Android Studio ストップウォッチアプリ制作(動画有り)

今回はストップウォッチアプリを制作しました。
YouTubeに作っている動画をアップしています。
https://www.youtube.com/watch?v=1c0LvmCW5EU&list=PLhg2PHSq8bjjjBpOPll39ZAUgtbnlDOvU

strings.xml

<resources>
    <string name="app_name">StopWatchApp</string>
    <string name="btn_start">START</string>
    <string name="btn_stop">STOP</string>
    <string name="btn_reset">RESET</string>
    <string name="btn_lap">LAP</string>
</resources>

list_items.xml

<?xml version="1.0" encoding="utf-8"?>
<TextView xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:gravity="center"
    android:textSize="24sp"></TextView>

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:weightSum="5"
    tools:context=".MainActivity">

    <TextView
        android:id="@+id/text_timer"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="1"
        android:text="00:00.000"
        android:gravity="center"
        android:textSize="36sp"></TextView>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:gravity="center"
        android:layout_weight="1">

        <Button
            android:id="@+id/btn_start"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_margin="2dp"
            android:onClick="startTimer"
            android:text="@string/btn_start"></Button>

        <Button
            android:id="@+id/btn_stop"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_margin="2dp"
            android:onClick="stopTimer"
            android:text="@string/btn_stop"></Button>

        <Button
            android:id="@+id/btn_reset"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_margin="2dp"
            android:onClick="resetTimer"
            android:text="@string/btn_reset"></Button>

    </LinearLayout>

    <ListView
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="2.5"
        android:id="@+id/list"></ListView>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:gravity="center"
        android:layout_weight="0.5">

        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@string/btn_lap"
            android:id="@+id/btn_lap"
            android:onClick="tapLap"></Button>

    </LinearLayout>

</LinearLayout>

MainActivity.xml

package com.example.stopwatchapp;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.os.Handler;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.ListView;
import android.widget.TextView;

import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Locale;

public class MainActivity extends AppCompatActivity {

    Handler handler = new Handler();
    Runnable runnable;
    Button btn_start, btn_stop, btn_reset;
    long startTime;
    TextView text_timer;
    long t, elapsedTime;
    ArrayList<String> lapTime;
    ListView listView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        btn_start = (Button) findViewById(R.id.btn_start);
        btn_stop = (Button) findViewById(R.id.btn_stop);
        btn_reset = (Button) findViewById(R.id.btn_reset);
        text_timer = (TextView) findViewById(R.id.text_timer);
        listView = findViewById(R.id.list);
        lapTime = new ArrayList<String>();

        btnState(true, false, false);
    }

    public void btnState(boolean start, boolean stop, boolean reset) {
        btn_start.setEnabled(start);
        btn_stop.setEnabled(stop);
        btn_reset.setEnabled(reset);
    }

    public void startTimer(View view) {
        startTime = System.currentTimeMillis();
        runnable = new Runnable() {
            @Override
            public void run() {
                t = System.currentTimeMillis() - startTime + elapsedTime;
                SimpleDateFormat sdf = new SimpleDateFormat("mm:ss.SSS", Locale.US);
                text_timer.setText(sdf.format(t));
                handler.removeCallbacks(runnable);
                handler.postDelayed(runnable, 10);
            }
        };
        handler.postDelayed(runnable, 10);
        btnState(false, true, false);
    }

    public void stopTimer(View view) {
        elapsedTime += System.currentTimeMillis() - startTime;
        handler.removeCallbacks(runnable);
        btnState(true, false, true);
    }

    public void resetTimer(View view) {
        elapsedTime = 0;
        t = 0;
        text_timer.setText("00:00.000");
        btnState(true, false, false);
        lapTime.clear();
        listView.setAdapter(new ArrayAdapter<String>(MainActivity.this,R.layout.list_items,lapTime));
    }

    public void tapLap(View view){
        lapTime.add(text_timer.getText().toString());
        ArrayAdapter<String> adapter =
                new ArrayAdapter<String>(MainActivity.this,R.layout.list_items,lapTime);
        listView.setAdapter(adapter);
    }
}

YouTubeに作っている動画をアップしています。
https://www.youtube.com/watch?v=1c0LvmCW5EU&list=PLhg2PHSq8bjjjBpOPll39ZAUgtbnlDOvU
以上