How to hide the title bar for an Activity in XML with existing custom theme

I now did the following.

I declared a style inheriting everything from my general style and then disabling the titleBar.

<style name="generalnotitle" parent="general">
    <item name="android:windowNoTitle">true</item>
</style>

Now I can set this style to every Activity in which I want to hide the title bar overwriting the application wide style and inheriting all the other style informations, therefor no duplication in the style code.

To apply the style to a particular Activity, open AndroidManifest.xml and add the following attribute to the activity tag;

<activity
    android:theme="@style/generalnotitle">

Do this in your onCreate() method.

//Remove title bar
this.requestWindowFeature(Window.FEATURE_NO_TITLE);

//Remove notification bar
this.getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN, WindowManager.LayoutParams.FLAG_FULLSCREEN);

//set content view AFTER ABOVE sequence (to avoid crash)
this.setContentView(R.layout.your_layout_name_here); 

this refers to the Activity.

You can modify your AndroidManifest.xml:

<activity android:name=".MainActivity"
          android:label="@string/app_name"
          android:theme="@android:style/Theme.Black.NoTitleBar.Fullscreen">

or use android:theme="@android:style/Theme.Black.NoTitleBar" if you don't need a fullscreen Activity.

Note: If you've used a 'default' view before, you probably should also change the parent class from AppCompatActivity to Activity.