Android Working with Relative Layout

main_activity.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/activity_main"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="iterator.ir.a003.MainActivity">


    <EditText
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:inputType="textPersonName"
        android:text=""
        android:hint="@string/firstNameHint"
        android:ems="10"
        android:layout_alignParentTop="true"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true"
        android:id="@+id/editTextFirstName" />

    <EditText
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:inputType="textPersonName"
        android:text=""
        android:hint="@string/lastNameHint"
        android:ems="10"
        android:layout_alignParentTop="true"
        android:layout_alignParentRight="true"
        android:layout_alignParentEnd="true"
        android:id="@+id/editTextLastName"
        android:layout_toRightOf="@+id/editTextFirstName"
        android:gravity="center"
        android:layout_toEndOf="@+id/editTextFirstName" />

    <Button
        android:text="@string/buttonSave_text"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@+id/editTextLastName"
        android:id="@+id/buttonSave"
        android:layout_toRightOf="@+id/editTextFirstName"
        android:layout_alignParentRight="true"
        android:layout_alignParentEnd="true" />
</RelativeLayout>

References
https://www.youtube.com/watch?v=QfF_oFPeh7c&list=PLshdtb5UWjSp0879mLeCsDQN6L73XBZTk&index=8
https://github.com/mhdr/AndroidSamples/tree/master/003