我有一个有效的音乐播放器应用程序，但到目前为止，该应用程序仅设置为在用户指定的顶级文件夹中查找音频文件。我想查看至少1级深度音频文件的子文件夹。
我的代码尝试在子文件夹中查找音频文件：
//search subfolders of root media directory for audio files
File[] someFiles = home.listFiles();
for (int i = 0; i < someFiles.length; i++) {
File selectedFile = someFiles[i];
if (selectedFile.isDirectory()) {
searchForAudioFiles(selectedFile, uniqueSongIDInt);
}
}
不幸的是我得到一个空指针异常。我运行调试器试图找出哪个变量为null，但还没有想出来。
logcat的： java.lang.RuntimeException：无法启动活动ComponentInfo {com.joshbgold.simplemusicplayer / com.joshbgold.simplemusicplayer.MainActivity}：java.lang.NullPointerException 在android.app.ActivityThread.performLaunchActivity（ActivityThread.java:2262） 在android.app.ActivityThread.handleLaunchActivity（ActivityThread.java:2316） 在android.app.ActivityThread.access $ 700（ActivityThread.java:158） 在android.app.ActivityThread $ H.handleMessage（ActivityThread.java:1296） 在android.os.Handler.dispatchMessage（Handler.java:99） 在android.os.Looper.loop（Looper.java:176） 在android.app.ActivityThread.main（ActivityThread.java:5365） at java.lang.reflect.Method.invokeNative（Native Method） 在java.lang.reflect.Method.invoke（Method.java:511） 在com.android.internal.os.ZygoteInit $ MethodAndArgsCaller.run（ZygoteInit.java:1102） 在com.android.internal.os.ZygoteInit.main（ZygoteInit.java:869） at dalvik.system.NativeStart.main（Native Method） 引起：java.lang.NullPointerException at com.joshbgold.simplemusicplayer.SongsManager.searchForAudioFiles（SongsManager.java:66） 在com.joshbgold.simplemusicplayer.SongsManager.getPlayList（SongsManager.java:56） 在com.joshbgold.simplemusicplayer.MainActivity.onCreate（MainActivity.java:126）
这是第66行，Logcat似乎表明可能导致异常：
if ((home.listFiles(new FileExtensionFilter()).length > 0)) {
这是完整的SongsManager类：
import android.content.Context;
import java.io.File;
import java.io.FilenameFilter;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Locale;
public class SongsManager {
public HashMap<String, String> song;
public String songTitle;
private String uniqueSongIDString = "0";
public Context mContext;
protected String lowerCaseName = "";
// SDCard Path
public String MEDIA_PATH = "/storage";
private ArrayList<HashMap<String, String>> songsList = new ArrayList<>();
private ArrayList<HashMap<String, String>> filteredSongsList = new ArrayList<>();
public SongsManager(Context context, String folderPath) {
mContext = context;
if (folderPath != null && !"".equals(folderPath)) {
MEDIA_PATH = folderPath;
}
}
/**
* Function to read all mp3 files from sdcard
* and store the details in ArrayList
*/
public ArrayList<HashMap<String, String>> getPlayList() {
File home = new File(MEDIA_PATH);
uniqueSongIDString = "0";
int uniqueSongIDInt = 0;
if (songsList != null) {
songsList.clear();
}
if (home.listFiles() != null) {
//search audio files in root media directory user has selected
//searchForAudioFiles(home, uniqueSongIDInt);
//search subfolders of root media directory for audio files
File[] someFiles = home.listFiles();
for (int i = 0; i < someFiles.length; i++) {
File selectedFile = someFiles[i];
if (selectedFile.isDirectory()) {
searchForAudioFiles(selectedFile, uniqueSongIDInt);
}
}
}
// return songs playlist_item array
return songsList;
}
private void searchForAudioFiles(File home, int uniqueSongIDInt) {
if ((home.listFiles(new FileExtensionFilter()).length > 0)) {
for (File file : home.listFiles(new FileExtensionFilter())) { //for each file that is an audio file in home directory
song = new HashMap<>(); //make a hashmap data structure to store song info
songTitle = file.getName();
//remove track numbers from song titles
songTitle = songTitle.replaceFirst("^\\d*\\s", ""); //replaces leading digits & following space
songTitle = songTitle.replaceFirst("^\\d*\\-\\d*", ""); //replaces leading digits, following hyphen, and following digits
song.put("songTitle", songTitle);
song.put("songPath", file.getPath());
song.put("songUniqueID", uniqueSongIDString);
uniqueSongIDInt++;
uniqueSongIDString = String.valueOf(uniqueSongIDInt);
// Adding each song to SongList
songsList.add(song);
}
}
}
/**
* Class to filter files which are having .mp3 extension
*/
class FileExtensionFilter implements FilenameFilter {
public boolean accept(File dir, String name) {
lowerCaseName = name.toLowerCase();
return (name.endsWith(".mp3") || name.endsWith(".wma") || name.endsWith(".wav") || name.endsWith(".m4a") || name.endsWith(".flac"));
}
}
// Filter Class
public ArrayList<HashMap<String, String>> filter(String searchString) {
searchString = searchString.toLowerCase(Locale.getDefault());
songsList.clear();
songsList = getPlayList();
//searchString is empty, so show all songs in results
if (searchString.length() == 0) {
if (filteredSongsList != null) {
filteredSongsList.clear();
}
filteredSongsList = songsList;
}
//only return songs that match the search string
else {
if (filteredSongsList != null) {
filteredSongsList.clear();
}
for (HashMap<String, String> song : songsList) {
if (song != null) {
String songTitle = song.get("songTitle");
if (songTitle.toLowerCase().contains(searchString)) {
filteredSongsList.add(song);
}
}
}
}
return filteredSongsList;
}
}
您可以从Github here
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