ProblematicActivity.java

/*
Sample problematic Activity to illustrate sub-optimal architecture

This Activity has the following issues:-

1. The Activity contains business logic to fetch, filter and transform data thus 
making it difficult to test business logic in isolation as it requires the 
Activity and views to function.

2. The Activity and its Views cannot be tested in isolation as it requires the 
API and business logic for data.

3. Overtime, since the Activity also hosts business logic, with new features
or enhancements, it can get difficult to manage changes as there will be
no logical seperation of code. Also with little to no regression testing, 
it will be difficult to make changes confidently.  

*/
public class ProblematicActivity extends AppCompatActivity {

    ImageView imageView;
    TextView textView;
    ApiClient apiClient;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        ... 
        getData();  
    }

    private void getData() {
        apiClient.getData(new Callback() {

            @Override
            public void onSuccess(Response response) {
                Data data = Data.parse(response)
                Data filtered = filter(data);
                Data transformed = transform(filtered);
                
                imageView.setImageBitmap(data.bitmap);
                textView.setText(data.text);
            }

            @Override
            public void onFailure(Throwable throwable) {
                Toast.make(this, "Error", Toast.LENGTH_LONG).show();
            }
        });
    }
    
    private Data filter() {...}
    
    private Data transform() {...}
}