AlertDialog.Builder cannot be resolved to a type error

I receive the error on the line below: ``` AlertDialog.Builder alrtDialog = new AlertDialog.Builder(this); ``` The Error show is : AlertDialog.Builder cannot be resolved to a type I import the following for this: ``` import android.content.DialogInterface; ``` the java code in the example(from android programming unleashed) includes android.app.AlertDialog but importing this results in message: conflicts with a type defined in the same file