title: How to make a callback between Activity and Fragment?
body: <p>I have this interface in my activity.</p>
  
  <pre><code>public interface LogoutUser {
      void logout();
  }
  </code></pre>
  
  <p>My fragment implements this interface, so in my fragment, I have this:</p>
  
  <pre><code>@Override
  public void logout() {
      // logout
  }
  </code></pre>
  
  <p>In my activity I call </p>
  
  <pre><code>mLogoutUser.logout();
  </code></pre>
  
  <p>Where <code>mLogoutUser</code> is of the type LogoutUser interface.</p>
  
  <p>My issue is the <code>mLogoutUser</code> object that is null. How can initialize it?</p>
  
  <p>Thank you!</p>