As discussed throughout this book, a Reactive Stream terminates with a completion or error event. Similarly to expectComplete, for a completion event, there is the expectError API to validate error events. The expectError API offers the following convenient methods to validate an error message or the exception class:
| Error name | Description |
| expectError() | The API only validates the occurrence of an error event. It does not validate any details about the error. |
| expectError(exceptionClass) | The API validates the underlying exception class wrapped in the error event. |
| expectErrorMessage(errorMessage) | The API validates the underlying exception message wrapped in the error event. |
| expectError(Predicate) | The API validates the ... |
