java 如何从 List<Object> 转换为 List<A> 或 List<？扩展 A>?
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How to cast from List<Object> to List<A> or List<? extends A>?
提问by reverse_engineer
Possible Duplicate:
How do you cast a List of objects from one type to another in Java?
Searched the internet a little, and found no nice way of doing it... My solution now is:
在互联网上搜索了一下，并没有找到好的方法......我现在的解决方案是：
public class A {}
List<Object> obj = new ArrayList<Object>();
obj.add(new A());
// Ugly solution here:
List<A> a = (List<A>) (List) obj;
But this is quite ugly and gets a warning. No "official" way of doing this?
但这很丑陋，并会收到警告。没有这样做的“官方”方式？
EDIT: To the guys who closed this: I was aware of the solution posted in How do you cast a List of objects from one type to another in Java?It is the same as the one I posted in my question (just adding the <?>after the first cast does exactly the same) I Was looking for something more "clean". In the direction of using the Class<?> clazz = listobj.get(0).getClassway of getting the class and casting to the correct class at runtime (but no idea if something like that works... Eclipse doesn't seem to like it anyway...)
编辑：致关闭此问题的人：我知道如何在 Java 中将对象列表从一种类型转换为另一种类型中发布的解决方案？它与我在我的问题中发布的相同（只是<?>在第一个演员之后添加完全相同）我正在寻找更“干净”的东西。在使用Class<?> clazz = listobj.get(0).getClass获取类的方式并在运行时转换为正确的类的方向上（但不知道这样的东西是否有效......反正 Eclipse 似乎并不喜欢它......）
回答by Subhrajyoti Majumder
List<Object> obj = new ArrayList<Object>();
obj.add(new A());
It is not the right way to write code. Basically you are creating a generic List and adding Object to it and it type unsafe and keep any Object type.
这不是编写代码的正确方法。基本上，您正在创建一个通用列表并向其中添加 Object，它的类型不安全并保留任何 Object 类型。
List<Object> obj = new ArrayList<Object>();
obj.add(new A());
obj.add(new String("str"));
obj.add(1);
It is recommended to create type-safe Listlike List<A> obj = new ArrayList<A>();
建议创建类型安全的List样List<A> obj = new ArrayList<A>();
you can do this in such a way -
你可以这样做 -
public <T>List<T> castCollection(List srcList, Class<T> clas){
List<T> list =new ArrayList<T>();
for (Object obj : srcList) {
if(obj!=null && clas.isAssignableFrom(obj.getClass()))
list.add(clas.cast(obj));
}
return list;
}
回答by Brian Agnew
So why isn't your list declared as
那么为什么你的列表没有被声明为
List<A> obj = new ArrayList<A>();
to begin with ?
开始？
It's really dangerous to do what you're trying to do. What you're saying is that your list contains As and subclasses thereof. But since it's originally a list of Objects (i.e. anything). You're likely to get a nasty surprise later on.
做你想做的事真的很危险。您的意思是您的列表包含As 及其子类。但是因为它最初是一个Objects列表（即任何东西）。稍后你可能会得到一个令人讨厌的惊喜。
What that compiler error really means is that somewhere you have a design issue.
编译器错误的真正含义是您在某个地方遇到了设计问题。
回答by Anders R. Bystrup
Perhaps the issue is that you want to put objects of different (but inheritance-related) types into the list? In that case, you read the Generics Tutorial, especially the parts about <? extends Something>and <? super Something>
也许问题是您想将不同（但与继承相关）类型的对象放入列表中？在那种情况下，你阅读泛型教程，特别是关于<? extends Something>和的部分<? super Something>
Cheers,
干杯，
