<?xml version="1.0" encoding="utf-8"?>
<paths xmlns:android="http://schemas.android.com/apk/res/android">
 <external-path name="external_files" path="."/>
</paths>

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
 xmlns:tools="http://schemas.android.com/tools"
 package="com.example.myapplication">
 <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
 <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>
 <application
  android:allowBackup="true"
  android:icon="@mipmap/ic_launcher"
  android:label="@string/app_name"
  android:roundIcon="@mipmap/ic_launcher_round"
  android:supportsRtl="true"
  android:requestLegacyExternalStorage="true"
  android:theme="@style/Theme.MyApplication">
  <provider
   android:name="androidx.core.content.FileProvider"
   android:authorities="包名"
   android:exported="false"
   android:grantUriPermissions="true">
   <meta-data
   android:name="android.support.FILE_PROVIDER_PATHS"
    android:resource="@xml/filepaths步驟2中的文件"
    />
  </provider>
  <activity android:name=".MainActivity">
   <intent-filter>
    <action android:name="android.intent.action.MAIN" />
    <category android:name="android.intent.category.LAUNCHER" />
   </intent-filter>
  </activity>
 </application>
</manifest>

public class MainActivity extends AppCompatActivity {
 Context mContext;
 private Button bt,bt0;
 private TextView tx;
 public static boolean isGrantExternalRW(Activity activity) {
 //權限判定
  if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M && activity.checkSelfPermission(
    Manifest.permission.WRITE_EXTERNAL_STORAGE) != PackageManager.PERMISSION_GRANTED) {
   activity.requestPermissions(new String[]{
     Manifest.permission.READ_EXTERNAL_STORAGE,
     Manifest.permission.WRITE_EXTERNAL_STORAGE
   }, 1);
   return false;
  }
  return true;
 }
 @Override
 protected void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_main);
  mContext = this;
  tx=findViewById(R.id.text);
  bt=findViewById(R.id.button);//安裝
  bt0=findViewById(R.id.button0);//卸載
//  Toast.makeText(this, ""+Environment.getExternalStorageDirectory().getAbsolutePath(), 0).show();
  if(!this.isGrantExternalRW(MainActivity.this)){return;}//判斷是否有權限修改，這很重要
  else{
  if(copyApkFromAssets(this, "app-debug.apk", Environment.getExternalStorageDirectory().getAbsolutePath()+"/app-debug.apk")){
   bt.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {
     new AlertDialog.Builder(mContext)
       .setIcon(R.drawable.ic_launcher)
       .setMessage("是否安裝？")
       .setPositiveButton("yes", new DialogInterface.OnClickListener() {
        @Override
        public void onClick(DialogInterface dialog, int which) {
         Intent intent = new Intent(Intent.ACTION_VIEW);
         intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
//         intent.setDataAndType(Uri.parse("file://" + Environment.getExternalStorageDirectory().getAbsolutePath()+"/app-debug.apk"),
//           "application/vnd.android.package-archive");
         File apkFile = new File(Environment.getExternalStorageDirectory()+"/app-debug.apk");
         if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
          intent.setFlags(Intent.FLAG_GRANT_READ_URI_PERMISSION);
          Uri uri = FileProvider.getUriForFile(MainActivity.this, "com.example.myapplication", apkFile);
          intent.setDataAndType(uri, "application/vnd.android.package-archive");
         } else {
          intent.setDataAndType(Uri.fromFile(apkFile), "application/vnd.android.package-archive");
         }
         mContext.startActivity(intent);

        }
       }).show();
    }
   });
   bt0.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {
     Uri packageURI = Uri.parse("package:com.example.a22");//package:+想要卸載的包名
     Intent intent =new Intent(Intent.ACTION_DELETE);
     intent.setData(packageURI);
     mContext.startActivity(intent);
    }
   });
  }

 }}
 public boolean copyApkFromAssets(Context context, String fileName, String path) {
 //將asserts中的apk文件復制到手機存儲中
 //可以從手機存儲劃到最下面，是單獨的文件，沒有設置文件夾
  boolean copyIsFinish = false;
  try {
   InputStream is = context.getAssets().open(fileName);
   File file = new File(path);
   file.createNewFile();
   FileOutputStream fos = new FileOutputStream(file);
   int length = is.available();
   byte[] temp = new byte[length];
   int i = 0;
   while ((i = is.read(temp)) > 0) {
    fos.write(temp, 0, i);
   }
   fos.close();
   is.close();

   copyIsFinish = true;
  } catch (IOException e) {
   e.printStackTrace();
  }
  return copyIsFinish;
 }
}