Program to display stop watch |Android Program|

Activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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"
    android:background="#000000"
    android:orientation="vertical">

    <TextView
        android:id="@+id/timerValue"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Stop watch"
        android:textSize="30sp"
        android:layout_marginTop="60sp"
        android:layout_gravity="center"
        app:layout_constraintBottom_toTopOf="@+id/pauseButton"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.388"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.535"
        android:textColor="#ffffff"/>

    <Button
        android:id="@+id/startButton"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="50sp"
        android:text="start"
        android:layout_gravity="center"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
        android:id="@+id/pauseButton"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="stop"
        android:layout_gravity="center"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="1.0"
        app:layout_constraintStart_toEndOf="@+id/startButton"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.512" />

    <Button
        android:id="@+id/resetButton"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:text="reset"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toEndOf="@+id/pauseButton"
        app:layout_constraintTop_toTopOf="parent" />
</LinearLayout>

MainActivity.java

package com.example.stopwatch;
import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
import android.os.Handler;
import android.os.SystemClock;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {
     Button startButton;
    Button pauseButton;
    Button resetButton;
     TextView timerValue;
     long startTime=0L;
     Handler customHandler=new Handler();
    long timeInMilliseconds=0L;
    long timeSwapBuff=0L;
    long updatedTime=0L;

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

        timerValue=(TextView)findViewById(R.id.timerValue);

        startButton=(Button)findViewById(R.id.startButton);

        startButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                startTime= SystemClock.uptimeMillis();
                customHandler.postDelayed(updateTimerThread,0);
            }
        });
        resetButton=findViewById(R.id.resetButton);
        resetButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

                 startTime=0L;
                 timeInMilliseconds=0L;
                timeSwapBuff=0L;
                 updatedTime=0L;
               



                timerValue.setText("00:00:00");
            }
        });
        pauseButton=findViewById(R.id.pauseButton);
        pauseButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

                timeSwapBuff+=timeInMilliseconds;
                customHandler.removeCallbacks(updateTimerThread);
            }
        });

    }

    private Runnable updateTimerThread=new Runnable() {
        @Override
        public void run() {

            timeInMilliseconds=SystemClock.uptimeMillis()-startTime;

            updatedTime=timeSwapBuff+timeInMilliseconds;

            int secs=(int)(updatedTime/1000);
            int mins=secs/60;
            secs=secs%60;
            int milliseconds=(int)(updatedTime%1000);
            timerValue.setText(""+mins+":"+String.format("%02d",secs)+":"+String.format("%03d",milliseconds));
            customHandler.postDelayed(this,0);
        }
    };
}

Design