MainActivity.java

package com.example.tanialeif.memoriaandroid;

import android.Manifest;
import android.content.pm.PackageManager;
import android.os.Environment;
import android.support.annotation.NonNull;
import android.support.v4.app.ActivityCompat;
import android.support.v4.content.ContextCompat;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.EditText;
import android.widget.RadioButton;
import android.widget.RadioGroup;
import android.widget.Toast;

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

public class MainActivity extends AppCompatActivity {

    EditText txtArchivo;
    RadioGroup optg;
    RadioButton optSeleccionado;
    final int READ_WRITE_PERMISSION = 777; //Este permiso deja a cualquiera leer y escribir
    boolean externalWritePermission = false;

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

        permisos();

        txtArchivo = (EditText) findViewById(R.id  .txtArchivo);
        optg = (RadioGroup) findViewById(R.id.optgAlmacenamiento);
        Log.d("CICLOVIDA", "onCreate");
    }

    public void btnGuardar_click(View v) {
        if(	optg.getCheckedRadioButtonId() == R.id.optExterna){
            if(externalWritePermission){
                saveExternal();;
            }
            else{
                Toast.makeText(this, "Permiso denegado", Toast.LENGTH_LONG).show();
            }
        }else{
            saveInternal();
        }
    }

    public void btnAbrir_click(View v) {
        if(optg.getCheckedRadioButtonId() == R.id.optExterna ){
            openExternal();
        }else {
            openInternal();
        }
    }

    public void saveExternal() {

        File dirAppPath =  getExternalFilesDir(null);

        Toast.makeText(getBaseContext(), Environment.getExternalStorageState(),
                Toast.LENGTH_LONG).show();

        Toast.makeText(getBaseContext(), dirAppPath.getAbsolutePath(),
                Toast.LENGTH_LONG).show();

        File miArchivo = new File(dirAppPath, "notita.txt");

        try {
            FileOutputStream fos = new FileOutputStream(miArchivo, true);
            OutputStreamWriter osw = new OutputStreamWriter(fos);

            osw.write(txtArchivo.getText().toString());

            osw.flush();
            osw.close();

            Toast.makeText(getBaseContext(), "GUARDADO EXTERNA", Toast.LENGTH_LONG).show();

        } catch (FileNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

    }

    public void saveInternal() {
        File dirAppPath = getFilesDir();
        Toast.makeText(getBaseContext(), dirAppPath.getAbsolutePath(), Toast.LENGTH_LONG).show();

        File miArchivo = new File(dirAppPath, "notita.txt");

        try {
            FileOutputStream fos = new FileOutputStream(miArchivo);
            OutputStreamWriter osw = new OutputStreamWriter(fos);

            osw.write(txtArchivo.getText().toString());

            osw.flush();
            osw.close();

            Toast.makeText(getBaseContext(), "GUARDADO INTERNA", Toast.LENGTH_LONG).show();

        } catch (FileNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

    }

    public void openInternal() {
        File rutaAppFile =  getFilesDir();

        File miArchivo = new File(rutaAppFile, "notita.txt");

        try {
            FileInputStream fis = new FileInputStream(miArchivo);
            InputStreamReader isr = new InputStreamReader(fis);

            char[] bloque = new char[100];
            String texto ="";
            while(isr.read(bloque)!=-1){

                texto += String.valueOf(bloque);
                bloque = new char[100];
            }

            txtArchivo.setText(texto);

            isr.close();
            fis.close();


            Toast.makeText(getBaseContext(), "LEIDO EXTERNA", Toast.LENGTH_LONG).show();

        } catch (FileNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

    public void openExternal() {

        File rutaAppFile =  getExternalFilesDir(null);

        File miArchivo = new File(rutaAppFile, "notita.txt");

        try {
            FileInputStream fis = new FileInputStream(miArchivo);
            InputStreamReader isr = new InputStreamReader(fis);

            char[] bloque = new char[100];
            String texto ="";
            while(isr.read(bloque)!=-1){

                texto += String.valueOf(bloque);
                bloque = new char[100];
            }

            txtArchivo.setText(texto);

            isr.close();
            fis.close();


            Toast.makeText(getBaseContext(), "LEIDO EXTERNA", Toast.LENGTH_LONG).show();

        } catch (FileNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

    }

    public void permisos(){

        //Para comprobar si tengo permisos para escribir en memoria externa
        int permissionCheck = ContextCompat.checkSelfPermission(this,
                        Manifest.permission.WRITE_EXTERNAL_STORAGE);

        //Si la app tiene permiso, el método muestra PackageManager.PERMISSION_GARANTED
        if(permissionCheck != PackageManager.PERMISSION_GRANTED) {
            ActivityCompat.requestPermissions(this,
                    new String[] { Manifest.permission.WRITE_EXTERNAL_STORAGE},
                    READ_WRITE_PERMISSION);
        }
    }


    //Se debe anular el método onRequestePermissionResult para averiguar si se otorgó permisos
    @Override
    public void onRequestPermissionsResult(int requestCode,
                                           @NonNull String[] permissions, @NonNull int[] grantResults) {
        switch (requestCode) {
            case READ_WRITE_PERMISSION: {
                if(grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                    externalWritePermission = true;
                } else {
                    externalWritePermission = false;
                }
            }
        }
    }
}