Bluej Java - 初学者 - 一次播放两首曲目问题

Question

我对 Java 比较陌生，只是在阅读朋友借给我的一本书。

问题说

如果您播放两首曲目而不停止第一首，则两者将同时播放。 这不是很有用。更改您的程序，以便自动播放曲目 当另一个轨道开始时停止

这是目前的样子：

public void playTrack(int index)
{
    if(indexValid(index)) {
        Track track = tracks.get(index);
        player.startPlaying(track.getFilename());
        track.incrementCount();
        System.out.println("Now playing: " + track.getArtist() + " - " + 
track.getTitle());
       }

我的计划是添加另一个 else if 语句，然后以某种方式判断一首歌曲是否正在播放，然后停止当前并开始下一首，但我不确定如何执行此操作。

抱歉，这是我第一次上这个论坛，我对 java 很陌生。

这里是这个类的完整代码

import java.util.ArrayList;

/**

 */
            public class MusicOrganizer
            {
                // An ArrayList for storing music tracks.
                private ArrayList<Track> tracks;
                // A player for the music tracks.
                private MusicPlayer player;
                // A reader that can read music files and load them as tracks.
                private TrackReader reader;

                /**
                 * Create a MusicOrganizer
                 */
                public MusicOrganizer()
                {
                    tracks = new ArrayList<>();
                    player = new MusicPlayer();
                    reader = new TrackReader();
                    readLibrary("../audio");
                    System.out.println("Music library loaded. " + getNumberOfTracks() + " tracks.");
                    System.out.println();
                }

                /**
                 * Add a track file to the collection.
                 * @param filename The file name of the track to be added.
                 */
                public void addFile(String filename)
                {
                    tracks.add(new Track(filename));
                }

                /**
                 * Add a track to the collection.
                 * @param track The track to be added.
                 */
                public void addTrack(Track track)
                {
                    tracks.add(track);
                }

                /**
                 * Play a track in the collection.
                 * @param index The index of the track to be played.
                 */
                public void playTrack(int index)
                {
                    if(indexValid(index)) {
                        Track track = tracks.get(index);
                        player.startPlaying(track.getFilename());
                        System.out.println("Now playing: " + track.getArtist() + " - " + track.getTitle());
                    }
                }

                /**
                 * Return the number of tracks in the collection.
                 * @return The number of tracks in the collection.
                 */
                public int getNumberOfTracks()
                {
                    return tracks.size();
                }

                /**
                 * List a track from the collection.
                 * @param index The index of the track to be listed.
                 */
                public void listTrack(int index)
                {
                    System.out.print("Track " + index + ": ");
                    Track track = tracks.get(index);
                    System.out.println(track.getDetails());
                }

                /**
                 * Show a list of all the tracks in the collection.
                 */
                public void listAllTracks()
                {
                    System.out.println("Track listing: ");

                    for(Track track : tracks) {
                        System.out.println(track.getDetails());
                    }
                    System.out.println();
                }

                /**
                 * List all tracks by the given artist.
                 * @param artist The artist's name.
                 */
                public void listByArtist(String artist)
                {
                    for(Track track : tracks) {
                        if(track.getArtist().contains(artist)) {
                            System.out.println(track.getDetails());
                        }
                    }
                }

                /**
                 * Remove a track from the collection.
                 * @param index The index of the track to be removed.
                 */
                public void removeTrack(int index)
                {
                    if(indexValid(index)) {
                        tracks.remove(index);
                    }
                }

                /**
                 * Play the first track in the collection, if there is one.
                 */
                public void playFirst()
                {
                    if(tracks.size() > 0) {
                        player.startPlaying(tracks.get(0).getFilename());
                    }
                }

                /**
                 * Stop the player.
                 */
                public void stopPlaying()
                {
                    player.stop();
                }

                /**
                 * Determine whether the given index is valid for the collection.
                 * Print an error message if it is not.
                 * @param index The index to be checked.
                 * @return true if the index is valid, false otherwise.
                 */
                private boolean indexValid(int index)
                {
                    // The return value.
                    // Set according to whether the index is valid or not.
                    boolean valid;

                    if(index < 0) {
                        System.out.println("Index cannot be negative: " + index);
                        valid = false;
                    }
                    else if(index >= tracks.size()) {
                        System.out.println("Index is too large: " + index);
                        valid = false;
                    }
                    else {
                        valid = true;
                    }
                    return valid;
                }

                private void readLibrary(String folderName)
                {
                    ArrayList<Track> tempTracks = reader.readTracks(folderName, ".mp3");

                    // Put all thetracks into the organizer.
                    for(Track track : tempTracks) {
                        addTrack(track);
                    }

Answer 1

在开始 index 指定的轨道之前，您必须通过执行以下操作来停止所有其他轨道：

for(int i = 0; i < tracks.size(); i++) {
    if(i != trackToPlay) {
        player.stop(tracks.get(i));
    }
}