1<TextView
2    android:id="@+id/clockTextView"
3    android:layout_width="wrap_content"
4    android:layout_height="wrap_content"
5    android:textSize="48sp"
6    android:text="00:00:00"
7    android:layout_gravity="center"/>

1import android.os.Bundle;
2import android.os.Handler;
3import android.widget.TextView;
4
5import java.text.SimpleDateFormat;
6import java.util.Calendar;
7import java.util.Locale;
8
9public class MainActivity extends AppCompatActivity {
10    private TextView clockTextView;
11    private Handler handler;
12
13    @Override
14    protected void onCreate(Bundle savedInstanceState) {
15        super.onCreate(savedInstanceState);
16        setContentView(R.layout.activity_main);
17
18        clockTextView = findViewById(R.id.clockTextView);
19        handler = new Handler();
20
21        // 時計の更新を開始
22        updateClock();
23    }
24
25    private void updateClock() {
26        handler.postDelayed(new Runnable() {
27            @Override
28            public void run() {
29                // 現在の時刻を取得
30                Calendar calendar = Calendar.getInstance();
31                SimpleDateFormat sdf = new SimpleDateFormat("HH:mm:ss", Locale.JAPAN);
32                String currentTime = sdf.format(calendar.getTime());
33
34                // TextViewに時刻をセット
35                clockTextView.setText(currentTime);
36
37                // 1秒ごとに時計を更新
38                updateClock();
39            }
40        }, 1000); // 1秒ごとに更新
41    }
42}