title: ConcurrentModificationException when adding items to adapter on Android
body: <p>In this method, I try to loop over <code>listSong</code> to find the singer ID:</p>
  
  <pre><code>private void getAllSongBySinger() {
      SongService songService = ServiceGenerator.createService(SongService.class);
      Call&lt;SongRespones&gt; callAllSong = songService.findAllSongBySinger();
      callAllSong.enqueue(new Callback&lt;SongRespones&gt;() {
          @Override
          public void onResponse(Call&lt;SongRespones&gt; call, Response&lt;SongRespones&gt; response) {
              listSong = new ArrayList&lt;&gt;();
              songRespones = response.body();
              listSong = songRespones.getSongs();
              for (SongRespones.Songs songs : listSong) {
                  if (songs.getSinger().getId() == getSingerId) {
                      listSong.add(songs);
                      adapter.addMoreItem(listSong);
                      rvSongBySinger.setAdapter(adapter);  
                  }
              }
          }
          @Override
          public void onFailure(Call&lt;SongRespones&gt; call, Throwable t) {
              t.printStackTrace();
          }
      });
  }
  </code></pre>
  
  <p>But it gives me this is error:</p>
  
  <pre><code>08-11 18:44:54.852 2280-2280/com.vichit.khmersong E/AndroidRuntime: FATAL EXCEPTION: main
  Process: com.vichit.khmersong, PID: 2280
  java.util.ConcurrentModificationException
      at java.util.ArrayList$ArrayListIterator.next(ArrayList.java:573)
      at com.vichit.khmersong.fragment.main.DetailSingerSongFragment$1.onResponse(DetailSingerSongFragment.java:76)
      at retrofit2.ExecutorCallAdapterFactory$ExecutorCallbackCall$1$1.run(ExecutorCallAdapterFactory.java:70)
      at android.os.Handler.handleCallback(Handler.java:739)
      at android.os.Handler.dispatchMessage(Handler.java:95)
      at android.os.Looper.loop(Looper.java:148)
      at android.app.ActivityThread.main(ActivityThread.java:5417)
      at java.lang.reflect.Method.invoke(Native Method)
      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
  </code></pre>
  
  <p>How can I add <code>listSong</code> to adapter after the loop?</p>