activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.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=".ui.MainActivity">

    <TextView
        android:id="@+id/tvVisualizadorOperacion"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:gravity="start|center"
        android:minHeight="100dp"
        android:padding="8dp"
        android:textColor="@android:color/white"
        android:textSize="26sp"
        app:layout_constraintBottom_toTopOf="@id/tvVisualizadorResultado"
        tools:text="2*3+6/2-9" />

    <TextView
        android:id="@+id/tvVisualizadorResultado"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:gravity="end|center"
        android:minHeight="50dp"
        android:padding="8dp"
        android:textColor="@android:color/white"
        android:textSize="22sp"
        app:layout_constraintBottom_toTopOf="@id/btnLimpiar"
        tools:text="0" />

    <Button
        android:id="@+id/btnLimpiar"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:text="@string/limpiar"
        app:layout_constraintBottom_toTopOf="@id/btnNueve"
        app:layout_constraintEnd_toStartOf="@id/btnBorrar"
        app:layout_constraintWidth_percent="0.25" />

    <Button
        android:id="@+id/btnBorrar"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:text="@string/borrar"
        app:layout_constraintBottom_toTopOf="@id/btnDivision"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintWidth_percent="0.25" />

    <Button
        android:id="@+id/btnDivision"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:text="@string/division"
        app:layout_constraintBottom_toTopOf="@id/btnMultiplicacion"
        app:layout_constraintStart_toEndOf="@id/btnNueve"
        app:layout_constraintWidth_percent="0.25" />

    <Button
        android:id="@+id/btnNueve"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:text="@string/nueve"
        app:layout_constraintBottom_toTopOf="@id/btnSeis"
        app:layout_constraintStart_toEndOf="@id/btnOcho"
        app:layout_constraintWidth_percent="0.25" />

    <Button
        android:id="@+id/btnOcho"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:text="@string/ocho"
        app:layout_constraintBottom_toTopOf="@id/btnCinco"
        app:layout_constraintStart_toEndOf="@id/btnSiete"
        app:layout_constraintWidth_percent="0.25" />

    <Button
        android:id="@+id/btnSiete"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:text="@string/siete"
        app:layout_constraintBottom_toTopOf="@id/btnCuatro"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintWidth_percent="0.25" />

    <Button
        android:id="@+id/btnMultiplicacion"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:text="@string/multiplicacion"
        app:layout_constraintBottom_toTopOf="@id/btnSuma"
        app:layout_constraintStart_toEndOf="@id/btnSeis"
        app:layout_constraintWidth_percent="0.25" />

    <Button
        android:id="@+id/btnSeis"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:text="@string/seis"
        app:layout_constraintBottom_toTopOf="@id/btnTres"
        app:layout_constraintStart_toEndOf="@id/btnCinco"
        app:layout_constraintWidth_percent="0.25" />

    <Button
        android:id="@+id/btnCinco"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:text="@string/cinco"
        app:layout_constraintBottom_toTopOf="@id/btnDos"
        app:layout_constraintStart_toEndOf="@id/btnCuatro"
        app:layout_constraintWidth_percent="0.25" />

    <Button
        android:id="@+id/btnCuatro"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:text="@string/cuatro"
        app:layout_constraintBottom_toTopOf="@id/btnUno"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintWidth_percent="0.25" />

    <Button
        android:id="@+id/btnSuma"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:text="@string/suma"
        app:layout_constraintBottom_toTopOf="@id/btnResta"
        app:layout_constraintStart_toEndOf="@id/btnTres"
        app:layout_constraintWidth_percent="0.25" />

    <Button
        android:id="@+id/btnTres"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:text="@string/tres"
        app:layout_constraintBottom_toTopOf="@id/btnIgual"
        app:layout_constraintStart_toEndOf="@id/btnDos"
        app:layout_constraintWidth_percent="0.25" />

    <Button
        android:id="@+id/btnDos"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:text="@string/dos"
        app:layout_constraintBottom_toTopOf="@id/btnCero"
        app:layout_constraintStart_toEndOf="@id/btnUno"
        app:layout_constraintWidth_percent="0.25" />

    <Button
        android:id="@+id/btnUno"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:text="@string/uno"
        app:layout_constraintBottom_toTopOf="@id/btnPunto"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintWidth_percent="0.25" />

    <Button
        android:id="@+id/btnResta"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:text="@string/resta"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintStart_toEndOf="@id/btnIgual"
        app:layout_constraintWidth_percent="0.25" />

    <Button
        android:id="@+id/btnIgual"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:text="@string/igual"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintStart_toEndOf="@id/btnCero"
        app:layout_constraintWidth_percent="0.25" />

    <Button
        android:id="@+id/btnCero"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:text="@string/cero"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintStart_toEndOf="@id/btnPunto"
        app:layout_constraintWidth_percent="0.25" />

    <Button
        android:id="@+id/btnPunto"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:text="@string/punto"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintWidth_percent="0.25" />

</androidx.constraintlayout.widget.ConstraintLayout>