andriod 动态显示当前时间

<?xml version="1.0" encoding="UTF-8"?>

<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="New Text"
        android:id="@+id/time"
        android:layout_gravity="center_horizontal" />
</LinearLayout>

package com.example.yanlei.mytk;

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

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

public class MainActivity extends AppCompatActivity  implements Runnable {

    private Handler handler;
    private TextView textView;
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        textView=(TextView) findViewById(R.id.time);
        handler = new Handler() {
            public void handleMessage(Message msg) {
                textView.setText((String)msg.obj);
            }
        };

        new Thread(this).start();
    }

    @Override
    public void run() {
        // TODO Auto-generated method stub
        try {
            while(true){
                SimpleDateFormat sdf=new SimpleDateFormat("yyyy年MM月dd日   HH:mm:ss");
                String str=sdf.format(new Date());
                handler.sendMessage(handler.obtainMessage(100,str));
                Thread.sleep(1000);
            }
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}