Throwing null pointer exception in Android

问题

I'm trying to fetch an audio file from the audio directory, for that I'm using below code. I'm not able to figure out why this condition if (home.listFiles(new FileExtensionFilter()).length > 0) is throwing an error. Code is given below.

Uri allsongsuri = MediaStore.Audio.Media.EXTERNAL_CONTENT_URI;
final String MEDIA_PATH = allsongsuri.toString();
private ArrayList<HashMap<String, String>> songsList = new ArrayList<HashMap<String, String>>();

public ArrayList<HashMap<String, String>> getPlayList(){
    System.out.println(" -- "+MEDIA_PATH);
    File home = new File(MEDIA_PATH);

    if (home.listFiles(new FileExtensionFilter()).length > 0) {
        for (File file : home.listFiles(new FileExtensionFilter())) {
            HashMap<String, String> song = new HashMap<String, String>();
        song.put("songTitle", file.getName().substring(0, (file.getName().length() - 4)));
            song.put("songPath", file.getPath());

            // Adding each song to SongList
        songsList.add(song);
        }
    }
    return songsList;
}

class FileExtensionFilter implements FilenameFilter {
    public boolean accept(File dir, String name) {
    return (name.endsWith(".mp3") || name.endsWith(".MP3"));
}

I need to store audio file in ArrayList using hashMap. I tried this example to store file in List http://android-er.blogspot.in/2012/02/list-audio-media-in-mediastoreaudiomedi.html . Using this example I tried a lot to store the list in ArrayList as my reqirement need that. But didn't succeed. I need help. I have to store Audio file in Arraylist.

回答1:

This must be home.listFiles returning null.

From Android developer reference on public File[] listFiles (FileFilter filter)

Gets a list of the files in the directory represented by this file. This list is then filtered through a FileFilter and matching files are returned as an array of files. Returns null if this file is not a directory. If filter is null then all files match.

So you need to make sure that MEDIA_PATH is a proper directory.

回答2:

The "listFiles()" method can return null if the file is not found. I would suggest putting in a check to see if it's null first, and only proceed if it is something other than null. Example:

public ArrayList<HashMap<String, String>> getPlayList(){
    System.out.println(" -- "+MEDIA_PATH);
    File home = new File(MEDIA_PATH);

    if (home.listFiles(new FileExtensionFilter()) != null){
        if (home.listFiles(new FileExtensionFilter()).length > 0) {
            for (File file : home.listFiles(new FileExtensionFilter())) {
                HashMap<String, String> song = new HashMap<String, String>();
                song.put("songTitle", file.getName().substring(0, (file.getName().length() - 4)));
                song.put("songPath", file.getPath());

                // Adding each song to SongList
                songsList.add(song);
            }
        }
    }

    return songsList;
}

You will probably want to put in an else statement as well, in order to make sure that songsList is not null when it is returned.

回答3:

public class SongsManager {
    final String MEDIA_PATH = Environment.getExternalStorageDirectory()
            .getPath() + "/";
    private ArrayList<HashMap<String, String>> songsList = new ArrayList<HashMap<String, String>>();
    private String mp3Pattern = ".mp3";

    // Constructor
    public SongsManager() {

    }

    /**
     * Function to read all mp3 files and store the details in
     * ArrayList
     * */
    public ArrayList<HashMap<String, String>> getPlayList() {
        System.out.println(MEDIA_PATH);
        if (MEDIA_PATH != null) {
            File home = new File(MEDIA_PATH);
            File[] listFiles = home.listFiles();
            if (listFiles != null && listFiles.length > 0) {
                for (File file : listFiles) {
                    System.out.println(file.getAbsolutePath());
                    if (file.isDirectory()) {
                        scanDirectory(file);
                    } else {
                        addSongToList(file);
                    }
                }
            }
        }
        // return songs list array
        return songsList;
    }

    private void scanDirectory(File directory) {
        if (directory != null) {
            File[] listFiles = directory.listFiles();
            if (listFiles != null && listFiles.length > 0) {
                for (File file : listFiles) {
                    if (file.isDirectory()) {
                        scanDirectory(file);
                    } else {
                        addSongToList(file);
                    }

                }
            }
        }
    }

    private void addSongToList(File song) {
        if (song.getName().endsWith(mp3Pattern)) {
            HashMap<String, String> songMap = new HashMap<String, String>();
            songMap.put("songTitle",
                    song.getName().substring(0, (song.getName().length() - 4)));
            songMap.put("songPath", song.getPath());

            // Adding each song to SongList
            songsList.add(songMap);
        }
    }


}

回答4:

with ref to Anup's and Destrom's answer I just want put the code with with Filename Filter

    public class SongsManager {
    // SDCard Path
//  final String MEDIA_PATH = new String("/sdcard/sound_recorder"); 

    final String MEDIA_PATH =Environment.getExternalStorageDirectory()+"/sound_recorder";
    private ArrayList<HashMap<String, String>> songsList = new ArrayList<HashMap<String, String>>();

    // Constructor
    public SongsManager(){

    }

    /**
     * Function to read all mp3 files from sdcard
     * and store the details in ArrayList
     * */
    public ArrayList<HashMap<String, String>> getPlayList(){

        Log.d("testsd",MEDIA_PATH);
        File home = new File(MEDIA_PATH);

    //  if (home.listFiles(new FileExtensionFilter()).length > 0) //don't use this to avoid null pointer exception !
        if (home.listFiles(new FileExtensionFilter())!=null) {
            for (File file : home.listFiles(new FileExtensionFilter())) {
                HashMap<String, String> song = new HashMap<String, String>();


                song.put("songTitle", file.getName().substring(0, (file.getName().length() - 4)));
                song.put("songPath", file.getPath());

                // Adding each song to SongList


                songsList.add(song);

            }
        }
        // return songs list array
        return songsList;
    }

    /**
     * Class to filter files which are having .mp3 extension
     * */
    class FileExtensionFilter implements FilenameFilter {
        public boolean accept(File dir, String name) {
            return (name.endsWith(".mp3") || name.endsWith(".MP3") || name.endsWith(".3gpp") || name.endsWith(".wav")); //add more extensions here
        }
    }
}

回答5:

Thanks a lote.....was getting mat with the moto g without a sdcard.....it emulate the physical sdcard....still not work.

But your code work perfect with another motorola that has the physical sdcard

i mean when the sdcard is emulated dontwork ..i try all the MEDIA_PATHS possibles

i need your advice...thanks. //final String MEDIA_PATH = Environment.getExternalStorageDirectory().getPath() + "/"; //private final String MEDIA_PATH = new String("/sdcard/res/raw/"); //final String MEDIA_PATH = Environment.getExternalStorageDirectory().getAbsolutePath() + "/"; //final String MEDIA_PATH = new String("/sdcard/"); //final String MEDIA_PATH = Environment.getExternalStorageDirectory().getPath() + "/"; // final String MEDIA_PATH =""; //final String MEDIA_PATH = new String("/sdcard/Musica/"); //final String MEDIA_PATH = Environment.getExternalStorageDirectory().getPath() + "/"; //final String MEDIAPATH = Environment.getExternalStorageDirectory().getAbsolutePath() + "/Musica/"; //final String MEDIAPATH = new String("/sdcard/"); //final String MEDIA_PATH = Environment.getExternalStorageDirectory().getPath() + "/"; final String MEDIA_PATH = Environment.getExternalStorageDirectory().getAbsolutePath() + "/";