ExternalStorage

activityMain.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:orientation="vertical"
    android:id="@+id/activity_main"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="karthik.saveexternaltorage.MainActivity">

   <EditText
       android:layout_width="match_parent"
       android:layout_height="wrap_content"
       android:id="@+id/edittext"
       android:hint="enter a message"/>
    <Button
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/button1"
        android:text="write message"/>
    <Button
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/button2"
        android:text="read message"/>
    <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/textview"
        android:text="helloworld"/>

</LinearLayout>

MainActivity.java

package karthik.saveexternaltorage;

import android.os.Environment;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStreamReader;

public class MainActivity extends AppCompatActivity {
    EditText editText;
    Button button1,button2;
    TextView textView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        editText = (EditText) findViewById(R.id.edittext);
        button1 = (Button) findViewById(R.id.button1);
        button2 = (Button) findViewById(R.id.button2);
        textView = (TextView) findViewById(R.id.textview);
        textView.setVisibility(View.GONE);
        button1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                //check sd card available or not
                String state;
                state = Environment.getExternalStorageState();
                if (Environment.MEDIA_MOUNTED.equals(state)) {
                    File root = Environment.getExternalStorageDirectory();
                    File dir = new File(root.getAbsolutePath() + "/MyAppFile");
                    if (!dir.exists()) {
                        dir.mkdir();
                        File file = new File(dir, "mymessage.txt");
                        String message = editText.getText().toString();
                        try {
                            FileOutputStream fileOutPutStream = new FileOutputStream(file);
                            fileOutPutStream.write(message.getBytes());
                            fileOutPutStream.close();
                            editText.setText("");
                            Toast.makeText(MainActivity.this, "message saved", Toast.LENGTH_SHORT).show();
                        } catch (FileNotFoundException e) {
                            e.printStackTrace();
                        } catch (IOException e) {
                            e.printStackTrace();
                        }

                    } else {
                        Toast.makeText(MainActivity.this, "sd card not found", Toast.LENGTH_SHORT).show();
                    }


                }
            }
        });
        button2.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                File root = Environment.getExternalStorageDirectory();
                File dir = new File(root.getAbsolutePath() + "/MyAppFile");
                File file = new File(dir, "mymessage.txt");
                String message;
                try {
                    FileInputStream fileInputStream = new FileInputStream(file);
                    InputStreamReader inputStreamReader = new InputStreamReader(fileInputStream);
                    BufferedReader bufferedReader = new BufferedReader(inputStreamReader);
                    StringBuffer stringBuffer = new StringBuffer();

                    while ((message = bufferedReader.readLine())!= null) {
                        stringBuffer.append(message + "/n");

                    }
                    textView.setText(stringBuffer.toString());
                    textView.setVisibility(View.VISIBLE);
                } catch (FileNotFoundException e) {
                    e.printStackTrace();
                } catch (IOException e) {
                    e.printStackTrace();
                }

            }
        });

    }

}

manifest.xml

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

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

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        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>