Simple RxJava mistakes and how to avoid them

1. Observable creation and error handling

Consider the following example:

public Observable<Book> getFavoriteBook(User user) {
	return Observable.just(user.getFavoriteBookId())
                .flatMap(bookId -> bookService.getById(bookId))
                .onErrorReturn(throwable -> DEFAULT_FAVORITE_BOOK);

Focus on the error handling part. In my experience in 95% of the cases the expectation behind the statement .onErrorReturn(...); is to ensure that the method getFavoriteBook() is “safe”, e.g. that an exception cannot be thrown from it at all, as if it was surrounded by a giant try-catch.

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