java - NullPointerException 和 NoSuchElementException

Question

我收到此错误

java.lang.NullPointerException
    at MidiPlayer.add_midi_files(MidiPlayer.java:59)
    at MidiPlayer.main(MidiPlayer.java:51)
Songs added!
Starting to play
Exception in thread "main" java.util.NoSuchElementException
    at java.util.LinkedList.removeFirst(LinkedList.java:268)
    at java.util.LinkedList.remove(LinkedList.java:683)
    at MidiPlayer.play_next_song(MidiPlayer.java:31)
    at MidiPlayer.start_playing(MidiPlayer.java:47)
    at MidiPlayer.main(MidiPlayer.java:54)

通过这些方法

public static void main(String[] args){
    Queue<String> song_queue = new LinkedList<String>();
    MidiPlayer.add_midi_files(song_queue,new File( "C:\\Users\\Kyle\\Desktop\\clieent\\Client\\cache\\Music\\runescape-7th realm.mid"));
    System.out.println("Songs added!");
    MidiPlayer player = new MidiPlayer(song_queue);
    player.start_playing();
};

public static void add_midi_files(Queue<String> queue, File top_dir){
    try {
        for (File f: top_dir.listFiles())
            if (f.isDirectory())
                add_midi_files(queue, f);
            else if (f.isFile() && f.getName().matches(".*midi?"))
            {
                System.out.println(f);
                queue.add(f.getAbsolutePath());
            }
            else
            {}
        //System.out.printf("%s doesn't match!", f.getName());
    } catch (NullPointerException e) {
        e.printStackTrace();
    }
}

老实说，我不明白问题出在哪里，有人可以帮助我吗？

任何正确方向的提示将不胜感激

如果您需要的话，这是整个类(class)，http://pastebin.com/0hVw83zn

Answer 1

File#listFiles() 可以返回 null。请参阅javadoc

Returns null if this abstract pathname does not denote a directory, or if an I/O error occurs.

您的catch语句吞下NullPointerException，然后

player.start_playing();

尝试从队列中获取数据但失败。