为什么总是提示错误

麻烦老师帮我看一下为什么p1.addToPlayList(song1);这句没办法正常执行，调试提示说：musicList error，见最下面图

Song.java

package imooc.com.player;

//Song类

public class Song{

private String id, name, singer;//属性：歌曲id、歌曲名、演唱者

public Song(String id, String name, String singer) {

this.setId(id);

this.setName(name);

this.setSinger(singer);

}

public String getId() {

return id;

}

public void setId(String id) {

this.id = id;

}

public String getName() {

return name;

}

public void setName(String name) {

this.name = name;

}

public String getSinger() {

return singer;

}

public void setSinger(String singer) {

this.singer = singer;

}

@Override

public int hashCode() {

final int prime = 31;

int result = 1;

result = prime * result + ((id == null) ? 0 : id.hashCode());

result = prime * result + ((name == null) ? 0 : name.hashCode());

result = prime * result + ((singer == null) ? 0 : singer.hashCode());

return result;

}

@Override

public boolean equals(Object obj) {

if(obj==this) {

return true;

}

if(obj.getClass()==Song.class) {

Song song=(Song)obj;

return song.getId().equals(id)&&song.getName().equals(name)&&song.getSinger().equals(singer);

}

return false;

}

@Override

public String toString() {

// TODO Auto-generated method stub

return "歌曲ID为："+this.getId()+"，歌曲名字为："+this.getName()+"，歌唱者为："+this.getSinger()+" ";

}

}

PlayList.java

package imooc.com.player;

import java.util.ArrayList;

import java.util.Collections;

import java.util.List;

//播放列表

public class PlayList {

private String playListName;// 播放列表名称

private List<Song> musicList;// 播放列表中的歌曲集合

// 构造方法

public PlayList(String playListName) {

this.setPlayListName(playListName);

this.musicList = new ArrayList<Song>();

}

// getter、setter

public String getPlayListName() {

return playListName;

}

public void setPlayListName(String playListName) {

this.playListName = playListName;

}

// 将歌曲添加到播放列表

public void addToPlayList(Song song) {

boolean flag = false;

for (Song song1:musicList) {

if (song1.equals(song)) {

flag = true;

System.out.println("啦啦啦啦啦");

break;

}

if(flag) {

System.out.println("该歌曲已存在");

}else {

musicList.add(song);

System.out.println("该歌曲添加成功！");

}

}

}

// 显示播放类别中所有歌曲

public void displayAllSong() {

System.out.println("列表中所有歌曲如下：");

for(Song song:this.musicList) {

System.out.println("123"+song);

}

}

// 通过ID查询歌曲

public Song searchSongById(String id) {

Song song=null;

for(Song song1:musicList) {

if(song1.getId().equals(id))

{

song=song1;

break;

}

}

return song;

}

//通过名称查询歌曲

public Song searchSongByName(String name) {

Song song=null;

for(Song song1:musicList) {

if(song1.getName().equals(name)) {

song=song1;

break;

}

}

return song;

}

//修改歌曲：将歌曲ID为id的歌修改为song

public void updateSong(String id, Song song) {

boolean flag=false;

for(Song song1:musicList) {

if(song1.getId().equals(id)) {

song1=song;

flag=true;

break;

}

if(flag){

System.out.println("修改成功！");

}else 

System.out.println("要修改的歌曲id不存在！");

}

}

//从播放列表删除歌曲

public void deleteSong(String id) {

boolean flag=false;

for(Song song1:musicList) {

if(song1.getId().equals(id)) {

musicList.remove(song1);

flag=true;

break;

}

if(flag){

System.out.println("删除成功！");

}else 

System.out.println("要删除的歌曲id不存在！");

}

}

//按歌曲名进行排序

public void sortBySongName() {

System.out.println("排序前：");

displayAllSong();

Collections.sort(musicList, new SongComparator());

System.out.println("排序后：");

displayAllSong();

}

}

TestClass.java

package imooc.com.player;

public class TestClass {

public void testSong() {

Song song1=new Song("s001","小太阳","小金人");

Song song2=new Song("s002","小月亮","小引人");

Song song3=new Song("s003","小地球","小铜人");

// Song song4=new Song("s003","小地球","小铜人");

// System.out.println(song4);

}

public void testPlayList() {

Song song1=new Song("s001","小太阳","小金人");

Song song2=new Song("s002","小月亮","小引人");

Song song3=new Song("s003","小地球","小铜人");

PlayList p1=new PlayList("播放列表1");

p1.addToPlayList(song1);

}

public static void main(String[] args) {

TestClass t=new TestClass();

t.testPlayList();

}

}

提示错误：