Q:

My notification doesn't show never

I create a notification in my app but when I click the button to show it, never shows and the log cat didn't show any error.  I don't know what to make. 
import android.support.v4.app. NotificationCompat;

Intent resultIntent = new Intent(Intent. ACTION_VIEW, Uri.parse("content://" + fileDir));
    PendingIntent resultPendingIntent =
            PendingIntent.getActivity(
                    this,
                    0,
                    resultIntent, /* <---- el intent que se realiza al hacer click*/
                    PendingIntent. FLAG_UPDATE_CURRENT
            );

    //TODO creamos notificacion
    NotificationCompat. Builder mBuilder =
             new NotificationCompat. Builder(this)
                    .setSmallIcon(R.mipmap.ic_launcher)
                    .setWhen(System.currentTimeMillis())
                    .setTicker("Downloader PRO")
                    .setContentTitle(getText(R.string.notificacion_titulo))
                    .setContentText("prueba")
                    .setContentIntent(resultPendingIntent)
                    .setContentInfo(getText(R.string.notificacion_informacion));

    mBuilder.setSound(RingtoneManager.getDefaultUri(RingtoneManager. TYPE_NOTIFICATION));

    //TODO mostramos la notificacion

    // Sets an ID for the notification
    int mNotificationId = 001;
    // Gets an instance of the NotificationManager service
    NotificationManager mNotifyMgr =
            (NotificationManager) getSystemService(NOTIFICATION_SERVICE);
    // Builds the notification and issues it. 
    mNotifyMgr.notify(mNotificationId, mBuilder.build());

FULL CLASS:
package com.toniapps.downloaderpro;

import android.content. DialogInterface;
import android.content. Intent;
import android.graphics. Bitmap;
import android.graphics. BitmapFactory;
import android.net. Uri;
import android.os. Bundle;
import android.os. Environment;
import android.provider. MediaStore;
import android.support.design.widget. FloatingActionButton;
import android.support.v7.app. AlertDialog;
import android.support.v7.app. AppCompatActivity;
import android.support.v7.widget. Toolbar;
import android.util. Patterns;
import android.view. Menu;
import android.view. MenuItem;
import android.view. View;
import android.widget. EditText;
import android.widget. ImageView;
import android.widget. Toast;

import com.loopj.android.http. AsyncHttpClient;
import com.loopj.android.http. FileAsyncHttpResponseHandler;

import java.io. ByteArrayOutputStream;
import java.io. File;
import java.io. FileOutputStream;
import java.io. IOException;
import java.util. ArrayList;
import java.util.regex. Matcher;
import java.util.regex. Pattern;

import cz.msebera.android.httpclient. Header;

public class MainActivity extends AppCompatActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);

    //recibir información
    Intent intent = getIntent();
    String action = intent.getAction();
    String type = intent.getType();

    if (Intent. ACTION_SEND.equals(action) && type ! = null) {
        if ("text/plain".equals(type)) {
            handleSendText(intent); // Handle text being sent
        } else if (type.startsWith("image/")) {
            handleSendImage(intent); // Handle single image being sent
        }
    }

   FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
    fab.setOnClickListener(new View. OnClickListener() {
        @Override
        public void onClick(View view) {
            descargarB(view);
        }
    });
}

void handleSendText(Intent intent) {
    String sharedText = intent.getStringExtra(Intent. EXTRA_TEXT);
    if (sharedText ! = null) {
        if(sharedText.contains("Tweet")){
            twitter(sharedText);
        }else{
            String enlace = decodificarUrl(sharedText);
            urlCompartida(enlace);
            buscar(enlace);
        }
    }
}

void handleSendImage(Intent intent) {
    Uri imageUri = (Uri) intent.getParcelableExtra(Intent. EXTRA_STREAM);
    if (imageUri ! = null) {
        try {
            Bitmap bitmap = MediaStore. Images. Media.getBitmap(this.getContentResolver(), imageUri);
            guardarImagen(bitmap);
        } catch (IOException e) {
            Toast.makeText(MainActivity.this, e.getMessage(), Toast. LENGTH_LONG).show();
        }
    }
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present. 
    getMenuInflater().inflate(R.menu.menu_main, menu);
    return false;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here.  The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml. 
    int id = item.getItemId();

    //noinspection SimplifiableIfStatement
    if (id == R.id.action_settings) {
        return true;
    }

    return super.onOptionsItemSelected(item);
}
    public void buscarB(View view){
        EditText urlT = (EditText) findViewById(R.id.url_text_view);
        String url = urlT.getText().toString();

        if (url.contains("instagram")){
            instagram();
        }else {
            buscar(url);
        }
    }
    private void buscar(String url){
        AsyncHttpClient client = new AsyncHttpClient();

        client.get(url, new FileAsyncHttpResponseHandler(/* Context */ this) {
            @Override
            public void onFailure(int statusCode, Header[] headers, Throwable throwable, File file) {
                Toast.makeText(MainActivity.this, throwable.getMessage(), Toast. LENGTH_LONG).show();
            }

            @Override
            public void onSuccess(int statusCode, Header[] headers, File file) {
                ImageView imagen = (ImageView) findViewById(R.id.imagen);
                imagen.setImageBitmap(BitmapFactory.decodeFile(file.getAbsolutePath()));
            }
        });
    }
    public void descargarB(View view){
        EditText urlT = (EditText) findViewById(R.id.url_text_view);
        String url = urlT.getText().toString();

        if (url.contains("instagram")){
            instagram();
        }else {
            descargar(url);
        }

