我的 android studio 应用程序在模拟器上成功运行，但在我真正的 android 智能手机上运行失败

Question

Logcat error is

2020-09-21 11:48:30.710 4698-4698/rishabh.example.aimusicplayer E/AndroidRuntime: FATAL EXCEPTION: main
    Process: rishabh.example.aimusicplayer, PID: 4698
    java.lang.RuntimeException: Unable to start activity ComponentInfo{rishabh.example.aimusicplayer/rishabh.example.aimusicplayer.MainActivity}: java.lang.NullPointerException: Attempt to get length of null array
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3307)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3446)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2043)
        at android.os.Handler.dispatchMessage(Handler.java:107)
        at android.os.Looper.loop(Looper.java:224)
        at android.app.ActivityThread.main(ActivityThread.java:7550)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:539)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:950)
     Caused by: java.lang.NullPointerException: Attempt to get length of null array
        at rishabh.example.aimusicplayer.MainActivity.readOnlyAudioSongs(MainActivity.java:66)
        at rishabh.example.aimusicplayer.MainActivity.displayAudioSongsName(MainActivity.java:86)
        at rishabh.example.aimusicplayer.MainActivity.access$000(MainActivity.java:21)
        at rishabh.example.aimusicplayer.MainActivity$1.onPermissionGranted(MainActivity.java:43)
        at com.karumi.dexter.MultiplePermissionsListenerToPermissionListenerAdapter.onPermissionsChecked(Unknown Source:35)
        at com.karumi.dexter.DexterInstance$1.run(Unknown Source:43)
        at com.karumi.dexter.MainThread.execute(Unknown Source:6)
        at com.karumi.dexter.DexterInstance.checkMultiplePermissions(Unknown Source:71)
        at com.karumi.dexter.DexterInstance.checkPermissions(Unknown Source:0)
        at com.karumi.dexter.Dexter.check(Unknown Source:10)
        at rishabh.example.aimusicplayer.MainActivity.appExternalStoragePermission(MainActivity.java:57)
        at rishabh.example.aimusicplayer.MainActivity.onCreate(MainActivity.java:33)
        at android.app.Activity.performCreate(Activity.java:7893)
        at android.app.Activity.performCreate(Activity.java:7880)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1307)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3282)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3446) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2043) 
        at android.os.Handler.dispatchMessage(Handler.java:107) 
        at android.os.Looper.loop(Looper.java:224) 
        at android.app.ActivityThread.main(ActivityThread.java:7550) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:539) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:950) 
2020-09-21 11:48:30.723 4698-4698/rishabh.example.aimusicplayer I/Process: Sending signal. PID: 4698 SIG: 9

My Main activity code is

package rishabh.example.aimusicplayer;

import androidx.appcompat.app.AppCompatActivity;

import android.Manifest;
import android.os.Bundle;
import android.os.Environment;
import android.widget.ArrayAdapter;
import android.widget.ListView;

import com.karumi.dexter.Dexter;
import com.karumi.dexter.PermissionToken;
import com.karumi.dexter.listener.PermissionDeniedResponse;
import com.karumi.dexter.listener.PermissionGrantedResponse;
import com.karumi.dexter.listener.PermissionRequest;
import com.karumi.dexter.listener.single.PermissionListener;

import java.io.File;
import java.util.ArrayList;

public class MainActivity extends AppCompatActivity
{
    private String[] itemsAll;
    private ListView mSongsList;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mSongsList = findViewById(R.id.songsList);

        appExternalStoragePermission();
    }

    public void appExternalStoragePermission() {
        Dexter.withContext(this)
                .withPermission(Manifest.permission.READ_EXTERNAL_STORAGE)
                .withListener(new PermissionListener() {
                    @Override
                    public void onPermissionGranted(PermissionGrantedResponse response)
                    {
                        displayAudioSongsName();
                    }

                    @Override
                    public void onPermissionDenied(PermissionDeniedResponse response)
                    {

                    }

                    @Override
                    public void onPermissionRationaleShouldBeShown(PermissionRequest permission, PermissionToken token)
                    {
                        token.continuePermissionRequest();
                    }
                }).check();
    }

    public ArrayList<File> readOnlyAudioSongs(File file)
    {
        ArrayList<File> arrayList = new ArrayList<>();

        File[] allFiles = file.listFiles();

        for (File individualFile : allFiles)
        {
            if (individualFile.isDirectory() && !individualFile.isHidden())
            {
                arrayList.addAll(readOnlyAudioSongs(individualFile));
            }
            else
            {
                if (individualFile.getName().endsWith(".mp3") || individualFile.getName().endsWith(".aac") || individualFile.getName().endsWith(".wav") || individualFile.getName().endsWith(".wma"))
                {
                    arrayList.add(individualFile);
                }
            }
        }

        return arrayList;
    }

    private void displayAudioSongsName()
    {
        final ArrayList<File> audioSongs = readOnlyAudioSongs(Environment.getExternalStorageDirectory());
        itemsAll = new String[audioSongs.size()];

        for (int songCounter=0; songCounter<audioSongs.size(); songCounter++)
        {
            itemsAll[songCounter] = audioSongs.get(songCounter).getName();
        }

        ArrayAdapter<String> arrayAdapter = new ArrayAdapter<String>(MainActivity.this, android.R.layout.simple_list_item_1, itemsAll);
        mSongsList.setAdapter(arrayAdapter);
    }
}

My Build Gradle is

apply plugin: 'com.android.application'

android {
    compileSdkVersion 30
    buildToolsVersion "30.0.2"

    defaultConfig {
        applicationId "rishabh.example.aimusicplayer"
        minSdkVersion 19
        targetSdkVersion 30
        versionCode 1
        versionName "1.0"

        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
    }

    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
    implementation fileTree(dir: "libs", include: ["*.jar"])
    implementation 'androidx.appcompat:appcompat:1.2.0'
    implementation 'androidx.constraintlayout:constraintlayout:2.0.1'
    implementation 'com.karumi:dexter:6.2.1'
    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'androidx.test.ext:junit:1.1.2'
    androidTestImplementation 'androidx.test.espresso:espresso-core:3.3.0'

}

My manifest code is

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

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

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

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity android:name=".SmartPlayerActivity">
        </activity>
    </application>

</manifest>

Answer 1

update this method!

private void displayAudioSongsName() {
    final ArrayList<File> audioSongs = readOnlyAudioSongs(Environment.getExternalStorageDirectory());
    if (audioSongs != null) {
        itemsAll = new String[audioSongs.size()];

        for (int songCounter = 0; songCounter < audioSongs.size(); songCounter++) {
            itemsAll[songCounter] = audioSongs.get(songCounter).getName();
        }

        ArrayAdapter<String> arrayAdapter = new ArrayAdapter<String>(MainActivity.this, android.R.layout.simple_list_item_1, itemsAll);
        mSongsList.setAdapter(arrayAdapter);
    } else Toast.makeText(this, "audio songs list is null!", Toast.LENGTH_SHORT).show();
}

Answer 2

File[] allFiles seem to have null, try to check null !!!

File[] allFiles = file.listFiles();

for (File individualFile : allFiles)