To show the output of the REST API, we need to create a custom list adapter and so we need to design an XML file of the custom list adapter. Here's the XML code for each row in the list:

<?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"
    android:padding="10dp">

    <TextView
        android:id="@+id/name"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:gravity="center_horizontal"
        android:padding="5dp"
        android:textAppearance="?android:textAppearanceMedium"
        android:textStyle="bold"
        app:layout_constraintBottom_toTopOf="@+id/contactNumber"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        tools:text="@tools:sample/full_names" />

    <TextView
        android:id="@+id/contactNumber"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:gravity="center_horizontal"
        android:padding="5dp"
        android:textAppearance="?android:textAppearanceSmall"
        app:layout_constraintBottom_toTopOf="@+id/email"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/name"
        tools:text="@tools:sample/cities" />

    <TextView
        android:id="@+id/email"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:gravity="center_horizontal"
        android:padding="5dp"
        android:textAppearance="?android:textAppearanceSmall"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/contactNumber"
        tools:text="@tools:sample/cities" />


</android.support.constraint.ConstraintLayout>

Here, we have a TextView of name, contactNumber, and email.

After that, we'll create the adapter, named UserListAdapter.kt, as follows:

class UserListAdapter(context: Context,
                      private val userList: List<UserModel>?) : BaseAdapter() {
    private val inflater: LayoutInflater = context.getSystemService(Context.LAYOUT_INFLATER_SERVICE)
            as LayoutInflater
    override fun getView(position: Int, convertView: View?, parent: ViewGroup?): View {
        val rowView = inflater.inflate(R.layout.user_list_item, parent, false)
        val name = rowView.findViewById(R.id.name) as TextView
        val email = rowView.findViewById(R.id.email) as TextView
        val contactNumber = rowView.findViewById(R.id.contactNumber) as TextView
        val userDetails = getItem(position) as UserModel
        name.text = userDetails.name
        email.text = userDetails.email
        contactNumber.text = userDetails.contactNumber
        return rowView
    }
    override fun getItem(position: Int): Any {
        return userList!![position]
    }
    override fun getItemId(position: Int): Long {
        return position.toLong()
    }
    override fun getCount(): Int {
        return userList!!.size
    }
}

This class extends BaseAdapter(), which will add several inherited functions. 

Then you need to add LayoutInflater, which converts the XML layout into corresponding ViewGroups and Widgets: 

• getView() creates a view for a row of the list. Here, you'll define all the UI-based information.
• getItem() returns the position of the list that's obtained from the server.
• getItemId() defines a unique ID for each row in the list.
• getCount() returns the size of the list.

Now, in getView(), you'll add the element of the layout, as follows:

 val name = rowView.findViewById(R.id.name) as TextView
        val email = rowView.findViewById(R.id.email) as TextView
        val contactNumber = rowView.findViewById(R.id.contactNumber) as TextView

You should never perform long-running tasks on the main thread. This will result in an Application Not Responding (ANR).