<?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"
    tools:context=".MainActivity">

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="ここに時計が表示される"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>

package com.example.yamato200605a

import android.icu.util.Calendar
import android.os.Build
import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.os.Handler
import androidx.annotation.RequiresApi
import kotlinx.android.synthetic.main.activity_main.*
import kotlin.concurrent.timer

class MainActivity : AppCompatActivity() {
    @RequiresApi(Build.VERSION_CODES.N)
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        val handler = Handler()
        timer(name = "testTimer", period = 1000) {
            val calendar = Calendar.getInstance()
            val hour = calendar.get(Calendar.HOUR_OF_DAY)
            val minute = calendar.get(Calendar.MINUTE)
            val second = calendar.get(Calendar.SECOND)
            handler.post {
                textView.text = "${hour}時 ${minute}分 ${second}秒"
            }
        }
    }
}