Java 在类NullPointerException内实例化类

我在playli1.firstSong=song行中得到一个nullPointerException错误在下面（第9行）。有什么想法吗

播放列表类别：

public class Playlist { 
Scanner console = new Scanner(System.in); 
private Playlist playlist1=null, playlist2=null; 

private Song firstSong;
private Song secondSong;
private Song thirdSong;

public void setSong(Song song) { 
    if (song != null) {
        if (playlist1.firstSong == null) {
            playlist1.firstSong = song;
            System.out.println("The song has been added to the playlist.");
        }

        else if (playlist1.secondSong == null) {
            playlist1.secondSong = song;
            System.out.println("The song has been added to the playlist.");
        }

        else if (playlist1.thirdSong == null) {
            playlist1.thirdSong = song;
            System.out.println("The song has been added to the playlist.");
        }
        else {
            System.out.println("This playlist is currently full with 3 songs. Please delete a song before attempting to add a new one.");
        }
    }
   }

addSongToPlaylist方法：

private void addSongToPlaylist() { 
  if (songCount <=3) { 

    System.out.println("Please enter the number of the song you'd like to be added to the playlist."); 
    System.out.println("");

    database.Display();

    int songNumber; 
    songNumber = console.nextInt(); 

    switch (songNumber) { 
        case 1:
            playlist.setSong(database.getSong(1)); 
            break;
        case 2:
            playlist.setSong(database.getSong(2)); 
            break;
        case 3:
            playlist.setSong(database.getSong(3)); 
            break;
        case 4:
            playlist.setSong(database.getSong(4)); 
            break;
        default:
            System.out.println("Please enter a valid song number."); 
            break;
    }
    songCount++; 
  }

---

非常感谢您的帮助，谢谢

因为您没有初始化播放列表对象。执行以下操作：

这样做

   private Playlist playlist1= new PlayList(), playlist2=new Playlist(); 

而不是

   private Playlist playlist1=null, playlist2=null; 

编辑：

class listMenu {
 // class that contains 3 playlists object. 

PlayList list1 = new Playlist(), list2 = new Playlist(), list3 = new PlayList(); 

addPlayList() {
  // whatever your logic for addition is.
}

}

你编写代码的方式是错误的，因为

class PlayList {
 PlayList list1 = new PlayList(), list2 = new Playlist();
 private Song song1 = null;
 private Song song2 = null;
 private Song song3 = null; 

void setSong(Song song){
   list1.song1 = song; // you are storing song in  of list1.song field.
}
}

当尝试访问歌曲时，您将再次收到NullPointerexception

class Menu{
 void main(// ) {
  PlayList list = new PlayList();
  list.setSong(new Song()); 
  list.getSong1.name(); // throw exception, because song is stored in the member object not in itselt. 



}
}

---

我希望你明白我的意思。。。你在播放列表课上被搞糊涂了。您的播放列表实际上存储了3首歌曲，仅此而已，您不需要使用playli1和playli2。 请尝试使用此选项：

public class Playlist { 
private Song firstSong;
private Song secondSong;
private Song thirdSong;

public void setSong(Song song) { 
    if (song != null) {
        if (this.firstSong == null) {
            this.firstSong = song;
            System.out.println("The song has been added to the playlist.");
        }

        else if (this.secondSong == null) {
            this.secondSong = song;
            System.out.println("The song has been added to the playlist.");
        }

        else if (this.thirdSong == null) {
            this.thirdSong = song;
            System.out.println("The song has been added to the playlist.");
        }
        else {
            System.out.println("This playlist is currently full with 3 songs. Please delete a song before attempting to add a new one.");
        }
    }
   }

设想一下，当您创建一个播放列表实例时，您可以在其中存储3首歌曲，仅此而已。如果你想要更多的播放列表，你可以创建更多

但是如果你想储存3首以上的歌曲呢？为此，您可以使用

数组来重构代码（或者任何类型的存储）。让我们试试向量：

public class Playlist { 

private Vector<Song> songList;

public void setSong(Song song) { 
    if (song != null) {
             songList.add(song);
        }
   }
public Song getSong(int nb) {
    if (nb > 0 && nb < songList.size()) //We don't want to check the song #-1 or a song that would be out of bonds
         return songList.elementAT(nb);
}
 } 

我找不到您在哪里初始化播放列表对象。您正在将其设置为null，但从未初始化它

无论如何，最好检查playList对象是否为null，然后设置字段（firstSong）的值。
@Mekap我不认为这是懒惰的，他们回答了OP提出的具体问题。也许还有其他的错误，但这个答案解决了这个问题。事实上，你们指出了正确的事情，+1来自我。他也不需要播放列表对象。我没有注意到班名：p@Lachie为什么在PlayList类中需要PlayList对象？弥补点是有效的。你不应该这样做，我建议你接受化妆answer@MuneebNasir我将其更改为您发布的内容，现在得到一个java.lan.stackOverflower错误。有什么想法吗？你的答案设计有缺陷，因为你想修正错误，而不是原来的问题。@StephenC我正在上传代码，请稍等。对于初学者，我想表明这比仍然使用playli1和playli2更合适。我需要有2个播放列表，用户可以将歌曲保存到其中。那么我不需要playli1和playli2对象吗？@Lachie你可以拥有同一类的2个实例！（这就是《公安条例》的权力）；我编辑了我的代码，所以你可以看到我在说什么。
public class Playlist { 

private Vector<Song> songList;

public void setSong(Song song) { 
    if (song != null) {
             songList.add(song);
        }
   }
public Song getSong(int nb) {
    if (nb > 0 && nb < songList.size()) //We don't want to check the song #-1 or a song that would be out of bonds
         return songList.elementAT(nb);
}
 } 

   Playlist firstPlaylist;
   Playlist secondPlaylist;