So, if we have decided to call Optional.get() to fetch the value wrapped in Optional, then we shouldn't do it as follows:

Optional<Book> book = ...; // this is prone to be empty

// Avoid
// if "book" is empty then the following code will
// throw a java.util.NoSuchElementException
Book theBook = book.get();

In other words, before fetching the value via Optional.get(), we need to prove that the value is present. A solution consists of calling isPresent() before calling get(). This way, we add a check that allows us to handle the missing value case:

Optional<Book> book = ...; // this is prone to be empty

// Prefer
if (book.isPresent()) {
  Book theBook = book.get();
  ... // do something with "theBook"
} else {
  ... // do something that does not call book.get()
}