[英]Why does the do-while loop not run as expected in this Java program?
我一直在編寫一個程序,它將采用一系列音樂文件的名稱並播放它們。 我成功地做到了這一點,但是,我想要修改一些東西,讓它更好一些。 我試圖讓音樂以隨機順序播放,但在整個列表播放之前不重復播放任何歌曲。 我幾乎能夠做到,但我認為我的do-while循環有問題。 該程序按預期運行大約八首歌曲,但隨后它停止播放音樂,JVM繼續運行。 我正在使用BlueJ,因為我仍然是AP Comp Sci學生,所以我意識到我可能無法完成這項任務,但任何幫助將不勝感激。 我有一個驅動程序,“MusicDriver”,它與另外兩個類別“有一個”關系:“MP3”和“音樂”。
我的MP3課程:
import java.io.BufferedInputStream;
import java.io.FileInputStream;
import javazoom.jl.player.Player;
public class MP3 {
String filename;
Player player;
public void stopMP3() { if (player != null) player.close(); }
// play the MP3 file to the sound card
public void playMP3(String filename) {
try {
FileInputStream fis = new FileInputStream(filename);
BufferedInputStream bis = new BufferedInputStream(fis);
player = new Player(bis);
}
catch (Exception e) {
System.out.println("Problem playing file " + filename);
System.out.println(e);
}
// run in new thread to play in background
new Thread() {
public void run() {
try { player.play(); }
catch (Exception e) { System.out.println(e); }
}
}.start();
}
}
我的音樂班:
import java.util.*;
public class Music{
private ArrayList<String> music;
public Music(){music = new ArrayList<String>();}
public int size(){return music.size();}
public void addSong(String song){music.add(song);}
public String getSong(){return music.get(music.size());}
public String getSong(int num){return music.get(num);}
public void removeSong(String song){
for(int i = 0; i < music.size(); i++){
if(music.get(i).equals(song)) {music.remove(i); return;}
}
}
public String toString(){
String s = "";
for(int i = 0; i < music.size(); i++){
s += music.get(i);
}
return s;
}
}
我的MusicDriver課程:
import java.util.*;
import java.io.*;
import javazoom.jl.player.Player;
import java.util.Random;
import java.util.Scanner;
import java.io.FileNotFoundException;
public class MusicDriver{
public static void main(String[] args) throws FileNotFoundException{
Random r = new Random();
Scanner s = new Scanner(System.in);
String line = "";
int number;
Music song = new Music();
song.addSong("1-01-overture.mp3");
song.addSong("1-03-fortune-teller-2.mp3");
song.addSong("1-07-prayer.mp3");
song.addSong("1-08-island-atlas.mp3");
song.addSong("1-12-warren-report.mp3");
song.addSong("1-13-avilla-hanya.mp3");
song.addSong("1-20-war-situation.mp3");
song.addSong("2-10-fog-of-phantom.mp3");
song.addSong("2-12-religious-precepts.mp3");
song.addSong("2-14-box-of-sentiment.mp3");
song.addSong("3-02-light-everlasting.mp3");
song.addSong("3-09-viking-spirits.mp3");
song.addSong("3-12-unsealed.mp3");
song.addSong("3-16-notice-of-death-reprise-.mp3");
//14 songs
ArrayList<Integer> songNums = new ArrayList<Integer>();
MP3 mp3 = new MP3();
do{
if(songNums.size() == song.size()) songNums.clear();
number = r.nextInt(song.size());
boolean done = false;
int counter = 0;
while(!done){
for(int i = 0; i < songNums.size(); i++){
if(number == songNums.get(i).intValue()) {number = r.nextInt(song.size()); counter++;}
}
if(counter == 0) done = true;
else done = false;
}
songNums.add(number);
mp3.playMP3(song.getSong(number));
System.out.println("Now Playing " + song.getSong(number));
System.out.println("Enter \"Stop\" to stop playing the song");
System.out.println("Enter \"n\" to play the next song");
line = s.nextLine();
mp3.stopMP3();
}while(line.equals("n"));
mp3.stopMP3();
}
}
我已經做了很多研究,為什么我的程序停止播放我的歌曲,但我找不到任何東西。 我做了,發現BlueJ程序沒有打開終端窗口(當你執行“System.out.print()”時出現的東西)如果你在輸出之前要求輸入但我不認為這個計划考慮到了這一點。 我還確保當我想播放下一首歌時我輸入了一個字符串“n”,對於前幾首歌曲,它確實有效,但是在第八首歌之后,它就停止了。 我很困惑。
我認為唯一的問題在於你用來改組列表的邏輯。
number = r.nextInt(song.size());
boolean done = false;
int counter = 0;
while(!done){
for(int i = 0; i < songNums.size(); i++){
if(number == songNums.get(i).intValue()) {number = r.nextInt(song.size()); counter++;}
}
if(counter == 0) done = true;
else done = false;
}
當生成的隨機數已存在於songNums列表中時,您將生成一個新的隨機數。 不會使用所有數量的songNums列表檢查此新隨機數。 以下更改應解決您的問題。
