我这里判断ID不能相等的语句，怎么循环执行

package com.imooc.playerDemo;

import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

public class PlayList {
  private String playListName;
  private List musicList;
  public PlayList(){
  
  }
public PlayList(String playListName, List musicList) {
 super();
 this.playListName = playListName;
 this.musicList = musicList;
}
Scanner in=new Scanner(System.in);
public String getPlayListName() {
 return playListName;
}
public void setPlayListName(String playListName) {
 this.playListName = playListName;
}
public List getMusicList() {
 return musicList;
}
public void setMusicList(List musicList) {
 this.musicList = musicList;
}
  public void addToPlayList(){
   System.out.println("请输入要添加的歌曲的数量");
  
   int n=in.nextInt();
   for(int i=0;i<n;i++)
   { 
  
    System.out.println("请输入第"+(i+1)+"首歌曲：");
    System.out.println("请输入歌曲id：");
    String id=in.next();
    System.out.println("请输入歌曲名称：");
    String name=in.next();
    System.out.println("请输入演唱者");
    String singer=in.next();
    Song song=new Song(id,name,singer);
    this.musicList.add(song);
    System.out.println("歌曲添加成功");
   }
  }
  public void addToPlayGeneralList(){//给普通播放列表添加歌曲
   System.out.println("请输入要添加的歌曲的数量");
   Scanner in=new Scanner(System.in);
   int n=in.nextInt();
   for(int i=0;i<n;i++)
   { 
    System.out.println("请输入第"+(i+1)+"首歌曲：");
    System.out.println("请输入歌曲id：");
    String id=in.next();
    System.out.println("请输入歌曲名称：");
    String name=in.next();
    System.out.println("请输入演唱者");
    String singer=in.next();
    Song song=new Song(id,name,singer);
    this.musicList.add(song);
    System.out.println("添加歌曲成功！");
   }
  }
  public void displayAllSong(){
   for(Song song:this.getMusicList())
    System.out.println(song);
//   for(int i=0;i<this.getMusicList().size();i++)
//    System.out.println(this.getMusicList().get(i));
  }
  public Song searchSongById(String id){
   for(int i=0;i<this.getMusicList().size();i++)
   {
    if((this.getMusicList().get(i))
     System.out.println(this.getMusicList().get(i));
   }
  }
  public Song searchSongByName(String name){
   for(Song song:musicList)
   {
    if(name.equals(song.getName()))
     System.out.println(song);
   }
  }
  public void updateSong(Song song){//修改信息，歌曲ID不变，只改变歌曲名称和演唱者
  
  }
  public void deleteSong(String id){
  
  }
  public void exportPlayList(){//导出歌单
  
  }
}

显示所有歌曲的时候我想用for-each循环，结果报错