Android java.lang.NullPointerException：尝试获取空数组的长度
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Android java.lang.NullPointerException: Attempt to get length of null array
提问by manpreet parmar
Sorry for my formatting I am newbie to android as well as to stackoverflow cant submit all of my logcat due to some formatting error
抱歉我的格式我是 android 的新手，并且由于一些格式错误，stackoverflow 无法提交我的所有 logcat
in short I am getting following error:
简而言之，我收到以下错误：
at com.youmasti.mp3test.MainActivity.findsongs(MainActivity.java:45)
at com.youmasti.mp3test.MainActivity.onCreate(MainActivity.java:27)
Here is my code:
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();
}
List item:
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)
回答by Derek Fung
You have to point to us where is the line 45.
你必须向我们指出第 45 行在哪里。
But I would guess it is this line
但我猜是这条线
for (File singlefile : file) {
if yes, the problem is because listFiles()return null
如果是，问题是因为listFiles()返回null
It can return null for several reasons, root not exists, or root is not a folder, or most likely, as you are working with Android, you missed to add the permission in your manifest
它可能由于多种原因返回 null，root 不存在，或者 root 不是文件夹，或者很可能，当您使用 Android 时，您错过了在清单中添加权限
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
EDIT:
编辑：
after your edit, I believe your manifest has some problem
编辑后，我相信您的清单有问题
please check a sample here
请在此处查看示例
https://developer.android.com/samples/BasicContactables/AndroidManifest.html
https://developer.android.com/samples/BasicContactables/AndroidManifest.html
EDIT2:
编辑2：
https://developer.android.com/preview/features/runtime-permissions.html
https://developer.android.com/preview/features/runtime-permissions.html
for android 6.0, you need to request permission
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;
}
回答by Andreas
Assume that MainActivity.java:45is this line:
假设这MainActivity.java:45是这一行：
for (File singlefile : file) {
It means that fileis null.
这意味着file是null。
According to the javadoc of File.listFiles():
根据File.listFiles()的 javadoc ：
Returns null if this abstract pathname does not denote a directory
如果此抽象路径名不表示目录，则返回 null
So value of rootis not a path name.
所以值root不是路径名。
回答by Androider
your
您的
listFiles()
method may return null and root is not any path (use as)
方法可能返回 null 并且 root 不是任何路径（用作）
Environment.getExternalStorageDirectory() + path like directory name
