Android的显示java.lang.NullPointerException:尝试获取空数组长度 [英] Android java.lang.NullPointerException: Attempt to get length of null array
问题描述
对不起,我格式化我是新手到Android以及对计算器
斜面提交所有我的logcat由于一些格式错误
总之我收到以下错误:
在com.youmasti.mp3test.MainActivity.findsongs(MainActivity.java:45)
在com.youmasti.mp3test.MainActivity.onCreate(MainActivity.java:27)
下面是我的code:
主要活动公共类MainActivity扩展AppCompatActivity {LV的ListView;
串的物品;
@覆盖
保护无效的onCreate(捆绑savedInstanceState){
super.onCreate(savedInstanceState);
的setContentView(R.layout.activity_main);
LV =(ListView控件)findViewById(R.id.lvPlaylist);
//文件FTEST = Environment.getExternalStorageDirectory();
ArrayList的<文件> mySongs = findsongs(Environment.getExternalStorageDirectory()); 的for(int i = 0; I< mySongs.size();我++){
吐司(mySongs.get(I).getName()的toString());
}
}公众的ArrayList<文件> findsongs(文件根目录){
ArrayList的<文件>人=新的ArrayList<文件>();
文件[] =文件root.listFiles(); 对于(文件singlefile:文件){
如果(singlefile.isDirectory()及&放大器;!singlefile.isHidden()){
al.addAll(findsongs(singlefile));
}其他{
如果(singlefile.getName()。的endsWith(MP3)){
al.add(singlefile);
}
}
}
返回人;
}公共无效吐司(字符串文本){
Toast.makeText(getApplicationContext(),文本,Toast.LENGTH_SHORT).show();
}
列表项:
<?XML版本=1.0编码=UTF-8&GT?;<清单的xmlns:机器人=http://schemas.android.com/apk/res/android
包=com.youmasti.mp3test>
<使用许可权的android:NAME =android.permission.READ_EXTERNAL_STORAGE/>
<使用许可权的android:NAME =android.permission.WRITE_EXTERNAL_STORAGE/><应用
机器人:allowBackup =真
机器人:图标=@的mipmap / ic_launcher
机器人:标签=@字符串/ APP_NAME
机器人:主题=@风格/ AppTheme>
<活动
机器人:名字=。MainActivity
机器人:标签=@字符串/ APP_NAME>
&所述;意图滤光器>
<作用机器人:名字=android.intent.action.MAIN/>
<类机器人:名字=android.intent.category.LAUNCHER/>
&所述; /意图滤光器>
< /活性GT;
< /用途>
LogCat中 10月8日至30日:39:30.227 2441年至2441年/? I /艺术:不晚启用-Xcheck:JNI(已上)
10月8日至30日:39:30.227 2441年至2441年/? I /艺术:晚启用JIT
10月8日至30日:39:30.229 2441年至2441年/? I /艺术:以$创建JIT C $ c_cache_capacity = 2MB compile_threshold = 1000
10月8日至30日:39:30.346 2441年至2441年/? W /系统:ClassLoader的引用未知的路径:/data/app/com.youmasti.mp3test-1/lib/x86
10月8日至30日:39:30.527 2441年至2441年/ com.youmasti.mp3test D / AndroidRuntime:关闭VM
---------崩溃的开始
10月8日至30日:39:30.528 2441年至2441年/ com.youmasti.mp3test E / AndroidRuntime:致命异常:主要
工艺:com.youmasti.mp3test,PID:2441
了java.lang.RuntimeException:无法启动活动ComponentInfo {com.youmasti.mp3test / com.youmasti.mp3test.MainActivity}:显示java.lang.NullPointerException:尝试获取空数组长度
在android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
在android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
在android.app.ActivityThread.-wrap11(ActivityThread.java)
在android.app.ActivityThread $ H.handleMessage(ActivityThread.java:1344)
在android.os.Handler.dispatchMessage(Handler.java:102)
在android.os.Looper.loop(Looper.java:148)
在android.app.ActivityThread.main(ActivityThread.java:5417)
在java.lang.reflect.Method.invoke(本机方法)
在com.android.internal.os.ZygoteInit $ MethodAndArgsCaller.run(ZygoteInit.java:726)
在com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
显示java.lang.NullPointerException:引起尝试获取空数组长度
在com.youmasti.mp3test.MainActivity.findsongs(MainActivity.java:45)
在com.youmasti.mp3test.MainActivity.onCreate(MainActivity.java:27)
在android.app.Activity.performCreate(Activity.java:6237)
在android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
在android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
在android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
在android.app.ActivityThread.-wrap11(ActivityThread.java)
在android.app.ActivityThread $ H.handleMessage(ActivityThread.java:1344)
在android.os.Handler.dispatchMessage(Handler.java:102)
您必须指向我们哪里是线45。
不过,我猜这是此行
的(文件singlefile:文件){
如果有,这个问题是因为 listFiles()
收益空
它可以返回空值以下几个原因,根不存在,或者根本不是一个文件夹或最有可能的,因为你是与Android合作,你错过添加的权限在你的清单
<使用许可权的android:NAME =android.permission.READ_EXTERNAL_STORAGE/>
编辑:
你的编辑后,我相信你的清单有一定的问题。
请点击这里查看样本
https://developer.android.com/samples/BasicContactables/AndroidManifest.html
EDIT2:
https://developer.android.com/$p$ PVIEW /功能/运行时permissions.html
为Android 6.0,你需要申请许可
如果(checkSelfPermission(Manifest.permission.READ_CONTACTS)
!= PackageManager.PERMISSION_GRANTED){ //我们应该显示一个解释?
如果(shouldShowRequestPermissionRationale(
Manifest.permission.READ_CONTACTS)){
//向用户说明为什么我们需要读取联系人
} requestPermissions(新的String [] {} Manifest.permission.READ_CONTACTS,
MY_PERMISSIONS_REQUEST_READ_CONTACTS); // MY_PERMISSIONS_REQUEST_READ_CONTACTS是
//应用程序定义的整型常量 返回;
}
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
in short I am getting following error:
at com.youmasti.mp3test.MainActivity.findsongs(MainActivity.java:45)
at com.youmasti.mp3test.MainActivity.onCreate(MainActivity.java:27)
Here is my code:
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:
<?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)
You have to point to us where is the line 45.
But I would guess it is this line
for (File singlefile : file) {
if yes, the problem is because listFiles()
return 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
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
EDIT:
after your edit, I believe your manifest has some problem
please check a sample here
https://developer.android.com/samples/BasicContactables/AndroidManifest.html
EDIT2:
https://developer.android.com/preview/features/runtime-permissions.html
for android 6.0, you need to request permission
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;
}
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