Map< String，Map< String，Boolean>> myMap = new HashMap< String，HashMap< String，Boolean>>（）; [英] Map<String, Map<String, Boolean>> myMap = new HashMap<String,HashMap<String,Boolean>>();

问题描述

为什么不在java中工作，但是这样做

Why doesn't that work in java, but this does

Map<String, Map<String, Boolean>> myMap = new HashMap<String,Map<String,Boolean>>();

为了澄清嵌套HashMap的下面的变化，显示了一个编译器错误，而上面没有; （不是hashmap）

Just to clarify the below alteration of the nested HashMap shows a compiler error, whereas the above does not not; with a Map (not hashmap)

Map<String, Map<String, Boolean>> myMap = new HashMap<String,HashMap<String,Boolean>>();

推荐答案

这是因为Java中的泛型是不变的即使B类是A， Collection 不是 a Collection A> 。

This is because generics in Java are invariant, i.e. even if class B is an A, a Collection<B> is not a Collection<A>.

这是一个很好的理由。如果您的示例合法，则可以这样做：

And this is for a good reason. If your example were legal, this would be possible:

Map<String, HashMap<String, Boolean>> myHashMap = new HashMap<String,HashMap<String,Boolean>>();
Map<String, Map<String, Boolean>> myMap = myHashMap;
myMap.put("oops", new TreeMap<String, Boolean>());
HashMap<String, Boolean> aHashMap = myMap.get("oops"); // oops - ClassCastException!

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