How the view are refereed under MainActivity In android

Here the XML code for simple text view

<TextView  
    android:layout_width="fill_parent" 
    android:layout_height="wrap_content" 
    android:text="@string/hello"
    android:id="@+id/test"/>

Then in MainActivity it is used like this

TextView test = (TextView) findViewById(R.id.test);
test.setText("test");

Here I have Doubt how the view accessed in Main class which is defined in xml layout can any explain how it happens