Android java.lang.NullPointerException:尝试获取null数组的长度

Question

对不起,我的格式化我是Android的新手,以及stackoverflow不能提交我的所有logcat由于一些格式错误

总之,我得到以下错误:

at com.youmasti.mp3test.MainActivity.findsongs(MainActivity.java:45)
at com.youmasti.mp3test.MainActivity.onCreate(MainActivity.java:27)

这是我的代码:

Main Activity

public class MainActivity extends AppCompatActivity {

ListView lv;
String items;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    lv= (ListView)findViewById(R.id.lvPlaylist);
    //File fTest = Environment.getExternalStorageDirectory();
    ArrayList<File> mySongs = findsongs(Environment.getExternalStorageDirectory());

    for (int i = 0; i < mySongs.size(); i++) {
        toast(mySongs.get(i).getName().toString());
    }
}

public ArrayList<File> findsongs(File root) {
    ArrayList<File> al= new ArrayList<File>();
    File[] file= root.listFiles();

    for (File singlefile : file) {
        if (singlefile.isDirectory() && !singlefile.isHidden()) {
            al.addAll(findsongs(singlefile));
        } else {
            if (singlefile.getName().endsWith(".mp3")) {
                al.add(singlefile);
            }
        }
    }
    return al;
}

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

项目清单:

<?xml version="1.0" encoding="utf-8"?>

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.youmasti.mp3test">    
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>

<application
    android:allowBackup="true"
    android:icon="@mipmap/ic_launcher"
    android:label="@string/app_name"
    android:theme="@style/AppTheme" >
    <activity
        android:name=".MainActivity"
        android:label="@string/app_name" >
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />
            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
</application>

LogCat

   08-30 10:39:30.227    2441-2441/? I/art? Not late-enabling -Xcheck:jni        (already on)
   08-30 10:39:30.227    2441-2441/? I/art? Late-enabling JIT
   08-30 10:39:30.229    2441-2441/? I/art? JIT created with                code_cache_capacity=2MB compile_threshold=1000
   08-30 10:39:30.346    2441-2441/? W/System? ClassLoader referenced        unknown path: /data/app/com.youmasti.mp3test-1/lib/x86
   08-30 10:39:30.527    2441-2441/com.youmasti.mp3test D/AndroidRuntime?        Shutting down VM
       --------- beginning of crash
   08-30 10:39:30.528    2441-2441/com.youmasti.mp3test E/AndroidRuntime?        FATAL EXCEPTION: main
       Process: com.youmasti.mp3test, PID: 2441
       java.lang.RuntimeException: Unable to start activity ComponentInfo       {com.youmasti.mp3test/com.youmasti.mp3test.MainActivity}:        java.lang.NullPointerException: Attempt to get length of null array
               at android.app.ActivityThread.performLaunchActivity       (ActivityThread.java:2416)
               at android.app.ActivityThread.handleLaunchActivity                     (ActivityThread.java:2476)
                      at android.app.ActivityThread.-wrap11                                                        (ActivityThread.java)
               at android.app.ActivityThread$H.handleMessage       (ActivityThread.java:1344)
               at android.os.Handler.dispatchMessage(Handler.java:102)
               at android.os.Looper.loop(Looper.java:148)
               at android.app.ActivityThread.main(ActivityThread.java:5417)
               at java.lang.reflect.Method.invoke(Native Method)
               at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run       (ZygoteInit.java:726)
               at com.android.internal.os.ZygoteInit.main       (ZygoteInit.java:616)
        Caused by: java.lang.NullPointerException: Attempt to get length of        null array
               at com.youmasti.mp3test.MainActivity.findsongs       (MainActivity.java:45)
               at com.youmasti.mp3test.MainActivity.onCreate       (MainActivity.java:27)
               at android.app.Activity.performCreate(Activity.java:6237)
               at android.app.Instrumentation.callActivityOnCreate       (Instrumentation.java:1107)
               at android.app.ActivityThread.performLaunchActivity       (ActivityThread.java:2369)
               at android.app.ActivityThread.handleLaunchActivity       (ActivityThread.java:2476)
               at android.app.ActivityThread.-wrap11(ActivityThread.java)
   at android.app.ActivityThread$H.handleMessage (ActivityThread.java:1344)
   at android.os.Handler.dispatchMessage(Handler.java:102)

Answer 1

你必须指出我们45号线在哪里.

但我猜它就是这条线

for (File singlefile : file) {

如果是的话,问题是因为listFiles()回归null

它可以返回null有几个原因,root不存在,或root不是文件夹,或者很可能,当你使用Android时,你错过了在清单中添加权限

<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />

编辑:

编辑后,我相信你的清单有一些问题

请在这里查看样品

https://developer.android.com/samples/BasicContactables/AndroidManifest.html

EDIT2:

https://developer.android.com/preview/features/runtime-permissions.html

对于android 6.0,您需要请求权限

if (checkSelfPermission(Manifest.permission.READ_CONTACTS)
        != PackageManager.PERMISSION_GRANTED) {

    // Should we show an explanation?
    if (shouldShowRequestPermissionRationale(
            Manifest.permission.READ_CONTACTS)) {
        // Explain to the user why we need to read the contacts
    }

    requestPermissions(new String[]{Manifest.permission.READ_CONTACTS},
            MY_PERMISSIONS_REQUEST_READ_CONTACTS);

    // MY_PERMISSIONS_REQUEST_READ_CONTACTS is an
    // app-defined int constant

    return;
}