Java通用问题(Java Generic Question)

 下面的代码编译，但如果我取消注释注释行，它不会，我很困惑为什么。 HashMap确实扩展了AbstractMap，并且声明map的第一行编译正常。 
 
import java.util.AbstractMap;
import java.util.HashMap;
import java.util.Map;

public class Test {

    public static void main(String args[]) {
        Map<String, ? extends AbstractMap<String, String>> map = new HashMap<String, HashMap<String, String>>();
        //map.put("one", new HashMap<String, String>());
    }
}
 
 而且，我知道“正确的方式”是这样的： 
 
import java.util.HashMap;
import java.util.Map;

public class Test {

    public static void main(String args[]) {
        Map<String, Map<String, String>> map = new HashMap<String, Map<String, String>>();
        map.put("one", new HashMap<String, String>());
    }
}

The following code compiles but if I uncomment the commented line, it does not and I am confused why. HashMap does extend AbstractMap and the first line where map is declared compiles fine.
 
import java.util.AbstractMap;
import java.util.HashMap;
import java.util.Map;

public class Test {

    public static void main(String args[]) {
        Map<String, ? extends AbstractMap<String, String>> map = new HashMap<String, HashMap<String, String>>();
        //map.put("one", new HashMap<String, String>());
    }
}
 
And, I know the "right way" is this:
 
import java.util.HashMap;
import java.util.Map;

public class Test {

    public static void main(String args[]) {
        Map<String, Map<String, String>> map = new HashMap<String, Map<String, String>>();
        map.put("one", new HashMap<String, String>());
    }
}

原文：https://stackoverflow.com/questions/5974065