Create button with round background

To create button with round background, create /res/drawable/roundbackground.xml to define round background drawable.

<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android"
    android:shape="rectangle" 
    android:padding="10dp">
    <solid android:color="@android:color/background_dark"/> 
    <corners
        android:bottomRightRadius="20dp"
        android:bottomLeftRadius="20dp"
        android:topLeftRadius="20dp"
        android:topRightRadius="20dp"/>
</shape>

Apply "roundbackground" to the background of button.

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent" >

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerHorizontal="true"
        android:layout_centerVertical="true"
        android:padding="@dimen/padding_medium"
        android:text="@string/hello_world"
        tools:context=".MainActivity" />
    <Button 
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_margin="5dp"
        android:background="@drawable/roundbackground"
        android:text="Button with round background"
        android:textColor="@android:color/white"
        />

</RelativeLayout>