How to set the Android permission on a folder/file on SD Card to be able to write to it?

This example demonstrates how do I set the Android permission on a folder/file on SD Card to be able to write to it.
Step 1 − Create a new project in Android Studio, go to File ⇒ New Project and fill all required details to create a new project.
Step 2 − Add the following code to res/layout/activity_main.xml.
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
   android:layout_width="match_parent"
   android:layout_height="match_parent"
   android:orientation="vertical"
   android:padding="12dp"
   tools:context=".MainActivity">
   <TextView
      android:textStyle="bold"
      android:textAlignment="center"
      android:layout_width="match_parent"
      android:layout_height="wrap_content"
      android:text="Reading and Writing to External Storage"
      android:textSize="24sp" />
   <EditText
      android:layout_marginTop="30dp"
      android:id="@+id/myInputText"
      android:layout_width="match_parent"
      android:layout_height="wrap_content"
      android:ems="10"
      android:gravity="top|start"
      android:lines="5"
      android:minLines="3">
   </EditText>
   <LinearLayout
      android:layout_width="match_parent"
      android:layout_height="wrap_content"
      android:layout_marginTop="20dp"
      android:orientation="horizontal"
      android:weightSum="1.0">
   <Button
      android:id="@+id/saveExternalStorage"
      android:layout_width="match_parent"
      android:layout_height="wrap_content"
      android:layout_weight="0.5"
      android:text="SAVE" />
   <Button
      android:id="@+id/getExternalStorage"
      android:layout_width="match_parent"
      android:layout_height="wrap_content"
      android:layout_weight="0.5"
      android:text="READ" />
   </LinearLayout>
   <TextView
      android:layout_marginTop="10dp"
      android:id="@+id/response"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:padding="5dp"
      android:text=""
      android:textAppearance="?android:attr/textAppearanceMedium" />
</LinearLayout>
Step 3 − Add the following code to src/MainActivity.java
import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
import android.os.Environment;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import java.io.BufferedReader;
import java.io.DataInputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStreamReader;
public class MainActivity extends AppCompatActivity {
   EditText editText;
   TextView textView;
   Button saveButton, readButton;
   File myExternalFile;
   String myData = "";
   @Override
   protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_main);
      editText = findViewById(R.id.myInputText);
      textView = findViewById(R.id.response);
      saveButton = findViewById(R.id.saveExternalStorage);
      saveButton.setOnClickListener(new View.OnClickListener() {
         @Override
         public void onClick(View v) {
            try {
               FileOutputStream fos = new FileOutputStream(myExternalFile);
               fos.write(editText.getText().toString().getBytes());
               fos.close();
            }
            catch (IOException e) {
               e.printStackTrace();
            }
            editText.setText("");
            textView.setText("SampleFile.txt saved to External Storage...");
         }
      });
      readButton = findViewById(R.id.getExternalStorage);
      readButton.setOnClickListener(new View.OnClickListener() {
         @Override
         public void onClick(View v) {
            try {
               FileInputStream fis = new FileInputStream(myExternalFile);
               DataInputStream in = new DataInputStream(fis);
               BufferedReader br = new BufferedReader(new InputStreamReader(in));
               String strLine;
               while ((strLine = br.readLine()) != null) {
                  myData = myData + strLine;
               }
               in.close();
            }
            catch (IOException e) {
               e.printStackTrace();
            }
            editText.setText(myData);
            textView.setText("SampleFile.txt data retrieved from Internal Storage...");
         }
      });
      if (!isExternalStorageAvailable() || isExternalStorageReadOnly()) {
         saveButton.setEnabled(false);
      }
      else {
         String fileName = "SampleFile.txt";
         String filePath = "MyFolder";
         myExternalFile = new File(getExternalFilesDir(filePath), fileName);
      }
   }
   private static boolean isExternalStorageReadOnly() {
      String extStorageState = Environment.getExternalStorageState();
      return Environment.MEDIA_MOUNTED_READ_ONLY.equals(extStorageState);
   }
   private static boolean isExternalStorageAvailable() {
      String extStorageState = Environment.getExternalStorageState();
      return Environment.MEDIA_MOUNTED.equals(extStorageState);
   }
}
Step 4 − Add the following code to androidManifest.xml
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
   package="app.com.sample">
   <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: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>
   </application>
</manifest>
Let's try to run your application. I assume you have connected your actual Android Mobile device with your computer. To run the app from the android studio, open one of your project's activity files and click Run  icon from the toolbar. Select your mobile device as an option and then check your mobile device which will display your default screen −
Click here to download the project code.