ListMap泛型使用问号编译无法通过

函数定义泛型List<Map<String, ?>> map，编译无法通过，写成List<? extends Map<String, ?>> map即可

package seaboot.test;

import java.io.IOException;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;

public class Test2 {
    public static void test(List<Map<String, ?>> map) {

    }

    public static void main(String[] args) throws IOException {
        List<Map<String, Object>> map = new ArrayList<>();
        test(map);
    }
}


package seaboot.test;

import java.io.IOException;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;

public class Test2 {
    public static void test(List<? extends Map<String, ?>> map) {

    }

    public static void main(String[] args) throws IOException {
        List<Map<String, Object>> map = new ArrayList<>();
        test(map);
    }
}