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"
    android:orientation="vertical">

    <TextView
        android:id="@+id/textView"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Simple Java Android Calculator"
        android:textAlignment="center"
        android:textColor="#ff000000"
        android:textSize="20sp"
        android:textStyle="bold"
        app:layout_constraintTop_toTopOf="parent" />

    <EditText
        android:id="@+id/num1edit_text"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="First number"
        app:layout_constraintTop_toBottomOf="@id/textView" />

    <EditText
        android:id="@+id/num2edit_text"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="Second number"
        app:layout_constraintTop_toBottomOf="@id/num1edit_text" />

    <EditText
        android:id="@+id/answer_textview"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:focusable="false"
        android:hint="Answer"
        app:layout_constraintTop_toBottomOf="@id/num2edit_text" />


    <LinearLayout
        android:id="@+id/linear"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal"
        app:layout_constraintTop_toBottomOf="@id/answer_textview">
        <Button
            android:id="@+id/buttonmultiply"
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:layout_weight="1"
            android:layout_margin="4dp"
            android:layout_marginEnd="7dp"
            android:background="@android:color/holo_orange_light"
            android:text="Multiply"/>

        <Button
            android:id="@+id/buttonadd"
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:layout_weight="1"
            android:layout_margin="4dp"
            android:layout_marginStart="26dp"
            android:layout_marginEnd="12dp"
            android:text="Add" />

        <Button
            android:id="@+id/buttonsubtract"
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:layout_weight="1"
            android:layout_margin="4dp"
            android:background="@android:color/holo_orange_light"
            android:text="Subtract" />

        <Button
            android:id="@+id/buttondivide"
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:layout_weight="1"
            android:layout_margin="4dp"
            android:layout_marginEnd="8dp"
            android:background="@android:color/holo_orange_light"
            android:text="Divide" />
    </LinearLayout>



    <Button
        android:id="@+id/buttonclear"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:background="@android:color/holo_blue_light"
        android:gravity="center"
        android:text="Clear"
        app:layout_constraintTop_toBottomOf="@id/linear" />


</androidx.constraintlayout.widget.ConstraintLayout>