我在下面的 playlist1.firstSong = song;
行(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++;
}
获取歌曲方法:
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!
最佳答案
因为你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();
代替
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.
}
}
您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,会再次出现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.
}
}
希望你明白我的意思
关于java - 在类 NullPointerException 中实例化一个类,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/30030371/