用jsp 做计算器时

用jsp 做计算器时，两个操作数是用double 类型的，可是运算结果有时小数点后面数太多，求有什么使运算结果精确点？

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Java code:
import javax.swing.*;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
import java.awt.*;
import java.util.Date;
import java.text.SimpleDateFormat;

/**
* 秒表功能
* @author Administrator
*
*/
public class StopWatch extends JFrame {
JButton btnStart, btnStop;
JLabel label;
Timer timer;

public StopWatch() {
label = new JLabel("00:00:00.000");
btnStart = new JButton("start");
btnStop = new JButton("stop");
final int delay = 100;
final Date startTime = new Date();
final SimpleDateFormat sdf = new SimpleDateFormat("HH:mm:ss.S");
final Action taskPerformer = new AbstractAction() {
public void actionPerformed(ActionEvent evt) {
// 显示时间
Date d = new Date(System.currentTimeMillis()
- startTime.getTime() - 28800000);
label.setText(sdf.format(d));
label.repaint();
}
};
btnStart.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent evt) {
startTime.setTime(new Date().getTime());