不兼容的通配符类型应该兼容

问题描述

继续从，它提供了一个解决方案，但没有解释它（不幸的是，答案中的链接现在已经失效）：

采取以下方法：

 超类型 Set< Map< String，Integer>> ，因为 Map< String，Integer> 是 Map的子类型 。因此，将 myMap.entrySet（）分配给 Set 类型的变量是可以的。扩展Map >< / code>。
 
 
 
请注意， String 和 Integer 在这里，但是 myMap 必须是的地图！ 
 
 
 
您可以写下：
 
 
 < K，V> void方法（Map< K，V> myMap）{
 Set< Map.Entry< K，V>> set = myMap.entrySet（）; 
 ... 
  
 
Following on from this question, which provides a solution but doesn't explain it (unfortunately, the links in the answers are now dead):
Take the following method:
void method(Map<?, ?> myMap) {
    Set<Map.Entry<?, ?>> set = myMap.entrySet();
    ...
}
Simple, no? However, this fails to compile on jdk1.7.0_25:
WTF? Map.entrySet() is specified as returning an object of type Set<Map.Entry<K, V>>, so in the example above, myMap.entrySet() returns a Set<Map.Entry<?, ?>>. But it doesn't compile!
Even weirder, from the linked question at the top, changing the method to this makes it compile:
void method(Map<?, ?> myMap) {
    Set<? extends Map.Entry<?, ?>> set = myMap.entrySet();
    ...
}
WTF??? Calling entrySet on a Map<?, ?> returns a Set<Map.Entry<K, V>>, which can't be assigned to a variable of type Set<Map.Entry<K, V>>, but it can to a variable of type Set<? extends Map.Entry<K, V>>?????
Can anyone shed light on what's going on here? And does this mean that, whenever I write a method using a wildcard type at least 2 levels deep, I have to remember to make it ? extends ... somewhere?
 解决方案 
Each of those ? can vary independently, so there's no guarantee that the <?,?> in the declaration of myMap matches the <?,?> in the declaration of set.
What this means is that once I have a Set<Map<?,?>>, I can put any type of Map into that set, because Map<?,?> is a supertype of all types of Map.  But this is not a property that Set<Map<String,Integer>> (for example) has - it's far more restrictive in terms of what types of map I can put into it.  So Set<Map<?,?>> is not a supertype of Set<Map<String,Integer>>.  But myMap.entrySet() could easily be a Set<Map<String,Integer>>, depending on what myMap is.  So the compiler has to forbid us from assigning it to a variable of type Set<Map<?,?>>, and that's what's happening.
On the other hand, Set<? extends Map<?,?>> is a supertype of Set<Map<String,Integer>>, because Map<String,Integer> is a subtype of Map<?,?>.  So it's OK to assign myMap.entrySet() to a variable of type Set<? extends Map<?,?>>.
Note that there's nothing special about String and Integer here, but myMap has to be a map of something!
You could write
<K, V> void method(Map<K, V> myMap) {
    Set<Map.Entry<K, V>> set = myMap.entrySet();
    ...
                        
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