activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout 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:background="@color/primary"
    tools:context=".MainActivity">

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="200dp"
        android:gravity="center"
        android:orientation="horizontal">

        <TextView
            android:id="@+id/hour"
            android:layout_width="50dp"
            android:layout_height="50dp"
            android:background="@drawable/round_back_black10_10"
            android:gravity="center"
            android:text="00"
            android:textColor="#FFFFFF"
            android:textSize="30sp"
            android:textStyle="bold" />

        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginStart="10dp"
            android:layout_marginEnd="10dp"
            android:text=":"
            android:textColor="#FFFFFF"
            android:textSize="30sp"
            android:textStyle="bold" />

        <TextView
            android:id="@+id/min"
            android:layout_width="50dp"
            android:layout_height="50dp"
            android:background="@drawable/round_back_black10_10"
            android:gravity="center"
            android:text="00"
            android:textColor="#FFFFFF"
            android:textSize="30sp"
            android:textStyle="bold" />

        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginStart="10dp"
            android:layout_marginEnd="10dp"
            android:text=":"
            android:textColor="#FFFFFF"
            android:textSize="30sp"
            android:textStyle="bold" />

        <TextView
            android:id="@+id/second"
            android:layout_width="50dp"
            android:layout_height="50dp"
            android:background="@drawable/round_back_black10_10"
            android:gravity="center"
            android:text="00"
            android:textColor="#FFFFFF"
            android:textSize="30sp"
            android:textStyle="bold" />

    </LinearLayout>

    <androidx.appcompat.widget.AppCompatButton
        android:id="@+id/startBtn"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true"
        android:layout_centerHorizontal="true"
        android:layout_marginBottom="100dp"
        android:background="@color/secondary"
        android:text="Start"
        android:textAllCaps="false"
        android:textColor="#FFFFFF" />
</RelativeLayout>

drawable/round_back_black10_10.xml

<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android"
    android:shape="rectangle">

    <solid android:color="#1AFFFFFF"/>
    <corners android:radius="10dp"/>

</shape>

MainActivity.java

import android.os.Bundle;
import android.os.CountDownTimer;
import android.view.View;
import android.widget.TextView;
import android.widget.Toast;

import androidx.appcompat.app.AppCompatActivity;
import androidx.appcompat.widget.AppCompatButton;

import java.util.Locale;
import java.util.concurrent.TimeUnit;

public class MainActivity extends AppCompatActivity {

    // set end time in seconds
    private int duration = 120;
    private boolean timerRunning = false;

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

        final TextView hour = findViewById(R.id.hour);
        final TextView min = findViewById(R.id.min);
        final TextView seconds = findViewById(R.id.second);
        final AppCompatButton startBtn = findViewById(R.id.startBtn);

        startBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                // check if countdown timer is already running or not
                if (!timerRunning) {
                    timerRunning = true;
                    new CountDownTimer(duration * 1000, 1000) {
                        @Override
                        public void onTick(long millisUntilFinished) {

                            runOnUiThread(new Runnable() {
                                @Override
                                public void run() {

                                    // getting time format on HH:MM:SS 
                                    String time = String.format(Locale.getDefault(), "%02d:%02d:%02d", TimeUnit.MILLISECONDS.toHours(millisUntilFinished),
                                            TimeUnit.MILLISECONDS.toMinutes(millisUntilFinished) -
                                                    TimeUnit.HOURS.toMinutes(TimeUnit.MILLISECONDS.toHours(millisUntilFinished)),
                                            TimeUnit.MILLISECONDS.toSeconds(millisUntilFinished) -
                                                    TimeUnit.MINUTES.toSeconds(TimeUnit.MILLISECONDS.toMinutes(millisUntilFinished)));

                                    final String[] hourMinSec = time.split(":");

                                    hour.setText(hourMinSec[0]);
                                    min.setText(hourMinSec[1]);
                                    seconds.setText(hourMinSec[2]);
                                }
                            });
                        }

                        @Override
                        public void onFinish() {

                            // reset timer duration
                            duration = 120;

                            timerRunning = false;
                        }
                    }.start();
                } else {
                    Toast.makeText(MainActivity.this, "Timer is already running", Toast.LENGTH_SHORT).show();
                }

            }
        });
    }
}

Output