why i cant remove title bar in android studio?

I want to do android program that contain no title bar. For some reason I cant remove title bar in android studio. I have already tried this coding.

this.requestWindowFeature(Window.FEATURE_NO_TITLE);

This one also I have tried but the program suddenly stop..

 android:icon="@drawable/icon" 
 android:label="@string/app_name" 
 android:theme="@android:style/Theme.NoTitleBar.Fullscreen"

How can I fix this?