title: <activity> without <intent-filter> in AndroidManifest.xml
body: <p>I know I need to use  to enable an activity to receive an Intent like this (not main activity).</p>
  
  <pre><code>&lt;activity android:name=".MyApp_2ndActivity"&gt;
      &lt;intent-filter&gt;
           &lt;action android:name="android.intent.action.VIEW" /&gt;
           &lt;category android:name="android.intent.category.DEFAULT" /&gt;
      &lt;/intent-filter&gt;
  &lt;/activity&gt;
  </code></pre>
  
  <p>However, I found it can be triggered too if "intent-filter" is removed, like this.</p>
  
  <pre><code>&lt;activity android:name=".MyApp_2ndActivity"&gt;
  &lt;/activity&gt;
  </code></pre>
  
  <p>I am wondering what difference is in these 2 formats?</p>