Question

我在下面的行playlist1.firstSong = song;（第9行）中收到nullPointerException错误。有什么想法吗？

播放列表类：

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++; 
  }

getSong方法：

public Song getSong(int songNumber) { 
    if (songNumber == 1){ 
        return song1; 
    }
    else if (songNumber == 2){ 
        return song2; 
    }
    else if (songNumber == 3){ 
        return song3; 
    }
    else if (songNumber == 4){ /
        return song4; 
    }

    else {
        return song1; 
}
}

非常感谢任何帮助，谢谢！

Answer 1

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;

修改

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. } }

你做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. } }

何时尝试访问该歌曲，您将再次获得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. } }

我希望你明白我的观点

Answer 2

嗯......你在播放列表课上感到困惑。您的播放列表实际上存储了3首歌曲，您不需要使用播放列表1＆amp; playlist2。 请尝试使用此代码：

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.");
        }
    }
   }

认为在创建播放列表实例时，您可以在那里存储3首歌曲，就是这样。如果您想要更多播放列表，可以创建更多播放列表。

但如果您想存储超过3首歌曲怎么办？为此，您可以使用array重构代码（或者任何类型的存储，实际上）。让我们尝试使用Vector：

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);
}
 }

你有更清洁的东西。（上面的代码肯定有一些拼写错误，我不能在这里检查它们，但它就是一个例子。）

作为评论的答案：如果你想使用2个播放列表，那很好，只需在你的主页中使用：

   Playlist firstPlaylist;
   Playlist secondPlaylist;

Answer 3

我无法找到初始化播放列表对象的位置。您将其设置为null，但从不初始化它。

无论如何，最好检查playList对象是否为null，然后设置字段的值（firstSong）。

在类NullPointerException中实现一个类

3 个答案: