The following test mocks out a service call to CloudService.  Does the test provide enough confidence that the service call is likely to work?

@Test public void uploadFileToCloudStorage() {    when(mockCloudService.write(            WriteRequest.newBuilder().setUserId("testuser").setFileType("plain/text")...))      .thenReturn(WriteResponse.newBuilder().setUploadId("uploadId").build());      CloudUploader cloudUploader = new CloudUploader(mockCloudService);        Uri uri = cloudUploader.uploadFile(new File("/path/to/foo.txt"));    // The uploaded file URI contains the user ID, file type, and upload ID. (Or does it?)    assertThat(uri).isEqualTo(new Uri("/testuser/text/uploadId.txt"));

Lots of things can go wrong, especially when service contracts get complex. For example, plain/text may not be a valid file type, and you can't verify that the URI of the uploaded file is correct.

If the code under test relies on the contract of a service, prefer exercising the service call instead of mocking it out. This gives you more confidence that you are using the service correctly:
@Test public void uploadFileToCloudStorage() {    CloudUploader cloudUploader = new CloudUploader(cloudService);    Uri uri = cloudUploader.uploadFile("/path/to/foo.txt");    assertThat(cloudService.retrieveFile(uri)).isEqualTo(readContent("/path/to/foo.txt));  }

 How can you exercise the service call?

  1. Use a fake.  A fake is a fast and lightweight implementation of the service that behaves just like the real implementation. A fake is usually maintained by the service owners; don't create your own fake unless you can ensure its behavior will stay in sync with the real implementation.  Learn more about fakes at testing.googleblog.com/2013/06/testing-on-toilet-fake-your-way-to.html.
  2. Use a hermetic server.  This is a real server that is brought up by the test and runs on the same machine that the test is running on. A downside of using a hermetic server is that starting it up and interacting with it can slow down tests.  Learn more about hermetic servers at testing.googleblog.com/2012/10/hermetic-servers.html.
If the service you are using doesn't have a fake or hermetic server, mocks may be the only tool at your disposal. But if your tests are not exercising the service call contract, you must take extra care to ensure the service call works, such as by having a comprehensive suite of end-to-end tests or resorting to manual QA (which can be inefficient and hard to scale).