MainActivity.java

public class MainActivity extends AppCompatActivity {
    // Removed members for example
    
    // Retrofit Service, we will move this out of the MainActivity
    GoogleBooksService service;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // Retrofit configuration, we will move this out too
        Retrofit retrofit = new Retrofit.Builder()
                // Base URL can change for endpoints (dev, staging, live..)
                .baseUrl("https://www.googleapis.com")
                // Takes care of converting the JSON response into java objects
                .addConverterFactory(GsonConverterFactory.create())
                .build();
        // Create the Google Book API Service
        service = retrofit.create(GoogleBooksService.class);

        // Init the views here, code removed for the example
     
        // Load the saved books list from the previous response on device rotation
        if (savedInstanceState != null) {
            Book[] books = (Book[]) savedInstanceState.getParcelableArray(SEARCH_RESULTS);
            adapter.addAll(books);
        }
    }

    private void performSearch() {
        String formatUserInput = getUserInput().trim().replaceAll("\\s+", "+");
        // Calling Retrofit inside the MainActivity
        service.search("search+" + formatUserInput)
                // enqueue runs the request on a separate thread
                .enqueue(new Callback<BookSearchResult>() {

                    // We receive a Response with the content we expect already parsed
                    @Override
                    public void onResponse(Call<BookSearchResult> call, Response<BookSearchResult> books) {
                        updateUi(books.body().getBooks());
                    }

                    // In case of error, this method gets called
                    @Override
                    public void onFailure(Call<BookSearchResult> call, Throwable t) {
                        t.printStackTrace();
                    }
                });
    }
  
    // More code has been removed from the example
}