This Activity already has an action bar supplied by the window decor

I think you're developing for Android Lollipop, but anyway include this line:

<item name="windowActionBar">false</item> 

to your theme declaration inside of your app/src/main/res/values/styles.xml.

Also, if you're using AppCompatActivity support library of version 22.1 or greater, add this line:

<item name="windowNoTitle">true</item>

Your theme declaration may look like this after all these additions:

<!-- Base application theme. --><style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar">    <!-- Customize your theme here. -->    <item name="colorPrimary">@color/colorPrimary</item>    <item name="colorPrimaryDark">@color/colorPrimaryDark</item>    <item name="colorAccent">@color/colorAccent</item>    <item name="windowActionBar">false</item>    <item name="windowNoTitle">true</item></style>

Another easy way is to make your theme a child of Theme.AppCompat.Light.NoActionBar like so:

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

Add single line android:theme="@style/AppTheme.NoActionBar" to activity in AndroidManifest and you've done.

AndroidManifest.xml:

<activity android:name=".activity.YourActivity"          android:theme="@style/AppTheme.NoActionBar"><!-- ADD THIS LINE -->

styles.xml

<style name="AppTheme.NoActionBar">        <item name="windowActionBar">false</item>        <item name="windowNoTitle">true</item></style>