更改包名称应用程序后给出错误“java.lang.NullPointerException：尝试获取空数组的长度”[duplicate]

Question

这个问题在这里已有答案：

• Android permission doesn't work even if I have declared it 12个答案
• listFiles() returns null on Android 6.0 emulator 3回答

注意： - 请停止投票我的问题，我不是Android开发人员。

我不是在Android编程中，无法找到错误，我已经购买了一个应用程序的源代码，您可以从中下载.mp3歌曲。我已根据开发人员指令更改了包名称，但之后我收到此错误“java.lang.NullPointerException：尝试获取null数组的长度”。但是，如果使用旧包名称运行旧应用程序它工作正常，我联系卖家但他没有回复所以我决定在这里发布我的问题。请帮我解决这个错误。下面是logcat显示错误的代码，还附带了截图供审阅。

    package com.mp3songsdownloader.searchonline;

    import android.app.AlertDialog;
    import android.content.DialogInterface;
    import android.content.Intent;
    import android.os.Bundle;
    import android.os.Environment;
    import android.support.v4.app.Fragment;
    import android.view.LayoutInflater;
    import android.view.View;
    import android.view.ViewGroup;
    import android.widget.AdapterView;
    import android.widget.ArrayAdapter;
    import android.widget.ListView;
    import android.widget.RelativeLayout;
    import android.widget.Toast;
    import java.io.File;
    import java.util.ArrayList;


    public class DownloadFragment extends Fragment implements  constants {
    View view;
    ListView lv;
    String[] items;
    RelativeLayout NoShow;
    ArrayAdapter<String> adp;
    ArrayList<File> mySongs;
    public DownloadFragment() {
        // Required empty public constructor
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
    }
    @Override
    public void setMenuVisibility(final boolean visible) {
        super.setMenuVisibility(visible);
        if (visible) {
            mySongs = findSongs(new File(Environment.getExternalStorageDirectory()+DOWNLOAD_DIRECTORY));

            setVal();
        }
    }
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment

        view = inflater.inflate(R.layout.fragment_two, container, false);
        lv=(ListView)view.findViewById(R.id.lvVideo);
        NoShow=(RelativeLayout)view.findViewById(R.id.NothingToShow);
        File dir  =  new File(Environment.getExternalStorageDirectory()+DOWNLOAD_DIRECTORY);


        if (!dir.exists() && !dir.isDirectory()) {
                    dir.mkdir();
        }
        mySongs = findSongs(dir);

        setVal();

        lv.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> arg0, View arg1, final int position, long arg3) {
                final CharSequence[] items = {"Play", "Delete"};

                final AlertDialog.Builder builder = new AlertDialog.Builder(getActivity());
                builder.setTitle("Options");
                builder.setItems(items, new DialogInterface.OnClickListener() {
                    public void onClick(DialogInterface dialog, int item) {


                        if (items[item].equals("Delete")) {
                            removeItemFromList(position);
                        } else if (items[item].equals("Play")) {


                            Intent intent1 = new Intent(getActivity(), PlayerActivity.class);
                            Bundle extras = new Bundle();
                            //intent.putExtra(ID, m.getID());

                            //extras.putString("TITLE", mySongs.get(position).getName().toString());
                            extras.putString("TITLE", mySongs.get(position).getName());
                            extras.putString("ARTWORK", "nothing");
                            extras.putString("STREAM", mySongs.get(position).getAbsolutePath());
                            intent1.putExtras(extras);

                            startActivity(intent1);
                        }

                    }
                });
                AlertDialog alert = builder.create();
                alert.show();

            }
        });
        return  view;
    }

    public void setVal(){
        items = new String[mySongs.size()];

        if(mySongs.size()>0){
            NoShow.setVisibility(NoShow.INVISIBLE);
            lv.setVisibility(lv.VISIBLE);
        }
        for(int i=0; i<mySongs.size(); i++){
            //toast(mySongs.get(i).toString());
        //    items[i]=mySongs.get(i).getName().toString().replace(".mp3","");
            items[i]=mySongs.get(i).getName().replace(".mp3","");

        }
        adp= new ArrayAdapter<String>(getActivity(),R.layout.song_down_layout,R.id.title,items);
        lv.setAdapter(adp);
    }

    protected void removeItemFromList(int position) {
        final int deletePosition = position;

        AlertDialog.Builder alert = new AlertDialog.Builder(
                getActivity());

        alert.setTitle("Delete");
        alert.setMessage("Do you want delete this item?");
        alert.setPositiveButton("YES", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int which) {
                File  f = new File(mySongs.get(deletePosition).getAbsolutePath());
                f.delete();
              mySongs.remove(deletePosition);
                setVal();
                adp.notifyDataSetChanged();
                adp.notifyDataSetInvalidated();
                Toast.makeText(getActivity(), "Song Deleted!", Toast.LENGTH_SHORT).show();

            }
        });
        alert.setNegativeButton("CANCEL", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int which) {
                // TODO Auto-generated method stub
                dialog.dismiss();
            }
        });

        alert.show();

    }
    public ArrayList<File> findSongs(File root){
        ArrayList<File> inFiles = new ArrayList<File>();
        File[] files = root.listFiles();
        for (File file : files) {
            if (file.isDirectory()) {
                inFiles.addAll(findSongs(file));
            } else {
                if(file.getName().endsWith(".mp3")){
                    inFiles.add(file);
                }
            }
        }
        return inFiles;

    }

    public void toast (String text){
        Toast.makeText(getActivity(),text, Toast.LENGTH_SHORT).show();

    }


}

Android Manifest.xml文件

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.mp3songsdownloader.searchonline" >

    <uses-permission android:name="android.permission.INTERNET" />
    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
    <uses-permission android:name="android.permission.WRITE_SETTINGS" />
    <uses-permission android:name="android.permission.WAKE_LOCK"/>
    <uses-permission android:name="android.permission.READ_PHONE_STATE" />
    <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />

    <application
        android:allowBackup="true"
        android:label="@string/app_name"
        android:theme="@style/MyMaterialTheme"
        android:name="com.mp3songsdownloader.searchonline.App" >
        <!--This meta-data tag is required to use Google Play Services.-->
        <meta-data android:name="com.google.android.gms.version" android:value="@integer/google_play_services_version" />
        <activity
            android:name="com.google.android.gms.ads.AdActivity"
            android:configChanges="keyboard|keyboardHidden|orientation|screenLayout|uiMode|screenSize|smallestScreenSize"/>
        <activity
            android:name="com.mp3songsdownloader.searchonline.MainActivity"
            android:label="@string/app_name"
            android:windowSoftInputMode="adjustNothing" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity
            android:name="com.mp3songsdownloader.searchonline.PlayerActivity"
            android:label="@string/app_name"
            android:theme="@style/MyDialogTheme" >
            </activity>
    </application>

</manifest>

Answer 1

例外意味着

 File[] files = root.listFiles();

将files设为null。这意味着listFiles()已经返回null。 javadoc的listFiles()说：

“如果此抽象路径名不表示目录，或者发生I / O错误，则返回null。”

所以你需要弄清楚你为你想要列出的目录形成File的方式有什么问题。

Answer 2

感谢所有人的帮助，我发现这只是因为没有外部存储的许可。