optional

This question already has answers here : Check if property is set in Core Data? (2 answers) Closed 4 years ago . I have a UITableViewController loading its entries from Core Data via a NSFetchedResultsController . Like this: let historyItem = fetchedResults.objectAtIndexPath(indexPath) as HistoryItem historyItem has a title property defined like this: @NSManaged var title: String So in cellForRowAtIndexPath the code says cell?.textLabel?.text = historyItem.title and that should all be fine. title is not an optional and does not need unwrapping. However, in the past, the stored Core Data has

1. 前言 对于 Java 开发者来说， null 是一个令人头疼的类型，一不小心就会发生 NPE （空指针） 问题。也是 Java 语言为人诟病的一个重要原因之一。在我们消除可恶的 NPE 问题之前我们要回顾一下 Java 中 null 的概念。 2. Java 中的 null 翻译自 Oracle Java 文档 Java语言中有两种类型，一种是 基本类型 ，另一种是 引用类型 。还有一种没有名字的特殊类型，即表达式 null 。 由于 null 类型没有名称，所以不可能声明为 null 类型的变量或者转换为 null 类型。 null 引用是 null 类型表达式唯一可能的值。 null 引用可以转换为任意引用类型。 事实上，程序员可以忽略 null 类型，可以认为 null 仅仅是一个可以成为任何引用类型的特殊符号。 从上面的描述我们可以了解到, 其实 null 仅仅是一个关键字标识量，既不是一种类型也不算对象，无法直接声明 null 和被转换为 null，仅仅只能被引用，null 可以转换为任何引用类型。当一个 Java 引用类型对象被引用为 null 时代表当前对象不引用对象，并没有为其分配内存。 这也是我们在没有引用的对象上调用方法出现空指针的根本原因。 大多数情况下 Java 开发者使用 null 是为了表示某种不存在的意思。 3. NPE 问题的解决

通过 DU/DF 可以查看磁盘使用情况 DU 通过搜索文件来计算每个文件的大小然后累加，du能看到的文件只是一些当前存在的，没有被删除的。他计算的大小就是当前他认为存在的所有文件大小的累加和。 用法：du [选项]... [文件]... 　或：du [选项]... --files0-from=F Summarize disk usage of the set of FILEs, recursively for directories. 必选参数对长短选项同时适用。 -0, --null end each output line with NUL, not newline -a, --all write counts for all files, not just directories --apparent-size print apparent sizes, rather than disk usage; although the apparent size is usually smaller, it may be larger due to holes in ('sparse') files, internal fragmentation, indirect blocks, and the like -B, --block-size=SIZE scale sizes by

Someone raised a question on another SO Answer about whether it is bad practice or inefficient to do this: Optional<User> user = ... user.ifPresent(u -> doSomethingWithoutUser()); instead of if (user.isPresent()) doSomethingWithoutUser(); Specifically, the fact that we're adapting a zero-arg method into a Consumer<User> which ignores its parameter u . As this isn't a Stream non-terminal operation, the fact doSomethingWithoutUser() likely has side-effects isn't a concern. I'm not bothered about the specifics of this one-line Optional example, it could the result of a long chain of functional

Update - there is no difference between Optional and optional types in Swift - they are the same thing. So in Swift they introduced the Type? construct, that creates an optional type which "forces" the programmer to check if the value actually exists. Also, in Swift there is no exception handling. But there is this built-in optionality mechanism. This optional feature ? is just an Optional<T> enum behind the scenes inspired from Haskell's Maybe . I was wondering if there was any way of passing error information through the optional type. Can "abc123".toInt() return error information through