JSP <input type= "checkbox" > Tagged null pointer exception

Today I see a piece of code in a book like this:

This is the code for the <input type= "checkbox" > tag

<BR>Choose the singer you like:<inputtype= "checkbox"name= "Singer"value= "Zhang">singer Zhang<inputtype= "checkbox"name= "Singer"value= "Lee">Lee singer<inputtype= "checkbox"name= "Singer"value= "King">Wang singer

The following is an array implementation

String personname[]=request.getparametervalues ("singer");//Gets the name of the singer, using an array to implement  if (Personname==null)     {        out.println ("One does not like");            }     else     {for           (int i=0;i<=personname. length;i++)       {          out.println ("<br > You like the singer "+personname[i");       }       }     

The result throws a null pointer exception, in the array where the contents of the previous selection box are assigned to the personname[] array, and then not every selection box is selected, this is a multi-box, optional, not selected, did not assign the array, so thrown a null pointer exception. That's how I solved it:

  if (personname==null)     {        out.println ("One does not like");            }     else     {     try{  //Why add a Try Catch statement without an error  without adding a system report null pointer exception for        (int i=0;i<= PersonName. length;i++)       {          out.println ("<br> your favorite singer" +personname[ I]);       }       }       catch (Exception e)       {         e.printstacktrace ();       }     }          Out.println ("<br> This is the hidden message you submitted" +secretmsg);

Catch the exception with a try Catch statement, and the result runs successfully! Although this is a very small problem, but I found a problem, these textbooks are not entirely correct, the true truth is in practice. More hands-on, more practice, more thinking ...

JSP <input type= "checkbox" > Tagged null pointer exception