我在JSP中调用了下面的函数，当传入参数guessNum的值为null时，程序就会出错，不知为何，请大家指教。
 public int[] checkNumber(String guessNum,String answerNum)
    {
      if(guessNum.length()!=4) return new int[] {-3,0,0};
      int num=0;
      try{
        num=Integer.parseInt(guessNum);
        }
      catch(NumberFormatException ex){
        return new int[] {-2,0,0};
        }
      int[] timesDigit={0,0,0,0,0,0,0,0,0,0};
      for (int i=0;i1)
          return new int[] {-1,0,0};
        }
        
      int a=0;
      int b=0;
      for (int j=0;j=0) b++;
       }
       
       if (a==4) return new int[] {0,a,b};
       else
        return new int[] {1,a,b};
   }

出错信息如下：
HTTP Status 500 - 

--------------------------------------------------------------------------------

type Exception report

message 

description The server encountered an internal error () that prevented it from fulfilling this request.

exception 

org.apache.jasper.JasperException
at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:248)
at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:289)
at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:240)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:853)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:247)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:193)

........
root cause 

java.lang.NullPointerException
at org.apache.jsp.debug_jsp.checkNumber(debug_jsp.java:25)
at org.apache.jsp.debug_jsp._jspService(debug_jsp.java:112)
at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:136)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:853)
at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:204)
at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:289)
at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:240)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:853)

.........

String guessNum

在判断语句中中加上

if(guessNum==null)

当guessNum为null的时间是要抛出NullPointerException的