ボタンを押して日時をテキストビューに表示する単純なもの

Design

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout
        xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:tools="http://schemas.android.com/tools"
        xmlns:app="http://schemas.android.com/apk/res-auto"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:context=".MainActivity">

    <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Hello World!"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintLeft_toLeftOf="parent"
            app:layout_constraintRight_toRightOf="parent"
            app:layout_constraintTop_toTopOf="parent" android:id="@+id/textView_01"/>
    <Button
            android:text="@string/button"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:id="@+id/button_01" android:layout_marginTop="29dp"
            app:layout_constraintTop_toBottomOf="@+id/textView_01" app:layout_constraintStart_toStartOf="parent"
            android:layout_marginLeft="167dp" android:layout_marginStart="167dp" android:layout_marginBottom="279dp"
            app:layout_constraintBottom_toBottomOf="parent" app:layout_constraintEnd_toEndOf="parent"
            android:layout_marginEnd="156dp" android:layout_marginRight="156dp"/>

</androidx.constraintlayout.widget.ConstraintLayout>

MainActivity.kt

package com.example.datepicker

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.widget.Button
import  android.widget.TextView
import java.text.SimpleDateFormat
import java.util.*

class MainActivity : AppCompatActivity() {

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

        val textButton: Button = findViewById(R.id.button_01)
        val messageView: TextView = findViewById(R.id.textView_01)

        textButton.setOnClickListener {

            val df = SimpleDateFormat("yyyy年MM月dd日 HH:mm")
            val dateNow = df.format(Date())
            messageView.text = dateNow

        }
    }
}