NullPointerException when filtering a SimpleAdapter

Question

问题：我想要的只是过滤或让ListView上的项目搜索，但我总是以此错误结束"java.lang.NullPointerException"。请帮我解决这个问题。

播放列表活动.java：

package com.name.musicplayer;
import java.util.ArrayList;
import java.util.HashMap;
import com.androidhive.musicplayer.PlayListActivity;
import android.app.ListActivity;
import android.content.Intent;
import android.os.Bundle;
import android.text.Editable;
import android.text.TextWatcher;
import android.view.View;
import android.widget.AdapterView;
import android.widget.AdapterView.OnItemClickListener;
import android.widget.ArrayAdapter;
import android.widget.EditText;
import android.widget.ListAdapter;
import android.widget.ListView;
import android.widget.SimpleAdapter;
public class PlayListActivity extends ListActivity {
// Songs list
public ArrayList<HashMap<String, String>> songsList = new ArrayList<HashMap<String, String>>();
//list view adapter
ArrayAdapter<String> adapter_l;
// Search EditText
EditText inputSearch;
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.playlist);
    inputSearch = (EditText) findViewById(R.id.inputSearch);
    ArrayList<HashMap<String, String>> songsListData = new ArrayList<HashMap<String, String>>();
    SongsManager plm = new SongsManager();
    // get all songs from sdcard
    this.songsList = plm.getPlayList();
    // looping through playlist
    for (int i = 0; i < songsList.size(); i++) {
        // creating new HashMap
        HashMap<String, String> song = songsList.get(i);
        // adding HashList to ArrayList
        songsListData.add(song);
    }
    // Adding menuItems to ListView
    ListAdapter adapter = new SimpleAdapter(this, songsListData,
            R.layout.playlist_item, new String[] { "songTitle" }, new int[] {
                    R.id.songTitle });
    setListAdapter(adapter);

    met_play_song_on_list();
    met_search();
}
private void met_play_song_on_list() {
    // TODO Auto-generated method stub
    // selecting single ListView item
            ListView lv = getListView();
            // listening to single listitem click
            lv.setOnItemClickListener(new OnItemClickListener() {
                public void onItemClick(AdapterView<?> parent, View view,
                        int position, long id) {
                    // getting listitem index
                    int songIndex = position;
                    // Starting new intent
                    Intent in = new Intent(getApplicationContext(),
                            AndroidBuildingMusicPlayerActivity.class);
                    // Sending songIndex to PlayerActivity
                    in.putExtra("songIndex", songIndex);
                    setResult(100, in);
                    // Closing PlayListView
                    finish();
                }
            });
}
private void met_search() {
    // TODO Auto-generated method stub
    /**
     * Enabling Search Filter
     * */
    inputSearch.addTextChangedListener(new TextWatcher() {
        public void onTextChanged(CharSequence cs, int arg1, int arg2, int arg3) {
            // When user changed the Text
            //PlayListActivity.this.adapter_l.getFilter().filter(cs);
        }
        public void beforeTextChanged(CharSequence arg0, int arg1, int arg2,
                int arg3) {
            // TODO Auto-generated method stub
        }
        public void afterTextChanged(Editable arg0) {
            // TODO Auto-generated method stub
            PlayListActivity.this.adapter_l.getFilter().filter(arg0);
        }
    });
}
}

这是歌曲经理.java类：

package com.name.musicplayer;
import java.io.File;
import java.io.FilenameFilter;
import java.util.ArrayList;
import java.util.HashMap;
import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;
import com.name.musicplayer.JSONParser;
public class SongsManager {
// SDCard Path
private ArrayList<HashMap<String, String>> songsList = new ArrayList<HashMap<String, String>>();

// url to make request
private static String url = "http://domain.com/android/songlist.php";
// JSON Node names
private static final String TAG_str_song = "str_song";
private static final String TAG_str_song_name = "str_song_name";
private static final String TAG_str_song_path = "str_song_path";
// contacts JSONArray
JSONArray songslist = null;
// Constructor
public SongsManager() {
}
/**
 * Function to read all mp3 files from sdcard and store the details in
 * ArrayList
 * */
public ArrayList<HashMap<String, String>> getPlayList() {
    // Creating JSON Parser instance
    JSONParser jParser = new JSONParser();
    // getting JSON string from URL
    JSONObject json = jParser.getJSONFromUrl(url);
    try {
        // Getting Array of Contacts
        songslist = json.getJSONArray(TAG_str_song);
        // looping through All Contacts
        for (int i = 0; i < songslist.length(); i++) {
            JSONObject c = songslist.getJSONObject(i);
            // Storing each json item in variable
            String str_song_name = c.getString(TAG_str_song_name);
            String str_song_path = c.getString(TAG_str_song_path);
            // creating new HashMap
            HashMap<String, String> map = new HashMap<String, String>();
            // adding each child node to HashMap key => value
            map.put("songTitle", (i+1) +". " + str_song_name);
            map.put("songPath", "http://domain.com/" + str_song_path);
            // adding HashList to ArrayList
            songsList.add(map);
        }
    } catch (JSONException e) {
        e.printStackTrace();
    }
    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"));
    }
}
}

Answer 1

在没有看到堆栈跟踪的情况下，我假设NullPointerException来自使用EditText来过滤适配器：

PlayListActivity.this.adapter_l.getFilter().filter(arg0);

上面的行将引发NullPointerException，因为adapter_l null，因为您没有在代码中的任何位置为其分配有效的引用。您应该像这样修改onCreate方法：

adapter_l = new SimpleAdapter(this, songsListData,
            R.layout.playlist_item, new String[] { "songTitle" }, new int[] {
                    R.id.songTitle });
setListAdapter(adapter_l);