// WidgetApi.java
public interface WidgetApi {
    @GET("/widget/{:id}")
    void getWidget(@Path("id") int id, Callback<Widget> callback);
}

// MockRetrofitClient.java
public class MockRetrofitClient implements Client {
    
    private int statusCode = 200;
    private String responseBody = "{}";
    
    public void setStatusCode(int statusCode) {
        this.statusCode = statusCode;
    }
    
    public void setResponseBody(String responseBody) {
        this.responseBody = responseBody;
    }
    
    @Override
    public Response execute(Request request) throws IOException {
        return new Response(request.getUrl(),
            statusCode,
            responseBody,
            Collections.EMPTY_LIST,
            new TypedByteArray("application/json", responseBody.getBytes()));
    }
}

// WidgetApiTest.java
@RunWith(RobolectricGradleTestRunner.class)
public class WidgetApiTest {
    
    private WidgetApi api;
    private MockRetrofitClient client;
    
    private Widget widget;
    
    @Before
    public void setUp() {
        client = new MockRetrofitClient();
        
        Executor executor = Executors.newSingleThreadExecutor();
        
        RestAdapter adapter = new RestAdapter.Builder()
            .setEndpoint("https://mock.com")
            .setClient(client)
            .setExecutors(executor, executor)
            .build();
            
        api = adapter.create(WidgetApi.class);
    }
    
    private String loadFixture(String relativePath) throws IOException {
        String path = "src/test/fixtures/" + relativePath;
        return FileUtils.readFileToString(new File(path));
    }
    
    @Test
    public void testGetWidget() throws Exception {
        client.setResponseBody(loadFixture("/api/widgets/widget-1.json"));
        final CountDownLatch latch = new CountDownLatch(1);
        
        api.getWidget(1, new Callback<Widget>() {
            @Override
            public void success(Widget widget, Response response) {
                this.widget = widget;
                latch.countDown();
            }
            
            @Override
            public void failure(RetrofitError error) {
                this.widget = null;
                latch.countDown();
            }
        });
        
        latch.await();
        
        assertEquals("test-widget-name", widget.getName());
    }
    
}