为什么选择< T>宣布为最后一堂课？ [英] Why is Optional<T> declared as a final class?

问题描述

我正在玩以下问题：使用Java 8的可选流程:: flatMap 并希望将方法添加到自定义可选< T> ，然后检查它是否有效。

更准确，我想要将 stream（）添加到我的 CustomOptional< T> ，如果没有值，则返回空流，或者如果它存在，则包含单个元素的流。

I was playing with the following question: Using Java 8's Optional with Stream::flatMap and wanted to add a method to a custom Optional<T> and then check if it worked.
More precise, I wanted to add a stream() to my CustomOptional<T> that returns an empty stream if no value is present, or a stream with a single element if it is present.

然而，我得出的结论是可选< T> 被宣布为最终。

However, I came to the conclusion that Optional<T> is declared as final.

为什么会这样？有很多类没有被声明为final，我个人认为没有理由声明 Optional< T> final。

Why is this so? There are loads of classes that are not declared as final, and I personally do not see a reason here to declare Optional<T> final.

作为第二个问题，为什么所有的方法都不能是最终的，如果担心它们会被覆盖，并且让这个课程不是最终的？

As a second question, why can not all methods be final, if the worry is that they would be overridden, and leave the class non-final?

推荐答案

根据这个Java SE 8 API文档的页面，可选< T> 是一个基于值的类。根据 API文档的此页面，基于值的类必须是不可变的。

According to this page of the Java SE 8 API docs, Optional<T> is a value based class. According to this page of the API docs, value-based classes have to be immutable.

声明中的所有方法可选< T> 作为final将阻止方法被覆盖，但这不会阻止扩展类添加字段和方法。扩展类并添加字段以及更改该字段值的方法将使该子类可变，因此将允许创建可变的可选< T> 。以下是如果 Optional< T> 不会被声明为final的话，可以创建这样一个子类的示例。

Declaring all the methods in Optional<T> as final will prevent the methods from being overridden, but that will not prevent an extending class from adding fields and methods. Extending the class and adding a field together with a method that changes the value of that field would make that subclass mutable and hence would allow the creation of a mutable Optional<T>. The following is an example of such a subclass that could be created if Optional<T> would not be declared final.

//Example created by @assylias
public class Sub<T> extends Optional<T> {
    private T t;

    public void set(T t) {
        this.t = t;
    }
}

声明可选< T> final会阻止创建类似上面的子类，因此保证 Optional< T> 始终是不可变的。

Declaring Optional<T> final prevents the creation of subclasses like the one above and hence guarantees Optional<T> to be always immutable.

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