實現步驟：

1.使用System.currentTimeMillis()可以獲取系統當前的時間
2.開啟一個執行緒，執行緒每隔一秒傳送一次訊息
3.通過handler更新TextView上顯示的時間。

程式碼示例：

public class DeomActivity extends Activity {

    private static final int KEY = 1;
    private TextView tv_time;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super
 
.onCreate(savedInstanceState);
        setContentView(R.layout.mian);
        tv_time = (TextView) findViewById(R.id.tv_time);
        new TimeThread().start();
    }

    public class TimeThread extends Thread {
        @Override
        public void run () {
            do {
                try {
                    Thread.sleep(1000
 
);
                    Message msg = new Message();
                    msg.what = KEY;
                    handler.sendMessage(msg);
                }
                catch (InterruptedException e) {
                    e.printStackTrace();
                }
            } while(true);
        }
    }

    private
 
 Handler handler = new Handler() {
        @Override
        public void handleMessage (Message msg) {
            super.handleMessage(msg);
            switch (msg.what) {
                case KEY :
                    long sysTime = System.currentTimeMillis();
                    CharSequence sysTimeStr = DateFormat.format("hh:mm:ss", sysTime);
                    tv_time.setText(sysTimeStr);
                    break;
                default:
                    break;
            }
        }
    };
}