Question on EL

[Veena Pointi]

In HFSJ chapter test question 6

   
<% List list=new ArrayList();
list.add("a");
list.add("2");
list.add("c");
request.setAttribute("list",list);
request.setAttribute("listIdx","1");
<%-- insert code here --%>

Which ,inserted at line 18 ,are valid and evaluate to c?

A.${list.2}
B.${list[2]}
C.${list.listIdx+1}
D.${list[listIdx+1]}
E.${list['listIdx'+1]}
F.${list[list[listIdx]]}

Answers are B,D,F .
 

My question is why not E....what is the difference between D and E?

Thanks
Veena

[Ankit Garg]

Do you remember the statement from the HFSJ book, whatever inside the [] which is not in quotes will be evaluated. Thus in ${list[listIdx+1]} listIdx will evaluate to 1 but in ${list['listIdx'+1]}, it will try to concatenate 1 to the String listIdx and since there's no string concatenation in EL, you'll get an error...

[Veena Pointi]

I understood if it is not within quotes it will be evaluated.But I didn't understand why option E is incorrect.The explanation given in the book is "Option E is incorrect because EL tries to coerce 'listIdx' to a Long which is invalid "....why it tries to coerce it to Long?

I get following exception for Option E

javax.servlet.ServletException: An exception occured trying to convert String "listIdx" to type "java.lang.Long"

[avi sinha]

Veena Pointi wrote:I understood if it is not within quotes it will be evaluated.But I didn't understand why option E is incorrect.The explanation given in the book is "Option E is incorrect because EL tries to coerce 'listIdx' to a Long which is invalid "....why it tries to coerce it to Long?

I get following exception for Option E

javax.servlet.ServletException: An exception occured trying to convert String "listIdx" to type "java.lang.Long"

in EL 'xyz' has the same meaning as that of "xyz" . hence here it will not look for any attribute named xyz in any scope.i.e. you are trying to add a string and a number here.
now EL will try to convert the string into a numeric value so that it can be added to the number provided which is not possible here.

avi sinha

[Chinmaya Chowdary]

Hi, Veena.

....why it tries to coerce it to Long?

I think, the designers used 'Long' inorder to support huge values. See coersion rules.

[Veena Pointi]

Thanks alot Avi,Chinmay for replieng.It cleared my doubts.

Thanks again
Veena