_Rot13StringTest.java

@RunWith(JMock.class)
public class _Rot13StringTest {
  private Mockery _mockery = new JUnit4Mockery();
  private PersistenceMechanism _fileSystem;
    
  @Before
  public void setup() {
    _fileSystem = _mockery.mock(PersistenceMechanism.class);
  }
  
  @Test
  public void saveAs() throws IOException {
    TransformableString string = new Rot13String("abc", _fileSystem);

    _mockery.checking(new Expectations() {{
      oneOf (_fileSystem).overwrite("filename", "abc");
    }});

    string.saveAs("filename");
  }
  
  @Test
  public void writeTo() {
    TransformableString string = new Rot13String("foo", _fileSystem);

    final Display display = _mockery.mock(Display.class);
    _mockery.checking(new Expectations() {{
      oneOf (display).write("foo");
    }});
    string.writeTo(display);
  }

_UITest.java

  @Test
  public void go_handlesIOExceptionNicely() throws IOException {
    _mockery.checking(new Expectations() {{
      oneOf (_stringFactory).load(_inputFile); 
        will(throwException(new FileNotFoundException("file not found")));
      oneOf (_display).write("file not found");
    }});
    
    _ui.go(new String[] {_inputFile, _outputFile});
  }

Rot13String-Hypothetical.java

// You might expect it to look like this:

public static Rot13String load(String filename) throws IOException {
  return new Rot13String(_fileSystem.read(filename));
}

public void saveAs(String filename) throws IOException {
  _fileSystem.overwrite(filename, _string);
}

Rot13String.java

public void saveAs(String filename) throws IOException {
  _fileSystem.overwrite(filename, _string);
}

Rot13StringFileSystemLoader.java

// But instead it looks like this:

public TransformableString load(String filename) throws IOException {
  return new Rot13String(_fileSystem.read(filename), _fileSystem);
}

UI.java

  public UI(PrintStream out, Configuration configuration) {
    this(new ConsoleDisplay(out), new Rot13StringFileSystemLoader(
                                          new FileSystem(configuration)));
  }
  
  public static void main(String[] args) {
    main(args, System.out, Configuration.production());
  }

  public static void main(String[] args, PrintStream out, Configuration configuration) {
    new UI(out, configuration).go(args);
  }