优化这段代码，使其运行结果只显示一行import java.text.SimpleDateFormat; import java.util.Date; class Timer implements Runnable { public void run() { while(true) { SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"); System.out.println(sdf.format(new Date())); try { Thread.sleep(1000); } catch(InterruptedException e) {e.printStackTrace(); } } } } class Watch { public static void main(String[] args) { Thread thread=new Thread(new Timer()); thread.start(); } }

可以将实例化SimpleDateFormat对象和输出语句移到while循环外，避免每次循环都创建对象，同时使用System.out.print()代替System.out.println()，使输出不换行。代码如下：

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

class Timer implements Runnable {
    private SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");

    public void run() {
        while (true) {
            System.out.print(sdf.format(new Date()) + " ");
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

class Watch {
    public static void main(String[] args) {
        Thread thread = new Thread(new Timer());
        thread.start();
    }
}