handler.postDelayed(runnable,1000);

package com.skex.handlerclock;

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;

public class MainActivity extends AppCompatActivity {
    private TextView tv_Clock;
    private Handler handler=new Handler();
    private Runnable runnable=new Runnable() {
        @Override
        public void run() {
            Date day=new Date();
            SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
            tv_Clock.setText(df.format(day));
            handler.postDelayed(runnable,1000);
        }
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        tv_Clock = (TextView) findViewById(R.id.tv_Clock);
        handler.post(runnable);

    }

    @Override
    protected void onDestroy(){
        handler.removeCallbacks(runnable);
        super.onDestroy();
    }
}