实现一个很简单的计时器类

计算程序从开始到结束使用了多少时间，是经常要做的事。

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class StopWatch {
    private final static Logger LOG = LoggerFactory.getLogger(StopWatch.class);
//  程序开始运行的时间
    private final long start;

    public StopWatch() {
        start = System.currentTimeMillis();
    }

    public double elapsedTime() {
        long now = System.currentTimeMillis();
        return (now - start) / 1000.0;
    }

    public void logElapsedTime(){
        LOG.info("used time "+elapsedTime()+"s");
    }
}