A simple working demonstration of using a Handler for the purposes of a count down timer

MainActivity.java

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

import java.util.Locale;

public class MainActivity extends AppCompatActivity implements View.OnClickListener {

    private TextView textView;
    private Button btnStart;
    private Handler countDownTimer;
    private Runnable runnable;
    private final int TIMER_DELAY = 1000;
    private int duration = 20;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        textView = (TextView)findViewById(R.id.text_view);
        btnStart = (Button)findViewById(R.id.button_start) ;

        btnStart.setOnClickListener(this);

        countDownTimer = new Handler();

        runnable = new Runnable() {
            @Override
            public void run() {
                if (duration > 0){
                    countDownTimer.postDelayed(this, TIMER_DELAY);
                    textView.setText(String.format(Locale.getDefault(), "%d seconds", duration));
                    duration--;
                }else{
                    countDownTimer.removeCallbacks(runnable);
                    textView.setText("Time's up!");
                    letsDoSomething();
                }
            }
        };
    }

    @Override
    public void onClick(View v) {
        if (v == btnStart){
            countDownTimer.post(runnable);
        }
    }
    
    private void letsDoSomething(){
        /* to do on timer finish */
    }
}