Android Working with Linear Layout

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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.a002.MainActivity"
    android:orientation="vertical">

    <EditText
        android:layout_width="match_parent"
        android:layout_height="wrap_content" 
        android:hint="@string/to_hint"/>
    
    <EditText
        android:layout_width="match_parent"
        android:layout_height="wrap_content" 
        android:hint="@string/subject_hint"/>
    
    <EditText
        android:layout_width="match_parent"
        android:layout_height="wrap_content" 
        android:hint="@string/message_hint"
        android:layout_weight="1"
        android:gravity="top|left"/>
    
    <Button
        android:layout_width="100dp"
        android:layout_height="wrap_content"
        android:hint="@string/buttonSend_text"
        android:layout_gravity="right"/>

</LinearLayout>

References
https://www.youtube.com/watch?v=DVRN3EuF9ys&index=7&list=PLshdtb5UWjSp0879mLeCsDQN6L73XBZTk
https://github.com/mhdr/AndroidSamples/tree/master/002