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区别在于，例如

List<HashMap<String,String>>

是一个

List<? extends Map<String,String>>

但不是

List<Map<String,String>>

所以：

void withWilds( List<? extends Map<String,String>> foo ){}void noWilds( List<Map<String,String>> foo ){}void main( String[] args ){    List<HashMap<String,String>> myMap;    withWilds( myMap ); // Works    noWilds( myMap ); // Compiler error}

你可能会认为一个

List

HashMap

List

Map

假设您可以执行以下操作：

List<HashMap<String,String>> hashMaps = new ArrayList<HashMap<String,String>>();List<Map<String,String>> maps = hashMaps; // Won't compile,         // but imagine that it couldMap<String,String> aMap = Collections.singletonMap("foo","bar"); // Not a HashMapmaps.add( aMap ); // Perfectly legal (adding a Map to a List of Maps)// But maps and hashMaps are the same object, so this should be the same ashashMaps.add( aMap ); // Should be illegal (aMap is not a HashMap)

所以这就是为什么

List

HashMap

List

Map