclude individual files or entire packages or source paths from App Engine validation.
The exclusion filters also understand wildcard parameters (e.g.