public void appendTimestampToResponse(OutputStream outputStream) throws IOException {
    AbstractMessageLite messageLite = protobufObjectGenerator.generateTimestampMessage(system2.now());
    messageLite.writeDelimitedTo(outputStream);
  }
@Test
  public void appendTimestampToResponse_outputStreamIsCalledAtLeastOnce() throws IOException {
    OutputStream outputStream = mock(OutputStream.class);

    underTest.appendTimestampToResponse(outputStream);

    verify(outputStream, atLeastOnce()).write(any(byte[].class), anyInt(), anyInt());
  }