title: Dagger injection for different activities
body: <p>So I am learning Dagger 2 and there is one thing I dont quite follow. 
  So I understand that in the component you need to specifiy the activity that can be used for injecting into:</p>
  
  <pre><code>Singleton
  @Component(modules={AppModule.class, NetModule.class})
  public interface NetComponent {
     void inject(MainActivity activity);
  }
  </code></pre>
  
  <p>First of all why do I need to specify this ? (I dont ever user the variable activity).</p>
  
  <p>But the main question is lets say I have 10 different activities or fragments do I need to list them all here ? Is this the correct way of doing it or perhaps could we inject into the Application ? What is the correct thing to do if I want to use the NetComponent throughout my application.</p>
  
  <p>Thanks</p>