title: Android-Notification doesn't appear
body: <p>This is a code for creating a notification by click a button and then turn to another class. The second class it totally empty. I don't know why it doesn't appear even it cannot turn to another class.</p>
  
  <p>I have already checked the "sittings" of this file and the "notification" is "on". Could anyone help me to check it?</p>
  
  <p>MainActivity.class</p>
  
  <pre><code>package com.example.ww.lab;
  
  import android.app.Notification;
  import android.app.PendingIntent;
  import android.content.Intent;
  import android.support.v4.app.NotificationManagerCompat;
  import android.support.v7.app.AppCompatActivity;
  import android.os.Bundle;
  import android.app.NotificationManager;
  import android.support.v4.app.NotificationCompat;
  import android.util.Log;
  import android.view.View;
  import android.widget.Button;
  
  public class MainActivity extends AppCompatActivity {
      private int id = 1;
      private NotificationCompat.Builder notification_builder;
      private NotificationManagerCompat notification_manager;
      Button button;
  
      @Override
      protected void onCreate(Bundle savedInstanceState) {
          super.onCreate(savedInstanceState);
          setContentView(R.layout.activity_main);
  
          button = (Button) findViewById(R.id.button);
          button.setOnClickListener(new View.OnClickListener() {
              @Override
              public void onClick(View v) {
                  notification_manager.notify(id, notification_builder.build());
              }
          });
  
          Intent open_activity_intent = new Intent(this, NotificationActivity.class);
          PendingIntent pending_intent = PendingIntent
              .getActivity(this, 0, open_activity_intent, PendingIntent.FLAG_CANCEL_CURRENT);
  
          notification_builder = new NotificationCompat.Builder(this)
              .setSmallIcon(R.drawable.ic_launcher)
              .setContentTitle("Notification Title")
              .setContentText("Notification Body")
              .setAutoCancel(true)
              .setContentIntent(pending_intent);
  
          notification_manager = NotificationManagerCompat.from(this);
      }
  }
  </code></pre>
  
  <p>activity_main.xml</p>
  
  <pre><code> &lt;?xml version="1.0" encoding="utf-8"?&gt;
      &lt;android.support.constraint.ConstraintLayout       
      xmlns:android="http://schemas.android.com/apk/res/android"
      xmlns:app="http://schemas.android.com/apk/res-auto"
      xmlns:tools="http://schemas.android.com/tools"
      android:layout_width="match_parent"
      android:layout_height="match_parent"
      tools:context="com.example.ww.lab.MainActivity"&gt;
  
      &lt;Button
          android:layout_width="wrap_content"
          android:layout_height="wrap_content"
          android:text="Do it!"
          android:id="@+id/button"
          android:onClick="sendNotification"/&gt;
      &lt;/android.support.constraint.ConstraintLayout&gt;
  </code></pre>