activity_main.xmlMainActivity.java
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout 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="com.codinginflow.countdowntimerexample.MainActivity">

    <TextView
        android:id="@+id/text_view_countdown"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerHorizontal="true"
        android:layout_centerVertical="true"
        android:text="00:00"
        android:textColor="@android:color/black"
        android:textSize="60sp" />

    <Button
        android:id="@+id/button_start_pause"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@+id/text_view_countdown"
        android:layout_centerHorizontal="true"
        android:text="start" />

    <Button
        android:id="@+id/button_reset"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@+id/text_view_countdown"
        android:layout_marginStart="11dp"
        android:layout_toEndOf="@+id/button_start_pause"
        android:text="reset"
        android:visibility="invisible"
        tools:visibility="visible" />

</RelativeLayout>

package com.codinginflow.countdowntimerexample;

import android.os.CountDownTimer;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

import java.util.Locale;

public class MainActivity extends AppCompatActivity {
    private static final long START_TIME_IN_MILLIS = 600000;

    private TextView mTextViewCountDown;
    private Button mButtonStartPause;
    private Button mButtonReset;

    private CountDownTimer mCountDownTimer;

    private boolean mTimerRunning;

    private long mTimeLeftInMillis = START_TIME_IN_MILLIS;

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

        mTextViewCountDown = findViewById(R.id.text_view_countdown);

        mButtonStartPause = findViewById(R.id.button_start_pause);
        mButtonReset = findViewById(R.id.button_reset);

        mButtonStartPause.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if (mTimerRunning) {
                    pauseTimer();
                } else {
                    startTimer();
                }
            }
        });

        mButtonReset.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                resetTimer();
            }
        });

        updateCountDownText();
    }

    private void startTimer() {
        mCountDownTimer = new CountDownTimer(mTimeLeftInMillis, 1000) {
            @Override
            public void onTick(long millisUntilFinished) {
                mTimeLeftInMillis = millisUntilFinished;
                updateCountDownText();
            }

            @Override
            public void onFinish() {
                mTimerRunning = false;
                mButtonStartPause.setText("Start");
                mButtonStartPause.setVisibility(View.INVISIBLE);
                mButtonReset.setVisibility(View.VISIBLE);
            }
        }.start();

        mTimerRunning = true;
        mButtonStartPause.setText("pause");
        mButtonReset.setVisibility(View.INVISIBLE);
    }

    private void pauseTimer() {
        mCountDownTimer.cancel();
        mTimerRunning = false;
        mButtonStartPause.setText("Start");
        mButtonReset.setVisibility(View.VISIBLE);
    }

    private void resetTimer() {
        mTimeLeftInMillis = START_TIME_IN_MILLIS;
        updateCountDownText();
        mButtonReset.setVisibility(View.INVISIBLE);
        mButtonStartPause.setVisibility(View.VISIBLE);
    }

    private void updateCountDownText() {
        int minutes = (int) (mTimeLeftInMillis / 1000) / 60;
        int seconds = (int) (mTimeLeftInMillis / 1000) % 60;

        String timeLeftFormatted = String.format(Locale.getDefault(), "%02d:%02d", minutes, seconds);

        mTextViewCountDown.setText(timeLeftFormatted);
    }
}