How to create a Stopwatch App using Android Studio

package com.gfg.stopwatch_app_java;

import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import androidx.appcompat.app.AppCompatActivity;
import androidx.core.graphics.Insets;
import androidx.core.view.ViewCompat;
import androidx.core.view.WindowInsetsCompat;

public class MainActivity extends AppCompatActivity {

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

        Button btnGoToStopwatch = findViewById(R.id.btnGoToStopwatch);
        btnGoToStopwatch.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                // Create an Intent to start StopwatchActivity
                Intent intent = new Intent(MainActivity.this, StopwatchActivity.class);
                startActivity(intent);
            }
        });
    }
}StopwatchActivity.javapackage com.gfg.stopwatch_app_java;

import android.app.Activity;
import android.os.Handler;
import android.view.View;
import android.os.Bundle;
import java.util.Locale;
import android.widget.TextView;

public class StopwatchActivity extends Activity {

    // Use seconds, running and wasRunning respectively
    // to record the number of seconds passed,
    // whether the stopwatch is running and
    // whether the stopwatch was running
    // before the activity was paused.

    // Number of seconds displayed
    // on the stopwatch.
    private int seconds = 0;

    // Is the stopwatch running?
    private boolean running;

    private boolean wasRunning;

    @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_stopwatch);
        if (savedInstanceState != null) {

            // Get the previous state of the stopwatch
            // if the activity has been
            // destroyed and recreated.
            seconds
                    = savedInstanceState
                    .getInt("seconds");
            running
                    = savedInstanceState
                    .getBoolean("running");
            wasRunning
                    = savedInstanceState
                    .getBoolean("wasRunning");
        }
        runTimer();
    }

    // Save the state of the stopwatch
    // if it's about to be destroyed.
    @Override
    public void onSaveInstanceState(
            Bundle savedInstanceState)
    {
        savedInstanceState
                .putInt("seconds", seconds);
        savedInstanceState
                .putBoolean("running", running);
        savedInstanceState
                .putBoolean("wasRunning", wasRunning);
    }

    // If the activity is paused,
    // stop the stopwatch.
    @Override
    protected void onPause()
    {
        super.onPause();
        wasRunning = running;
        running = false;
    }

    // If the activity is resumed,
    // start the stopwatch
    // again if it was running previously.
    @Override
    protected void onResume()
    {
        super.onResume();
        if (wasRunning) {
            running = true;
        }
    }

    // Start the stopwatch running
    // when the Start button is clicked.
    // Below method gets called
    // when the Start button is clicked.
    public void onClickStart(View view)
    {
        running = true;
    }

    // Stop the stopwatch running
    // when the Stop button is clicked.
    // Below method gets called
    // when the Stop button is clicked.
    public void onClickStop(View view)
    {
        running = false;
    }

    // Reset the stopwatch when
    // the Reset button is clicked.
    // Below method gets called
    // when the Reset button is clicked.
    public void onClickReset(View view)
    {
        running = false;
        seconds = 0;
    }

    // Sets the NUmber of seconds on the timer.
    // The runTimer() method uses a Handler
    // to increment the seconds and
    // update the text view.
    private void runTimer()
    {

        // Get the text view.
        final TextView timeView
                = (TextView)findViewById(
                R.id.time_view);

        // Creates a new Handler
        final Handler handler
                = new Handler();

        // Call the post() method,
        // passing in a new Runnable.
        // The post() method processes
        // code without a delay,
        // so the code in the Runnable
        // will run almost immediately.
        handler.post(new Runnable() {
            @Override

            public void run()
            {
                int hours = seconds / 3600;
                int minutes = (seconds % 3600) / 60;
                int secs = seconds % 60;

                // Format the seconds into hours, minutes,
                // and seconds.
                String time
                        = String
                        .format(Locale.getDefault(),
                                "%d:%02d:%02d", hours,
                                minutes, secs);

                // Set the text view text.
                timeView.setText(time);

                // If running is true, increment the
                // seconds variable.
                if (running) {
                    seconds++;
                }

                // Post the code again
                // with a delay of 1 second.
                handler.postDelayed(this, 1000);
            }
        });
    }
}