java - java中ListIterator中next()和previous()方法的问题

问题描述

import java.util.Scanner;
public class Main_songs {
public static Scanner scanner=new Scanner(System.in);
public static PlayList play=new PlayList();
    public static void main(String[] args) {
     boolean quit=false;
     while(!quit) {
     System.out.println("Enter your choice");
     System.out.println("0.quit\n"+
                        "1.Add song in playlist\n"+
                        "2.remove song from playlist\n"+
                        "3.forward song\n"+
                        "4.backward song\n"+
                        "5.replay current song\n"+
                        "6.list all songs\n"+
                        "7.create new album\n"+
                        "8.Add songs to album");
     int choice=scanner.nextInt();
     scanner.nextLine();
     switch(choice) {
     case 0:
         quit=true;
         break;
     case 1:
         System.out.println("Enter the album name");
         String album_name=scanner.nextLine();
         Album album=play.findAlbum(album_name);
        if(album!=null) {
         System.out.println("Enter the song tilte");
         String title=scanner.nextLine();
         Song song=album.findSong(title);
         if(song!=null) {
         play.addSongs(album_name, song);
         System.out.println("Song added in the playlist");}
         else {
             System.out.println("Song not found in the album");}}
         else
             System.out.println("album not found");
         break;
     case 2:
         System.out.println("Enter the song title");
         String title=scanner.nextLine();
         play.removeSong(title);
         break;
     case 3:
        play.skipForward();
         break;
     case 4:
         play.skipBackward();
         break;
     case 5:
         play.Replay();
         break;
     case 6:
         play.listSongs();
         break;
     case 7:
         System.out.println("Enter the album name");
         String album_title=scanner.nextLine();
         play.createNewAlbum(album_title);
         break;
     case 8:
         System.out.println("Enter the album name");
         String albumName=scanner.nextLine();
         System.out.println("Enter the song name");
         String song_name=scanner.nextLine();
         System.out.println("Enter the song duration");
         double duration=scanner.nextDouble();
         play.addSongtoAlbum(albumName, song_name, duration);
         break;

     }
     }

}

package linkedList;
import java.util.ArrayList;
import java.util.LinkedList;
import java.util.ListIterator;
import java.util.Iterator;
public class PlayList {

private LinkedList<Song> playlist=new LinkedList<Song>();
private ArrayList<Album> album_list;
private ListIterator<Song> list=playlist.listIterator();

public PlayList() {
    this.album_list=new ArrayList<Album>();

}
public LinkedList<Song> getPlaylist(){
    return this.playlist;
}
public ArrayList<Album> getAlbum_list(){
    return this.album_list;
}

public void addSongs(String album_name,Song song) {

        playlist.add(song);

}
public void listSongs() {
    for(int i=0;i<playlist.size();i++) {
        Song song=playlist.get(i);
        System.out.println("Songs are "+song.getTitle() +" and duration is "+song.getDuration());}
}

public void createNewAlbum(String album_name) {
    Album album=findAlbum(album_name);
    if(album==null) {
        album_list.add(new Album(album_name));
        System.out.println("New album created succesfully");
    }else
        System.out.println("This album is already exist");
}
public Album findAlbum(String title) {
    for(int i=0;i<album_list.size();i++) {
        Album album=album_list.get(i);
        if(album.getAlbum_name().equals(title)) {
            return album;
        }
    }return null;}
public void skipForward() {
    list = playlist.listIterator();
    if (list.hasNext()) {
        list.next();
        if (list.hasNext()) {
            System.out.println("Playing next song " + list.next().getTitle());
        } else {
            System.out.println("Currently playing last song");
        }
    }
}

public void skipBackward() {
    list = playlist.listIterator();
    if (list.hasPrevious()) {
        list.previous();
        if (list.hasPrevious()) {
            System.out.println("Playing previous song " + list.previous().getTitle());
        } else {
            System.out.println("Currently playing last song");
        }
    }
}

public void Replay() {

    boolean goingForward=true;
    if(goingForward) {
        if(list.hasPrevious()) {
            System.out.println("Now replaying "+list.previous().getTitle());

        goingForward=false;
       }else
           System.out.println("Now playlist at the start");}
    else
     if(list.hasNext()) {
        System.out.println("Now replaying "+list.next().getTitle());
        goingForward=true;
    }else
        System.out.println("Playlist come to end");

}
public void removeSong(String title) {

    if(playlist.size()>0) {
        list.remove();
        System.out.println("Deleted current song");
    if(list.hasNext()) {
        System.out.println("Now you are playing "+list.next().getTitle());
    }
    if(list.hasPrevious()) {
        System.out.println("Now you are playing "+list.previous().getTitle());
    }
}
}
public void addSongtoAlbum(String album_name,String title,double duration) {
    Album album=findAlbum(album_name);
    if(album!=null) {
        if(album.getAlbum_name().equals(album_name)) {
            album.addSongs(title, duration);
        }else
            System.out.println("Album not exist");
    }else
        System.out.println("This album not exist");
}

}


package linkedList;
import java.util.ArrayList;
public class Album {
private String album_name;
private ArrayList<Song> song_list=new ArrayList<Song>();
public Album(String album_name) {
    this.album_name=album_name;

}
public String getAlbum_name() {
    return album_name;
}
public ArrayList<Song> getSongs(){
    return this.song_list;
}
public boolean addSongs(String title,double duration) {
    Song song=findSong(title);
    if(song==null) {
        song_list.add(new Song(title,duration));
        System.out.println("song added successfully");
        return true;
    }else {
        System.out.println("This song already exist in the list");
        return false;
    }

}
public Song findSong(String title) {
    for(int i=0;i<song_list.size();i++) {
        Song song=song_list.get(i);
        if(song.getTitle().equals(title)) {
            return song;
        }
    }return null;
}

}

package linkedList;

public class Song {
private String title;
private double duration;
public Song(String title,double duration) {
    this.title=title;
    this.duration=duration;
}
public String getTitle() {
    return title;
}
public double getDuration() {
    return duration;
}
}

嗨，我发布了部分代码。该程序基于带有列表迭代器概念的链表。创建一张专辑。需要从该专辑中选择歌曲并放入播放列表。在这里，我遇到了前进和后退功能的问题。转发功能只转发一次。然后它重复着同一首歌。后退功能不起作用。它只是打印 No song in previous。任何人都可以帮助我并建议我一些想法。谢谢你。

在我的代码中，它创建专辑并添加歌曲，然后将专辑中的歌曲添加到播放列表，最后打印播放列表中的所有歌曲。之后在 forward() 方法中，它只转发一首歌曲。然后它重复同一首歌。在backward（）方法中，它只是检查hasPrevious（）。在它无法继续进行之后。Replay方法也一样，就是说list的开始。Remove()方法就是说Exception非法状态异常。

任何人都可以检查我的代码并给我反馈。

标签： javalinked-listiterator

每次next通话之前hasNext。对于previous.

        Song sing = list.next();
        System.out.println("Playing next song "+sing.getTitle());

应该

        Song sing = list.next();
        System.out.println("Playing next song "+sing.getTitle());

或（因为那个“跳过”）

        list.next();
        if (list.hasNext()) {
            System.out.println("Playing next song "+list.next().getTitle());
        }

java - java中ListIterator中next()和previous()方法的问题

问题描述

解决方案

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