请老师帮我看一下这是什么原因？

这么做的话，出现了一个问题，倒数第三行报错了：

//测试播放器

public static void testPlayListCollection() {

Song s1 = new Song("a001","星晴","周杰伦");

Song s2 = new Song("a002","十年","陈奕迅");

Song s3 = new Song("a003","妹妹你大大滴往前走","不知道");

//音乐打包

List<Song> playList = new ArrayList<Song>();

playList.add(s1);

playList.add(s2);

playList.add(s3);

PlayList pl1 = new PlayList("我的收藏",playList);

//列表打包

Map<String,PlayList> playListMap = new HashMap<String,PlayList>();

playListMap.put(pl1.getPlayListName(), pl1);

PlayListCollection plc1 = new PlayListCollection(playListMap);

PlayList pl2 = new PlayList("我的收藏2",playList);

plc1.addPlayList(playList);//

System.out.println("显示所有播放列表名称：");

plc1.displayPlayListName();

}

这是PlayListCollection类的内容：

package zuoye;
 
import java.util.Iterator;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Set;
 
public class PlayListCollection {
 
    //为什么选Map类型
    private Map playListMap;
 
    public PlayListCollection(Map playListMap) {
        this.playListMap = playListMap;
        this.playListMap.put("主列表",null);
    }
 
    public Map getPlayListMap() {
        return playListMap;
    }
 
    public void setPlayListMap(Map playListMap) {
        this.playListMap = playListMap;
    }
     
    //添加播放列表
    public void addPlayList(PlayList playList) {
        playListMap.put(playList.getPlayListName(),playList);
    }
     
    //删除播放列表
    public void deletePlayList(PlayList playList) {
         
    }
     
    //通过名字查询
    public PlayList searchPlayListByName(String playListName) {
         
    }
     
    //显示所有播放列表名称
    public void displayPlayListName() {
         
        Set<Entry<String,PlayList>> entrySet = playListMap.entrySet();
        for(Entry<String,PlayList> entry:entrySet) {
            System.out.println(entry.getKey());
        }
    }
     
     
}