jsp问题

在netBeans上写了一个简单的网页计算器，但是javaBean里有个错误，哪位大侠帮帮改一下~~~
代码如下：
package Bean;
public class SimpleCalculator{
 private String first;
 private String second;
 private double result;
 private String operator;
 public void setFirst(String first){
  this.first=first;
}
.........
//下面是操作
public void calculate(){
 double one=Double.parseDouble(first); // 说这一行有错误：java.long.NullPointerException
 double two=Double.parseDouble(second);
try{
  if(operator.equals("+"))result=one+two;
  if(operator.equals("-"))result=one-two;
  if(operator.equals("*"))result=one*two;
  if(operator.equals("/"))result=one/two;
}
catch(Exception e){}
}
}
高手帮忙