    }

private void descargar(String url) {
    AsyncHttpClient client = new AsyncHttpClient();

    client.get(url, new FileAsyncHttpResponseHandler(/* Context */ this) {
        @Override
        public void onFailure(int i, Header[] headers, Throwable throwable, File file) {
            Toast.makeText(MainActivity.this, throwable.getMessage(), Toast. LENGTH_LONG).show();
        }

        @Override
        public void onSuccess(int i, Header[] headers, File file) {

            ImageView imagen = (ImageView) findViewById(R.id.imagen);
            imagen.setImageBitmap(BitmapFactory.decodeFile(file.getAbsolutePath()));

            Bitmap imagenSC = BitmapFactory.decodeFile(file.getAbsolutePath());

            ByteArrayOutputStream bytes = new ByteArrayOutputStream();
            imagenSC.compress(Bitmap. CompressFormat. PNG, 100, bytes);
            File path = Environment.getExternalStoragePublicDirectory(Environment. DIRECTORY_PICTURES + "/Downloader_PRO/");
            path.mkdirs();
            File imageFile = new File(path, (file.getName() + ".png"));
            try {
                imageFile.createNewFile();
                FileOutputStream fo = new FileOutputStream(imageFile);
                fo.write(bytes.toByteArray());
                fo.flush();
                fo.close();
                Toast.makeText(MainActivity.this, imageFile.getAbsolutePath(), Toast. LENGTH_LONG).show();
            } catch (IOException e) {
                e.printStackTrace();
                Toast.makeText(MainActivity.this, e.getMessage(), Toast. LENGTH_LONG).show();
                }
            }
        });
    }
    private void guardarImagen(Bitmap bitmap) {
        ByteArrayOutputStream bytes = new ByteArrayOutputStream();
        bitmap.compress(Bitmap. CompressFormat. PNG, 100, bytes);
        File path = Environment.getExternalStoragePublicDirectory(Environment. DIRECTORY_PICTURES + "/Downloader_PRO/");
        path.mkdirs();
        File imageFile = new File(path, (bitmap.getByteCount()+".png"));
        try {
            imageFile.createNewFile();
            FileOutputStream fo = new FileOutputStream(imageFile);
            fo.write(bytes.toByteArray());
            fo.flush();
            fo.close();
            crearNotificacionDescarga(imageFile.getAbsolutePath());
            Toast.makeText(MainActivity.this, imageFile.getAbsolutePath(), Toast. LENGTH_LONG).show();
        } catch (IOException e) {
            e.printStackTrace();
            Toast.makeText(MainActivity.this, e.getMessage(), Toast. LENGTH_LONG).show();
        }
    }
    private void urlCompartida(String url){
        EditText urlT = (EditText) findViewById(R.id.url_text_view);
        urlT.setText(url);
    }
    private String decodificarUrl(String urlBase){

        ArrayList links = new ArrayList();
        String urlStr ="";
        Pattern p = Patterns. WEB_URL;
        Matcher m = p.matcher(urlBase);
        while(m.find()) {
            urlStr = m.group();
            if (urlStr.startsWith("(") && urlStr.endsWith(")")) {
                urlStr = urlStr.substring(1, urlStr.length() - 1);
            }
            links.add(urlStr);
        }
        return urlStr;
    }
    private void twitter(final String tweetText){
        new AlertDialog. Builder(this)
                .setTitle(getText(R.string. Twitter_titulo))
                .setMessage(getText(R.string. Twitter_mensaje_uno) + "\n" + "\n" + getText(R.string. Twitter_mensaje_dos))
                .setPositiveButton(android. R.string.ok, new DialogInterface. OnClickListener() {
                    public void onClick(DialogInterface dialog, int which) {
                        String enlace = decodificarUrl(tweetText);
                        Intent browserIntent = new Intent(Intent. ACTION_VIEW, Uri.parse(enlace));
                        startActivity(browserIntent);
                        dialog.dismiss();
                    }
                })
                .setNegativeButton(android. R.string.cancel, new DialogInterface. OnClickListener() {
                    public void onClick(DialogInterface dialog, int which) {
                        // do nothing
                    }
                })
                .setIcon(R.drawable.ic_warning_black_48dp)
                .show();
    }
private void instagram(){
    new AlertDialog. Builder(this)
            .setTitle(getText(R.string.instagram_titulo))
            .setMessage(getText(R.string.instagram_mensaje_1) + "\n" + "\n" + getText(R.string.instagram_mensaje_2))
            .setPositiveButton(android. R.string.ok, new DialogInterface. OnClickListener() {
                public void onClick(DialogInterface dialog, int which) {

                }
            })
            .setIcon(R.drawable.ic_warning_black_48dp)
            .show();
}
private void crearNotificacionDescarga(String fileDir){

    new Intent(Intent. ACTION_VIEW, Uri.parse("content://" + fileDir));

}
}

IDK what to make now, in any device works....

A:

Replace the line:
NotificationManager mNotifyMgr = (NotificationManager) getSystemService(NOTIFICATION_SERVICE);

with:
NotificationManager mNotifyMgr = (NotificationManager) getSystemService(Context. NOTIFICATION_SERVICE);

Select Apps in the Settings, select the app you are interested in and set the Show notifications checkbox.