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());
		}
	}
	
	
}

专业躺鸡 2017-08-25

源自：Java常用工具练习 1-3 详细设计