How do I remove the title bar from my app?

In my app there is this title bar at the top where the overflow menu would be, but I don't need settings and only have one screen. When I change the theme like described in many other questions I get the old 2.2 theme. I want to have the modern theme just without the bar at the top.

Go to styles.xml and change .DarkActionBar for .NoActionBar

<style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar">
        <item name="colorPrimary">@color/colorPrimary</item>
        <item name="colorPrimaryDark">@color/colorPrimaryDark</item>
        <item name="colorAccent">@color/colorAccent</item>
</style>

becomes

<style name="AppTheme" parent="Theme.AppCompat.Light.NoActionBar">
        <item name="colorPrimary">@color/colorPrimary</item>
        <item name="colorPrimaryDark">@color/colorPrimaryDark</item>
        <item name="colorAccent">@color/colorAccent</item>
</style>

if colors are irrelevant to your app, you can actually go for

<style name="AppTheme" parent="Theme.AppCompat.Light.NoActionBar" />

In the manifest file Change:

    android:theme="@style/AppTheme" >
    android:theme="@style/Theme.AppCompat.NoActionBar"

this.requestWindowFeature(Window.FEATURE_NO_TITLE);

works in onCreate() when put before setContentView() is invoked.

otherwise it crashes

Check this

 ActionBar actionBar = getSupportActionBar();
    actionBar.hide();

In styles.xml file, change DarkActionBar to NoActionBar

    <style name="AppTheme" parent="Theme.AppCompat.Light.NoActionBar">
    <item name="colorPrimary">@color/colorPrimary</item>
    <item name="colorPrimaryDark">@color/colorPrimaryDark</item>
    <item name="colorAccent">@color/colorAccent</item>