小编Lac*_*hie的帖子

我在playlist1.firstSong = song;下面的行(第9行)中收到nullPointerException错误.有任何想法吗？

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 …

我已经尝试查看有关此问题的帖子,但我的代码中仍然遇到此错误的问题.所以在第四行中,我创建了一个名为SongDatabase访问SongDatabase该类的实例变量.但是当我到达线SongDatabase.addNewSong();下时case 1,我得到一个java.lang.NullPointerException:null错误.

接口类:

public class Interface 
{
    Scanner console = new Scanner(System.in);
    private SongDatabase SongDatabase;

    public static void main(String[] args) {
        Interface intFace = new Interface();
        intFace.run();
    }

    private void run() {
        switch (userInput) {
        case 1: 
            SongDatabase.addNewSong();
            break;
        case 2:
            SongDatabase.removeSong();
            break;
        case 3:
            SongDatabase.sortSongs();
            break;
        default:
            System.out.println("Please enter a valid number.");
            break;

    }
}

SongDatabase类:

public class SongDatabase {
    Scanner console = new Scanner(System.in);  
    private Song song1, song2, song3, song4; …