boolean done = false;
while(!done){
number = r.nextInt(song.size());
if(!songNum.contains(number)) done = true;
}
或者,您可以在評論中使用Sasha的建議來改組列表(Collections.shuffle())。
現有算法的實際問題是,當您發現已播放的歌曲時,您不會重置counter
。 因此,只要你重復一次,就會陷入無限循環 - done
將永遠不會成真。
(實際上,它不會是無限的 - 一旦counter
達到Integer.MAX_VALUE
,它將回繞到Integer.MIN_VALUE
並最終再次達到0
,所以如果你把它留下足夠長的時間它最終會播放另一首歌)
這里有一些關於代碼改進的有用建議,我不會在這里重復它們,但是修改你所擁有的最小變化是在循環內將counter
的初始化移動到0
:
boolean done = false;
while(!done){
int counter = 0; // reset counter every time
for(int i = 0; i < songNums.size(); i++){
if(number == songNums.get(i).intValue()) {number = r.nextInt(song.size()); counter++;}
}
if(counter == 0) done = true;
else done = false;
}
Sasha在評論中說:使用Collections.shuffle()。 在實踐中看起來像這樣的東西:
在Music類中有一個獲取所有歌曲的方法:
public List<String> getSongs() {return music;}
MusicDriver中的循環將遵循:
List<String> songs = song.getSongs();
do{
Collections.shuffle(songs);
for (String songToPly: songs) {
mp3.playMP3(song.getSong(number));
System.out.println("Now Playing " + song.getSong(number));
System.out.println("Enter \"Stop\" to stop playing the song");
System.out.println("Enter \"n\" to play the next song");
mp3.stopMP3();
line = s.nextLine();
if (!line.equals("n")) break;
}
}while(line.equals("n"));
在變量命名注釋上,將Music類的實例命名為“song”(單數)有點令人困惑。 也許稱之為“音樂”或至少稱為“歌曲”。
我會做的是:
public class MainClass() {
public static void main(String[] args) {
PlayerWrapper player = new PlayerWrapper();
}
}
public class PlayerWrapper() {
private List<MP3> playlist;
private Scanner userInputReader;
private String currentUserInput;
public PlayerWrapper() {
userInputReader = new Scanner(System.in());
System.out.println("Filepath to playlist?");
String playlistFileName = userInputReader.nextLine();
playlist = PlayListExtractor.extractPlaylist(playlistFileName);
start();
}
public void start() {
playlistCopy = new ArrayList<MP3>(playlist);
shufflePlayList(playlistCopy);
Iterator<MP3> songIterator = playlistCopy.iterator();
while (songIterator.hasNext()) {
MP3 song = songIterator.next();
songIterator.remove();
player = new Player(song.toStream());
player.play();
displayCurrentSongAndCommands(song);
currentUserInput = userInputReader.nextLine();
if ("Stop".equals(currentUserInput )) {
player.close();
break;
} else if ("n".equals(currentUserInput )) {
player.close();
continue;
}
}
if("Stop".equals(currentUserInput)) {
System.out.println("Playlist stopped. Press q to quit or c to continue");
currentUserInput = userInputReader.nextLine();
if ("q".equals(currentUserInput)) {
System.exit(0);
} else if ("c".equals(currentUserInput)) {
start();
}
}
start();
}
private void shufflePlayList(final List<MP3> playlistToBeShuffled) {
long seed = System.nanoTime();
Collections.shuffle(playlistToBeShuffled, new Random(seed));
}
private void displayCurrentSongAndCommands(final MP3 currentSong) {
System.out.println("Now Playing " + currentSong.toString());
System.out.println("Enter \"Stop\" to stop playing the song");
System.out.println("Enter \"n\" to play the next song");
}
}
public static class PlayListExtractor() {
private PlayListExtractor();
public static List<MP3> extractPlayList(final String playListFileName) {
List<MP3> result = new ArrayList<>();
try (BufferedReader br = new BufferedReader(new FileReader(file))) {
String line;
while ((line = br.readLine()) != null) {
result.add(new MP3(line));
}
return result;
} catch (IOException e) {
System.out.println("Problem parsing playlist");
}
}
}
public class MP3 {
private String filename;
public MP3(final String filename) {
this.filename = filename;
}
public BufferedInputStream toStream() {
try {
FileInputStream fis = new FileInputStream(filename);
return new BufferedInputStream(fis);
}
catch (Exception e) {
System.out.println("Problem playing file " + filename);
System.out.println(e);
}
}
public String toString() {
return filename;
}
}
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