optional

问题 (I say "more" in my title because this question is an appendix to my How does Swift ReferenceWritableKeyPath work with an Optional property?.) In my code, self.iv is an outlet property: @IBOutlet weak var iv: UIImageView! Now, will this compile? let im = UIImage() let kp = \UIImageView.image self.iv[keyPath:kp] = im // error No! We are told that something here must be unwrapped, although it is not entirely obvious what it is: Value of optional type 'UIImage?' must be unwrapped to a value of

问题 I have read a lot of Java 8 Optional and I do understand the concept, but still get difficulties when trying to implement it myself in my code. Although I have serached the web for good examples, I didn't found one with good explanation. I have the next method: public static String getFileMd5(String filePath) throws NoSuchAlgorithmException, IOException { AutomationLogger.getLog().info("Trying getting MD5 hash from file: " + filePath); MessageDigest md = MessageDigest.getInstance("MD5");

问题 Java 8 introduces Optional to deal with NPE. In practical application，I can‘t understand a problem. I have the method A public void doSomethingA(String para) { Optional<String> name = Optional.of(para); if (name.isPresent()) { //do } } But if para = null, it will throw NPE. method B public void doSomethingB(String para) { if (para != null) { //do } } if I check para is not null, What's the difference between A and B. Where is the meaning of Optional. 回答1: Use Optional.ofNullable if you are

问题 Closed. This question is off-topic. It is not currently accepting answers. Want to improve this question? Update the question so it's on-topic for Stack Overflow. Closed 3 years ago . I have a Rest Application using Jersey framework and maven. My application works well. Now I have decided to re-factor my code and use Java 8 features wherever possible. I started with Optional class. But when I use Optional.ifPresent method in my code, tomcat fails to start with Exception. Note: There are no

问题 I have a tiny playground example here fn main() { let l = Some(3); match &l { None => {} Some(_x) => {} // x is of type &i32 } } I'm pattern matching on &Option and if I use Some(x) as a branch, why is x of type &i32 ? 回答1: The type of the expression &l you match against is &Option<i32> , so if we are strict the patterns should be &None and &Some(x) , and if we use these patterns, the type of x indeed is i32 . If we omit the ampersand in the patterns, as you did in your code, it first looks

问题 I'm working through a book on Swift and I understand the idea of scope in functions so what I'd like to understand next is why we set global variables using optional types in classes. Honestly it looks like we don't set these variables per say but just let the class know that there will be a variable of a specific type somewhere throughout the code base: var sut: ItemManager! . From what I understand the variable sut is an unwrapped optional of the type ItemManger which definitely has a valid

问题 I have an certificate based authentication: import java.security.cert.X509Certificate; import java.util.Optional; import java.util.logging.Level; import java.util.logging.Logger; import javax.inject.Inject; import javax.inject.Named; import javax.naming.InvalidNameException; import javax.naming.ldap.LdapName; import javax.naming.ldap.Rdn; @Named public class UsernameService { private static final String MISSING_NAME = null; private static final Logger LOG = Logger.getLogger(UsernameService

问题 How do I convert an object of type (NSObject, AnyObject) to the type String ? At the end of the first line of the method below, as String causes the compiler error: '(NSObject, AnyObject)' is not convertible to 'String' Casting street to NSString instead of String compiles, but I'm casting street to String because I want to compare it to placemark.name , which has the type String! , not NSString . I know name and street are optionals, but I'm assuming they're not nil for now because all the

问题 public ValueA map(ValueB valueB, Date date) { Optional<ValueC> valueCOpt = find(valueB); if (valueCOpt.isPresent()) { ValueC valueC = valueCOpt.get(); // call many getters on valueC and do a lot of logic with it. return map(/*some parameters*/); } return null; } This seems quite ugly. The advantage of optionals is completely gone in here. I read that one should rather use map or flatMap instead of get . But is it really a benefit if I replace every getter like valueC.getFieldA() with

问题 Ground of Being : It will help, before reading, to know that you cannot assign a UIImage to an image view outlet's image property through the keypath \UIImageView.image . Here's the property: @IBOutlet weak var iv: UIImageView! Now, will this compile? let im = UIImage() let kp = \UIImageView.image self.iv[keyPath:kp] = im // error No! Value of optional type 'UIImage?' must be unwrapped to a value of type 'UIImage' Okay, now we're ready for the actual use case. What I'm actually trying to