免费闲来麻将作弊器:如何把String 转换成double类型?

我在上一个页面填入数据，在这个页面想读取并进行计算
因为有小数，我应该如何转换成才能计算？
double a=Double.parseDouble(request.getParameter("qzdghl"));
我这样写不对，不知道问题在哪？老报错，请各位帮帮忙，谢了
String a=request.getParameter("qzdghl");
double a1=Double.parseDouble(a);
String b=request.getParameter("qprice");
double b1=Double.parseDouble(b);
String c=request.getParameter("qservice");
double c1=Double.parseDouble(c);
String d=request.getParameter("qmoral");
double d1=Double.parseDouble(d);
double e=a1+b1+c1+d1;
报错：java.lang.NullPointerException
at java.lang.FloatingDecimal.readJavaFormatString(Unknown Source)
at java.lang.Double.parseDouble(Unknown Source)
at org.apache.jsp.tlast$jsp._jspService(tlast$jsp.java:120)
at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:107)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:853)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:243)
at java.lang.Thread.run(Unknown Source)