To access Google Books API or any other REST APIs directly at HTTP level, you can use Volley if you are willing to write asynchronous code, or OkHttp for simpler synchronous requests. And there's also Android Asynchronous Http Client.
But what's even better, you can use Feign or Retrofit that abstract away the HTTP-level implementation details and provide fluent typesafe APIs on top of auto-generated implementation. Retrofit is the most used network library in Android, but Feign is used more in the wider Java ecosystem.
Here's an example using Feign for Google Books API, Retrofit is very similar.
API interface, implementation is auto-generated by Feign:
public interface GoogleBooksApi {
@RequestLine("GET /books/v1/volumes")
Results findBookByISBN(@QueryMap Map<String, Object> queryParameters);
}
API client code:
public class BookLookupService {
public Book fetchBookByISBN(String isbn) throws BookLookupException {
final GoogleBooksApi googleBooksApi = connect();
final Map<String, Object> queryParameters = new HashMap<>();
queryParameters.put("q", "isbn:" + isbn);
final Results apiResponse = googleBooksApi.findBookByISBN(queryParameters);
if (apiResponse == null || apiResponse.getTotalItems() < 1) {
throw new BookLookupException("No books found for ISBN " + isbn);
}
final List<Result> results = apiResponse.getItems();
if (results == null || results.size() < 1) {
throw new BookLookupException("Invalid items list for ISBN " + isbn);
}
final Book book = results.get(0).getBook();
return book;
}
private static GoogleBooksApi connect() {
return Feign.builder()
.decoder(new GsonDecoder())
.logger(new Logger.ErrorLogger())
.logLevel(Logger.Level.BASIC)
.target(GoogleBooksApi.class, "https://www.googleapis.com");
}
}
Entities that model the API response structure:
public class Results {
int totalItems;
List<Result> items;
public int getTotalItems() {
return totalItems;
}
public List<Result> getItems() {
return items;
}
}
public class Result {
// the JSON field is named volumeInfo
Book volumeInfo;
public Book getBook() {
return volumeInfo;
}
}
public class Book {
private String title;
private List<String> authors;
public String getTitle() {
return title;
}
public List<String> getAuthors() {
return authors;
}
}
And last not least, test:
@RunWith(AndroidJUnit4.class)
public class BookLookupServiceAndroidTest {
private BookLookupService bookLookupService = new BookLookupService();
@Test
public void whenMultipleLookupResultsThenReturnsFirst() throws Exception {
assertThat(bookLookupService.fetchBookByISBN("9780321356680").getTitle(),
is(equalTo("Effective Java, 2nd Edition")));
}
}
Note that you need to wrap the code in AsyncTask to make it asynchronous as network requests are not allowed on main thread. The AsyncTask should update the UI in onPostExecute().
Here's an example:
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
final Button fetchBookButton = (Button) findViewById(R.id.FetchBookButton);
fetchBookButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) { new FetchBookTask().execute(getISBN()); }
});
}
private String getISBN() {
final EditText isbnField = (EditText) findViewById(R.id.BookIsbnField);
return isbnField.getText().toString();
}
private void showMessage(String message) {
Toast.makeText(getApplicationContext(), message, Toast.LENGTH_LONG).show();
}
class FetchBookTask extends AsyncTask<String, Void, Book> {
@Override
protected Book doInBackground(String... params) {
final String isbn = params[0];
try {
return new BookLookupService().fetchBookByISBN(isbn);
} catch (Exception e) {
Log.e("fetchBookByISBN", e.toString());
return null;
}
}
@Override
protected void onPostExecute(Book book) {
if (book != null) {
showMessage("Got book: " + book.getTitle());
} else {
showMessage("Failed to fetch book");
}
}
}
}
