我可以使用包含A或List& lt; A& gt;的类型安全的映射吗? [英] Can I have a type safe map that either contains A or List<A>?

问题描述

我需要一个包含

项目对象

或

List< Item>对象

作为它的价值，我可以不用铸造就可以得到它，这有可能吗?

as its value, and I can get it out without casting, is it possible?

推荐答案

简短答案:否.

联盟类型在Java中不存在.为了进行编译时类型检查，您可以做的最接近的事情是创建一些自定义包装类的列表，这些包装类包含 ， A 或 List< A> ，如下所示:

Union types don't exist in Java. The closest thing you could do in order to get compile-time type checking would be to create list of some custom wrapper class which contained either an A or a List<A>, something like the following:

public class UnionListWrapper<A> {
    private final A item;
    private final List<A> list;

    public UnionListWrapper(A value) {
        item = value;
        list = null;
    }

    public UnionListWrapper(List<A> value) {
        item = null;
        list = value;
    }

    public Object getValue() {
        if (item != null) return item;
        else return list;
    }
}

至少您不能创建此类的实例，这些实例不是 A 或 List< A> ，但是取回值仍然必须是 Object ，并带有关联的强制转换.这可能很笨拙，并且总体上可能不值得.

At least you wouldn't be able to create instances of this class that weren't either an A or a List<A>, but getting the value back out would still have to just be Object, with associated casting. This could get quite clumsy, and on the whole it's probably not worth it.

在实践中，我可能只会有一个 List< Object> ，并带有一些注释，以非常谨慎地接受哪些数据类型.问题在于，甚至无法进行运行时检查(次优)，因为Java的泛型擦除意味着您无法在运行时执行 instanceof A 检查(双重如此) List 的通用参数.

In practice I'd probably just have a List<Object> with some comments around being very careful about what data types are accepted. The problem is that even run-time checks (sub-optimal) aren't going to be possible, since Java's generic erasure means that you can't do an instanceof A check at runtime (doubly so on the generic parameter of the List).

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