.doOnError() is a bit of a different beast. It is used to intercept errors that haven't reached .subscribe() yet. You may wonder how this is possible; is because the flow can be intercepted and recovered from the exception. In such a case, the original exception will never reach the error handler if it is implemented in the .subscribe() section.

For example, we can build on a previous case with .onExceptionResumeNext():

Observable.<String>error(new RuntimeException("Crash!"))
        .doOnError(e -> log("doOnNext", e))
        .onExceptionResumeNext(Observable.just("Second"))
        .subscribe(item -> {
            log("subscribe", item);
        }, e -> log("subscribe", e));

Here, we've added a line:

        .doOnError(e -> log("doOnNext", e))

It will intercept the exception before it reaches the .onExceptionResumeNext() block. So the output, in this case, will be as shown:

doOnNext:main: error
java.lang.RuntimeException: Crash!
subscribe:main:Second

Again, it's a super useful tool when there is a need to display notifications in the Android UI or just log that something failed in general.