从Java中的Object进行转换，而不会获取未选中的警告 [英] Casting from Object in Java without getting an unchecked warning

问题描述

我写了一个类，它有一个< String，Object> 的映射。我需要它来持有任意对象，但在同一时间有时我需要投射一些对象，所以我会做一些像

I wrote a class that has a map of <String, Object>. I need it to hold arbitrary objects, but at the same time sometimes I need to cast some of those objects, so I'll do something like

HashMap<String, Object> map = new HashMap<String, Object>();                                                                                 
Object foo = map.get("bar");                                                                                                                                                                                                         
if (foo instanceof HashMap) {                                                                                                                                                                                                        
    ((HashMap<String, Integer>) foo).put("a", 5);                                                                                                                                                                                    
}            

这会产生警告

Stuff.java:10: warning: [unchecked] unchecked cast
found   : java.lang.Object
required: java.util.HashMap<java.lang.String,java.lang.Integer>
        ((HashMap<String, Integer>) foo).put("a", 5);

我怀疑它与泛型的使用有关。我可以摆脱的错误使用@SupressWarnings（未选中），但我想知道是否有一个更好的方法来做到这一点。或者也许事实，我得到警告意味着我应该重新考虑我在做什么。

I suspect it has to do with the use of generics. I can get rid of the error using @SupressWarnings("unchecked"), but I was wondering if there was a better way to do it. Or maybe the fact that I'm getting the warning means I should reconsider what I'm doing. Is there anything I could do, or should I just use @SupressWarnings?

推荐答案

已编辑（基于

投射到 HashMap< String，Integer> （btw，using 映射而不是 HashMap 可以说是一个更好的选择）是一个不同的故事。很遗憾，没有办法避免在这种情况下由于类型擦除的未经检查的警告。你可以使用它作为非通用地图：

Casting to HashMap<String, Integer> (btw, using Map instead of HashMap is arguably a better choice) is a different story. There's sadly no way to avoid an unchecked warning in that case due to type erasure. You can, however, use it as non-generic map:

if (foo instanceof Map) {                                                                                                                                                                                                        
  ((Map) foo).put("a", 5);                                                                                                                                                                                    
}

你显然必须投射在gets ）类型安全，但不会有未经检查的警告。

You'll obviously have to cast on "gets" and you lose (perceived) type safety but there'll be no unchecked warning.

必须有更多的故事。以下代码：

There must be more to this story. The following code:

Map<String, Object> map = Maps.newHashMap(); // or new HashMap<String, Object>();
Object foo = map.get("bar");
if (foo instanceof Widget) {
  ((Widget) foo).spin();
}

会不为我生成未选中的警告。我也不能想象为什么会。如果你事先知道bar总是返回一个窗口小部件，这样做：

does NOT generate an unchecked warning for me. Nor can I imagine why would it. If you know beforehand that "bar" would always return a widget, doing this:

Widget widget = (Widget) map.get("bar");
widget.spin();

我在这里缺少什么吗？

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