实例化类型的泛型类<？>

问题描述

示例代码实例化了两个通用集合：

p>

 列表<？> list = new ArrayList<？>（）; 
 List< ;? extends Object> list2 = new ArrayList< ;? extends Object>（）; 
 

问题的正确答案是这个代码会编译，但添加到任何一个集合将产生运行时错误。

当我尝试编译这样的代码，我只是得到错误。 Java教程甚至不显示这种类型的代码，它通常使用通配符作为上传的一部分。

  Collection< > c = new ArrayList< String>（）; 
  

上述两个通用集合是否甚至是合法代码？第二个由我的逻辑只会禁止接口。第一个看起来完全没用。为什么使用一个通用的，没有尝试在控制？

解决方案

查看优秀的。更具体地，关于通配符的部分包含对您的问题的回答，我引用

 集合<？& c = new ArrayList< String>（）; 
 c.add（new Object（））; 
  

I'm studying for the SCJP/OCPJP and I came across a sample question that seams strange to me.

The sample code instantiated two generic collections:

List<?> list = new ArrayList<?>();
List<? extends Object> list2 = new ArrayList<? extends Object>();

The "correct" answer to the question was that this code would compile but adding to either collection would produce a runtime error.

When I try to compile code like this I just get errors. The Java tutorial does not even show this type of code, it instead usually uses wildcards as a part of an upcast.

Collection<?> c = new ArrayList<String>();

Are the two generic collections above even legitimate code? The second by my logic would only disallow interfaces. The first one looks completely useless. Why use a generic that makes no attempt at control?

解决方案

Check out the excellent Java generics tutorial PDF. More specifically the section about wildcards contains the answer to your question, and I quote

Collection<?> c = new ArrayList<String>();
c.add( new Object() );

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