ClockActivity.java

package com.example.beginner1;

import android.os.Handler;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.TextView;

import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.Locale;

public class ClockActivity extends AppCompatActivity {

    private TimerProcessor timerProcessor;

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

        timerProcessor = new TimerProcessor(new Handler(),
                (TextView) findViewById(R.id.clockView));
    }

    @Override
    protected void onStart() {
        super.onStart();
        timerProcessor.start();
    }

    @Override
    protected void onStop() {
        super.onStop();
        timerProcessor.stop();
    }

    public static class TimerProcessor implements Runnable {
        private static final SimpleDateFormat CLOCK_FORMAT
                = new SimpleDateFormat("HH:mm:ss", Locale.getDefault());
        private final Handler handler;
        private final TextView view;

        public TimerProcessor(Handler handler, TextView view) {
            this.handler = handler;
            this.view = view;
        }

        public void start() {
            this.handler.post(this);
        }

        public void stop() {
            this.handler.removeCallbacks(this);
        }

        @Override
        public void run() {
            Date date = new Date();
            this.view.setText(CLOCK_FORMAT.format(date));
            this.handler.postDelayed(this, 1000);
        }
    }
}