Java 8可选.为什么of和of可以为空? [英] Java 8 Optional. Why of and ofNullable?

问题描述

我对Java 8的可选，其目的是解决NullPointerException异常.

I have a question regarding Java 8's Optional, the purpose of which is to tackle NullPointerException exceptions.

问题是，让我们选择这两种类型的原因是什么?

The question is, what is the reason for having both types to let us choose:

Optional.of(T value)     <-----non-null value, null value will throw NPE
Optional.ofNullable(T value)   <----- nullable value

因为我期望的是，当我使用时:

Because what I expect is, when I use:

Optional.of(nullValue);

它不会抛出NullPointerException.

在回答后扩大了我的问题:

Expanded my question after some replies:

为什么人们会选择Optional而不是普通的if-else进行空检查?

Why would people opt for Optional instead of normal if-else for null checking?

推荐答案

The javadoc of Optional.of reads that explicitly :

@throws NullPointerException if value is null

，这就是使用Optional.ofNullable处理您所期望的案件的要求，这是一小段代码，例如:

and that is where the requirement of handling the cases as expected by you comes into picture with the use of Optional.ofNullable which is a small block of code as :

public static <T> Optional<T> ofNullable(T value) {
    return value == null ? empty() : of(value); // 'Optional.of'
}

也就是说，在应用程序设计中仍然会选择[strong] 来决定选择一个选项还是另一个选项.

That said, the decision of choosing one over the other would still reside with the application design as if your value could possibly be null or not.

在您的期望中，这不是Optional的实际意图. API注释进一步阐明了这一点(格式化我的格式):

On your expectation part, that was not what the Optional was actually intended for. The API note clarifies this further (formatting mine):

Optional主要用于用作方法返回类型，其中 显然需要代表没有结果"，在哪里使用 null可能会导致错误.类型为Optional的变量绝不能本身为null ；它应该始终指向Optional实例.

Optional is primarily intended for use as a method return type where there is a clear need to represent "no result," and where using null is likely to cause error. A variable whose type is Optional should never itself be null; it should always point to an Optional instance.

---

可选"的目的是解决NullPointerException异常.

purpose of Optional is to tackle NullPointerException exception.

旁边:只是为了清楚地指出，选择当然会隐式地让您定义是否应在运行时抛出NPE.它不是在编译时确定的.

Aside: Just to call it out clearly, that the choice would of course implicitly let you define if an NPE should be thrown at runtime or not. It's not determined at the compile time though.

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