我们是否可以在实例化期间使用通配符 [英] Are we allowed to use wildcard during instantiation

问题描述

为什么这些声明在Java中无效?

why are these declaration invalid in Java?

1. List< Number>测试=新的ArrayList< ;?扩展Number>();

列表测试=新的ArrayList< ;?扩展Number>();

在实例化期间我们是否不允许使用通配符.并且通配符仅对将它们传递给方法有用吗?

are we not allowed to use wildcard during instantiation. and if the wildcards are only useful for passing them to methods?

和 List< Object>test = new ArrayList< Integer>(); 是非法的，因为泛型不是协变正确的?

and List<Object> test = new ArrayList<Integer>(); is illegal because generics are not covariant correct?

推荐答案

?通配符表示未知"而不是任何".实例化一个未知内容的新容器没有任何意义，您将在其中放置什么?它真的不能用于任何东西！

The ? wildcard character means "unknown" not "any". It doesn't make any sense to instantiate a new container of unknown contents, what would you put in there? It can't really be used for anything!

那么声明 new ArrayList< ;?扩展Number>()的意思是某些扩展数字的特定事物，但我不知道是什么."不是是指任何扩展数字的内容."
您分配给它的 List< Number> 将允许同时添加Double和Integer，但是 List< ;?的实际内容是?扩展Number> 可能是Float！(或其他任何内容).请考虑一下，如果通配符充当"Any"，那么在代码中会发生什么:

So the declaraion new ArrayList<? extends Number>() Means "some specific thing that extends number, but I don't know what." It does not mean "anything that extends number."
The List<Number> you assigned it to would allow both Double and Integer to be added to it, but the actual contents of a List<? extends Number> might be Float! (or whatever else.) Consider what would happen in this code if the wildcard worked as an "Any":

    List<Integer> listInteger = new ArrayList<Integer>();
    listInteger.add(Integer.valueOf(1));
    List<? extends Number> listWildCard = listInteger;
    listWildCard.add(Double.valueOf(1.0)); //This does not compile
    Integer integer = listInteger.get(1);//because this would throw a ClassCastException

关于第二个示例的脚注:使用原始类型调用不带类型参数的参数化类型.这被认为是编程错误.该语法仅是合法的，因此仍可以编译在Java 5之前编写的代码.如果您的方案与Java 5之前的版本不具有向后兼容性，请不要这样做.

Footnote regarding your second example: Declaring a paramaterized type with no type parameter is called using the Raw Type. This is considered a programming error. The syntax is only legal so that code written before java 5 still compiles. Just don't do it if your scenario isn't backward compatability with pre-java 5.

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