LoginViewExample_step1_activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="it.enricocandino.loginviewexample.MainActivity">

    <LinearLayout
        android:layout_width="200dp"
        android:layout_height="wrap_content"
        android:orientation="vertical"
        android:background="#EEEEEE"
        android:padding="10dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent">

        <EditText
            android:id="@+id/username_edittext"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:hint="username" />

        <EditText
            android:id="@+id/password_edittext"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:hint="password"
            android:inputType="textPassword" />

        <Button
            android:id="@+id/login_button"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="Login" />

    </LinearLayout>

</android.support.constraint.ConstraintLayout>