Ejemplo de Android + Firebase 101

activity_main.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: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=".activities.MainActivity">

    <EditText
        android:id="@+id/editText1"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_alignParentTop="true"
        android:layout_centerHorizontal="true"
        android:hint="@string/edit_text_hint" />

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/button_label"
        android:layout_below="@id/editText1"
        android:layout_centerHorizontal="true"  />

    <TextView
        android:id="@+id/textView1"
        android:layout_marginTop="@dimen/activity_vertical_margin"
        android:layout_below="@id/button1"
        android:text="@string/text_view_label"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />
    <TextView
        android:id="@+id/textView2"
        android:layout_marginTop="@dimen/activity_vertical_margin"
        android:text="@string/text_view_initial"
        android:textSize="20sp"
        android:layout_below="@id/textView1"
        android:layout_centerHorizontal="true"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />
</RelativeLayout>