This `anyString` and `anyInt` verifications allow these tests to emit multiple `onNext` calls and still pass.

Perhaps you could consider assertions such as the following that ensure we receive only what we expect in the order we expect?

```java
InOrder inOrder = inOrder(observer);
inOrder.verify(observer, times(1)).onNext("0123");
inOrder.verify(observer, times(1)).onCompleted();
inOrder.verifyNoMoreInteractions();
```