Checkboxes: Layout File
CheckBox class
A checkbox is a specific type of two-states button that can be either checked or unchecked.
There are four IDs declared in this layout file:
Selecting
iPhone &
Android and

pushing
DISPLAY

⇒

MyApplication/app/src/main/res/layout/activity_main.xml 
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
  xmlns:android="http://schemas.android.com/apk/res/android"
  android:layout_width  = "match_parent"
  android:layout_height = "match_parent"
  android:orientation   = "vertical"
  android:gravity       = "center">
  <CheckBox
    android:id            = "@+id/chkIos"
    android:layout_width  = "wrap_content"
    android:layout_height = "wrap_content"
    android:text          = "iPhone" />
  <CheckBox
    android:id            = "@+id/chkAndroid"
    android:layout_width  = "wrap_content"
    android:layout_height = "wrap_content"
    android:text          = "Android"
    android:checked       = "true" />
  <CheckBox
    android:id            = "@+id/chkWindows"
    android:layout_width  = "wrap_content"
    android:layout_height = "wrap_content"
    android:text          = "Windows Phone" />
  <Button
    android:id            = "@+id/btnDisplay"
    android:layout_width  = "wrap_content"
    android:layout_height = "wrap_content"
    android:text          = "DISPLAY" />
</LinearLayout>