mainActivity.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=".MainActivity">


    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical">

        <ImageView
            android:id="@+id/imageView"
            android:layout_margin="25dp"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:src="@mipmap/ic_launcher"
            />

        <Button
            android:id="@+id/randomButton"
            android:layout_margin="25dp"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Random"
            android:layout_gravity="center"
            />

    </LinearLayout>

</android.support.constraint.ConstraintLayout>
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