求救大佬，为什么roomMap前面是HashMap<String,String>，而不是String？

/**
 * 针对多个映射关系时：
 * yureban 01 zhangsan
 * yureban 02 lisi
 * jiuyeban 01 wangwu
 * jiuyeban 02 zhaoliu
 * */
import java.util.*; 

public class MapDemo {

    public static void main(String[] args) {
        HashMap<String,String> yure=new HashMap<String,String>();
        yure.put("01", "zhangsan");
        yure.put("02", "lisi");
        HashMap<String,String> jiuye=new HashMap<String,String>();
        jiuye.put("01", "wangwu");
        jiuye.put("02", "zhaoliu");
        HashMap<String,HashMap<String,String>> czbk=new HashMap<String,HashMap<String,String>>();
        czbk.put("yureban", yure);
        czbk.put("jiuyeban", jiuye);
        //getStudentInfo(yure);
        Iterator<String> it=czbk.keySet().iterator();
        while(it.hasNext())
        {
            String roomName=it.next();
            HashMap<String,String> room=czbk.get(roomName);
            System.out.println(roomName);
            getStudentInfo(room);
        }
    }
    public static void getStudentInfo(HashMap<String,String> roomMap)
    {
        /*
        Set<String> me=roomMap.keySet();
        Iterator<String> it=me.iterator();
        */
        Iterator<String> it=roomMap.keySet().iterator();
        while(it.hasNext())
        {
            String id=it.next();
            String name=roomMap.get(id);
            System.out.println(id+":"+name);
        }
    }
}

roomMap是Map类型，Map类型的两个泛型参数一个是key的类型，一个是value的类型
所以前面必须是HashMap<A,B>
这种形式的

而roomMap.get(id);，返回的是根据key找到的那个value条目，所以是HashMap<A,B>中的那个B，具体到你这里就是String