android kotlin - How to filter string

MainActivity.kt

package com.cfsuman.kotlintutorials

import android.os.Bundle
import android.text.method.ScrollingMovementMethod
import android.widget.TextView
import androidx.appcompat.app.AppCompatActivity


class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        // get the widgets reference from XML layout
        val textView = findViewById<TextView>(R.id.textView)

        // make textview content scrollable
        textView.movementMethod = ScrollingMovementMethod()


        val string = "Lorem Ipsum is simply dummy text of the printing" +
                " and typesetting industry. Lorem Ipsum has been the" +
                " industry's standard dummy text ever since the 1500s," +
                " when an unknown printer took a galley of type" +
                " and scrambled it to make a type specimen book."

        textView.text = string

        // Returns a string containing only those characters
        // from the original string that match the given predicate.

        // Filter digits from string.
        // Return a string with only digits.
        val filteredText = string.filter { it.isDigit() }
        textView.append("\n\nFilter digits.\n")
        textView.append(filteredText)


        // Returns a string containing only those characters from
        // the original string that do not match the given predicate.

        // Filter not digits from string.
        // Return a string without digits
        val filterNotText = string.filterNot{ it.isDigit() }
        textView.append("\n\nFilter not digits.\n")
        textView.append(filterNotText)
    }
}

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:background="#F8F8F8"
    android:padding="24dp">

    <TextView
        android:id="@+id/textView"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:fontFamily="sans-serif"
        android:textSize="22sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>