Q:

Can bad stuff happen when calling a class from an Activity? 

I am calling a class from my Activity.   The class I am calling doesn't extend a Service or a Activity.   It is just a worker class that I am using to process a BufferedReader object.   I am concerned that if the app loses focus during the processing of the BufferedReader I could be left with messy data.   So what happens when my Activity calls a class to perform database entries and the Activity loses focus.   I am thinking of making my worker class a Service so that I will have access to the OnPause and OnResume methods.   Not sure what I should be doing.    Does anyone else have any clues about this.  

A:

You have to leverage the Android Activity Lifecycle.  
class MyActivity extends Activity {
    private MyWorkerClass myWorkerClass;

    void onCreate() {
      myWorkerClass = new MyWorkerClass();
      myWorkerClass.startLoading();
    }

    void onPause() {
      // You know the activity is going into the background
      // so act appropriately in your worker class
      myWorkerClass.stopLoading();
    }
}

The issues you describe above are very generic, so explaining any specific implementation wouldn't help. 
If you wanted your worker class to complete the work even if your Activity goes into onPause then yes you would want to do the work in another Thread and possibly in a Service.  You can still have your worker class though for nice OO & adhering to the SRP.