如何证明为什么未经检查的强制转换是可以的，关于Copyable getObjectCopy（） [英] How to justify why an unchecked cast is okay, regarding Copyable getObjectCopy()

问题描述

（这是对我的上一个问题的跟进。）

(This is a follow-up to my previous question.)

我有一个名为可复制的接口，它有一个功能

I have an interface called Copyable, which has a single function

Copyable getObjectCopy();

许多其他类使用它。因为此函数始终返回可复制，所以会导致未经检查的强制转换。示例：

This is used by many other classes. Because this function always returns a Copyable, it results in unchecked casts. Example:

@SuppressWarnings("unchecked")  //Copy of itself is the same type.
ValidateValue<L> vvo = (ValidateValue<O>)this_toCopy.getValidator().getObjectCopy();
vvBlkA = vvo;

我的问题与Josh Bloch的建议有关（在Effective Java，第2版，第24项） ：

My question relates to Josh Bloch`s recommendation (in Effective Java, 2nd ed., item 24):

每次使用@SuppressWarnings（未选中）注释时，请添加注释，说明为什么这样做是安全的。

Every time you use an @SuppressWarnings("unchecked") annotation, add a comment saying why it's safe to do so.

他的例子是

// This cast is correct because the array we're creating
// is of the same type as the one passed in, which is T[].
@SuppressWarnings("unchecked")
T[] result = (T[]) Arrays.copyOf(elements, size, a.getClass());
return  result;

（参见第9/117页底部： http://www.infoq.com/resource/articles/bloch-effective-java -2e / en / resources / Bloch_Ch05.pdf ）

(see the bottom of page 9 / 117: http://www.infoq.com/resource/articles/bloch-effective-java-2e/en/resources/Bloch_Ch05.pdf)

我喜欢这个想法，我希望用 getObjectCopy（）

I like this idea, and I want to it with getObjectCopy()

@SuppressWarnings("unchecked")  //Copy of itself is the same type.
ValidateValue<L> vvo = (ValidateValue<O>)this_toCopy.getValidator().getObjectCopy();
vvBlkA = vvo;

我的评论似乎很蹩脚，但我想不出更好的事情。这就是我的问题：为什么这个未经检验的演员合理？什么是一个有意义的评论，实际上将帮助未来的开发人员，这意味着有效的，只相信我？

My comment seems lame, but I can't think of anything better. And that's my question: Why is this unchecked-cast justifiable? What's a meaningful comment that would actually help future developers, that means something more than, effectively, "just trust me"?

推荐答案

我们现在在Java 5+世界！用户泛型。

We are in Java 5+ world now! User Generics.

您可以将可复制的签名更改为：

You can change the signature of Copyable to something like:

interface Copyable<T extends Copyable> {
    T getObjectCopy();
}

现在你的 ValidateValue< L> value将类似于：

Now your ValidateValue<L> value would be something like:

puvlic class ValidateValue<L> implements Copyable<ValidateValue<L>> {
    ...
}

并且每个人（包括编译器）都会快乐！

and everyone (including the compiler) would be happy!

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