<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:gravity="center"
    android:orientation="vertical">

    <TextView
        android:textSize="25sp"
        android:gravity="center"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:text="Internal storage" />

    <EditText
        android:id="@+id/myInputText"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:gravity="center">
        <requestFocus />
    </EditText>

    <LinearLayout
        android:gravity="center"
        android:layout_gravity="center"
        android:orientation="vertical"
        android:layout_width="match_parent"
        android:layout_height="wrap_content">
        <Button
            android:layout_weight="1"
            android:id="@+id/btnSave"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Lưu vào" />

        <Button
            android:layout_weight="1"
            android:id="@+id/btnDisplay"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Lấy dữ liệu " />
    </LinearLayout>

    <TextView
        android:id="@+id/responseText"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:padding="5dp"
        android:text=""
        android:textAppearance="?android:attr/textAppearanceMedium" />

</LinearLayout>

public class MainActivity extends AppCompatActivity implements View.OnClickListener {
    Button btnSave, btnDisplay;
    EditText myInputText;
    TextView responseText;

    private String filename = "internalStorage.txt";
    private String filepath = "vncoder.vm";
    File myInternalFile;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        initView();
        ContextWrapper contextWrapper = new ContextWrapper(
                getApplicationContext());
        File directory = contextWrapper.getDir(filepath, Context.MODE_PRIVATE);
        myInternalFile = new File(directory, filename);

    }
    private void initView() {
        myInputText = (EditText) findViewById(R.id.myInputText);
        responseText = (TextView) findViewById(R.id.responseText);
        btnSave = (Button) findViewById(R.id.btnSave);
        btnSave.setOnClickListener(this);
        btnDisplay = (Button) findViewById(R.id.btnDisplay);
        btnDisplay.setOnClickListener(this);
    }

    public void onClick(View v) {

        String myData = "";
        switch (v.getId()) {
            case R.id.btnSave:
                try {
                    FileOutputStream fos = new FileOutputStream(myInternalFile);
                    fos.write(myInputText.getText().toString().getBytes());
                    fos.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
                myInputText.setText("");
                responseText
                        .setText("Đã được lưu vào bộ nhớ trong");
                break;

            case R.id.btnDisplay:
                try {
                    FileInputStream fis = new FileInputStream(myInternalFile);
                    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();
                }
                myInputText.setText(myData);
                responseText
                        .setText("Lấy dữ liệu từ bộ nhớ trong");
                break;
        }
    }
}

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width = "fill_parent"
    android:layout_height = "fill_parent"
    android:orientation = "vertical"
    android:gravity = "center">

    <TextView
        android:gravity = "center"
        android:layout_width = "fill_parent"
        android:layout_height = "wrap_content"
        android:text = "External Storage " />

    <EditText

        android:layout_gravity="center"
        android:id = "@+id/myInputText"
        android:layout_width = "match_parent"
        android:layout_height = "wrap_content"
        android:ems = "10"
        android:gravity = "center"
        android:inputType = "textMultiLine"

        >
        <requestFocus />
    </EditText>

    <Button
        android:id="@+id/btnSave"
        android:layout_width = "wrap_content"
        android:layout_height = "wrap_content"
        android:text = "Lưu vào bộ nhớ ngoài" />

    <Button
        android:id = "@+id/btnDisplay"
        android:layout_width = "wrap_content"
        android:layout_height = "wrap_content"
        android:text = "Lấy dữ liệu từ bộ nhớ ngoài" />

    <TextView
        android:id = "@+id/responseText"
        android:layout_width = "wrap_content"
        android:layout_height = "wrap_content"
        android:padding = "5dp"
        android:text = ""
        android:textAppearance = "?android:attr/textAppearanceMedium" />
</LinearLayout>

public class MainActivity extends Activity implements OnClickListener {
    Button btnSave, readFromExternalStorage;
    private String filename = "vncoder.txt";
    private String filepath = "vncoder";
    TextView responseText;
    EditText myInputText;
    File myExternalFile;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        initView();
         if (!isExternalStorageAvailable() || isExternalStorageReadOnly()) {
            btnSave.setEnabled(false);
        } else {
            myExternalFile = new File(getExternalFilesDir(filepath), filename);
        }

    }

    private void initView() {
        myInputText = (EditText) findViewById(R.id.myInputText);
        responseText = (TextView) findViewById(R.id.responseText);
        btnSave = (Button) findViewById(R.id.btnSave);
        btnSave.setOnClickListener(this);
        readFromExternalStorage = (Button) findViewById(R.id.btnDisplay);
        readFromExternalStorage.setOnClickListener(this);
    }
    public void onClick(View v) {

        String myData = "";
        switch (v.getId()) {
            case R.id.btnSave:
                try {
                    FileOutputStream fos = new FileOutputStream(myExternalFile);
                    fos.write(myInputText.getText().toString().getBytes());
                    fos.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
                myInputText.setText("");
                responseText.setText("Dữ liệu đã được lưu vào bộ nhớ ngoài");
                break;

            case R.id.btnDisplay:
                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();
                }
                myInputText.setText(myData);
                responseText.setText("Được lấy ra từ bộ nhớ ngoài");
                break;
        }
    }


    private static boolean isExternalStorageReadOnly() {
        String extStorageState = Environment.getExternalStorageState();
        if (Environment.MEDIA_MOUNTED_READ_ONLY.equals(extStorageState)) {
            return true;
        }
        return false;
    }


    private static boolean isExternalStorageAvailable() {
        String extStorageState = Environment.getExternalStorageState();
        if (Environment.MEDIA_MOUNTED.equals(extStorageState)) {
            return true;
        }
        return false;
    }
}