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"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <Button
        android:id="@+id/signup"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="148dp"
        android:layout_marginTop="8dp"
        android:layout_marginEnd="148dp"
        android:layout_marginBottom="59dp"
        android:onClick="goToSignUp"
        android:text="@string/signup"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/passWord" />

    <EditText
        android:id="@+id/userName"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="84dp"
        android:layout_marginEnd="85dp"
        android:layout_marginBottom="115dp"
        android:background="@color/colorPrimaryDark"
        android:ems="10"
        android:hint="@string/username"
        android:importantForAutofill="no"
        android:inputType="textVisiblePassword"
        app:layout_constraintBottom_toTopOf="@+id/signup"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <EditText
        android:id="@+id/passWord"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="83dp"
        android:layout_marginTop="13dp"
        android:layout_marginEnd="86dp"
        android:layout_marginBottom="121dp"
        android:background="@color/colorPrimaryDark"
        android:ems="10"
        android:hint="@string/password"
        android:importantForAutofill="no"
        android:inputType="textPassword"
        app:layout_constraintBottom_toTopOf="@+id/signup"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="1.0"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/userName" />

    <Button
        android:id="@+id/login"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="148dp"
        android:layout_marginTop="88dp"
        android:layout_marginEnd="148dp"
        android:layout_marginBottom="13dp"
        android:onClick="handleLogin"
        android:text="@string/login"
        app:layout_constraintBottom_toTopOf="@+id/signup"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.0"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/passWord" />
</android.support.constraint.ConstraintLayout>