java – 为什么stream average()方法返回OptionalDouble而不是double？

  public final static Function<List<FileCheckOut>, Double> getAverageDefectCntFunction =
        (ruleResults) ->
        {
            if (ruleResults.isEmpty()) {
                return 0.0000;
            }
            OptionalDouble a = ruleResults.stream().parallel()
                .mapToDouble(p -> p.getDefectCnt()).average();
            logger.info("[getAsDouble---->]" + a.toString());
            if (a.isPresent()){
                return a.getAsDouble();
            }else {
                return 0.0000;
            }
        };

public class User {
    private Address address;
    public Optional<Address> getAddress() {
        return Optional.ofNullable(address);
    }
    // ..
}
public class Address {
    private Country country;
    public Optional<Country> getCountry() {
        return Optional.ofNullable(country);
    }
    //.. 
}

if (user != null) {
    Address address = user.getAddress();
    if (address != null) {
        Country country = address.getCountry();
        if (country != null) {
            String isocode = country.getIsocode();
            if (isocode != null) {
                isocode = isocode.toUpperCase();
            }
        }
    }
}

@Test
public void whenChaining_thenOk() {
    User user = new User("anna@gmail.com", "1234");
    String result = Optional.ofNullable(user)
      .flatMap(u -> u.getAddress())
      .flatMap(a -> a.getCountry())
      .map(c -> c.getIsocode())
      .orElse("default");
    assertEquals(result, "default");
}