Retrieving data from an ArrayList and presenting them in a JSP

Question

Dear Fellow Java Developers:
I am developing a catalogue site that would give the user the option of viewing items on a JSP page. The thing is, a user may request to view a certain item of which there are several varieties, for example "shirts". There may be 20 different varieties, and the catalogue page would present the 20 different shirts on the page. However, I want to give the user the option of either viewing all the shirts on a single page, or view a certain number at a time, and view the other shirts on a second or third JSP page.
See the following link as an example:
http://www.eddiebauer.com/eb/cat_default.asp?nv=2|21472|9&tid=&c=&sc=&cm_cg=&lp=n2f
I am able to retrieve the data from the database, and present all of them on a JSP, however I am not sure how to implement the functionality so that they can be viewed a certain number at a time. So if I want to present say 12 items on a page and the database resultset brings back 30 items, I should be able to present the first 12 items on page1, the next 12 items on page2, and the remaining 6 items on page3. How would I do this? Below is my scriplet code that I use to retrieve information from the ArrayList that I retrieve from my database and present them in their entirety on a single JSP page:
<table>
<tr>
<%String product=request.getParameter("item");
ArrayList list=aBean.getCatalogueData(product);
int j=0, n=2;
for(Iterator i=list.iterator(); i.hasNext(); j++){
if(j>n){
out.print("</tr><tr>");
j=0;
Integer id=(Integer)i.next();
String name=(String)i.next();
String productURL=(String)i.next();
out.print("a bunch of html with the above variables embedded inside")
%>
</tr>
</table>
where aBean is an instace of a JavaBean that retrieves the data from the Database.
I have two ideas, because each iteration of the for loop represents one row from the database, I was thinking of introducing another int variable k that would be used to count all the iterations in the for loop, thus knowing the exact number. Once we had that value, we would then be able to determine if it was greater than or less than or equal to the maximum number of items we wanted to present on the JSP page( in this case 12). Once we had that value would then pass that value along to the next page and the for loop in each subsequent JSP page would continue from where the previous JSP page left off. The other option, would be to create a new ArrayList for each JSP page, where each JSP page would have an ArrayList that held all the items that it would present and that was it. Which approach is best? And more importantly, how would I implement it?
Just wondering.
Thanks in advance to all that reply.
Sincerely;
Fayyaz

Answer 1

-You said to pass two parameters in the request,
"start", and "count". The initial values for "start"
would be zero, and the inital value for "count" would
be the number of rows in the resultSet from the
database, correct?Correct.
-I am a little fuzzy about the following block of code
you gave:
//Set start and count for next page
start += count; // If less than count left in array, send the number left to next next page
count = ((start*3)+(count*3) < list.size()) ? (count) : ((list.size() - (start*3))/3)
Could you explain the above block of code a little
further please?Okay, first, I was using the ternary operators (boolean) ? val_if_true : val_if_false;
This works like an if() else ; statement, where the expression before the ? represents the condition of the if statement, the result of the expression directly after the ? is returned if the condition is true, and the expression after the : is returned if the condition is false. These two statments below, one using the ternary ? : operators, and the other an if/else, do the same thing:
count = ((start*3)+(count*3) < list.size()) ? (count) : ((list.size() - (start*3))/3);
if ((start*3)+(count*3) < list.size()) count = count;
else count = ((list.size() - (start*3))/3);Now, why all the multiplying by 3s? Because you store three values in your list for each product, 1) the product ID, 2) the product Name, and 3) the product URL. So to look at the third item, we need to be looking at the 9th,10th, and 11th values in the list.
So I want to avoid an ArrayIndexOutOfBounds error from occuring if I try to access the List by an index greater than the size of the list. Since I am working with product numbers, and not the number of items stored in the list, I need to multiply by three, both the start and count, to make sure their sum does not exceed the value stored in the list. I test this with ((start*3)+(count*3) < list.size()) ?. It could have been done like: ((start + count) * 3 < list.size()) ?.
So if this is true, the next page can begin looking through the list at the correct start position and find the number of items we want to display without overstepping the end of the list. So we can leave count the way it is.
If this is false, then we want to change count so we will only traverse the number of products that are left in the list. To do this, we subtract where the next page is going to start looking in the list (start*3) from the total size of the list, and devide that by 3, to get the number of products left (which will be less then count. To do this, I used: ((list.size() - (start*3))/3), but I could have used: ((list.size()/3) - start).
Does this explain it enough? I don't think I used the best math in the original post, and the line might be better written as:
count = ((size + count)*3 < list.size()) ? (count) : ((list.size()/3) - start);All this math would be unnecessary if you made a ProductBean that stored the three values in it.
>
- You have the following code snippet:
//Get the string to display this same JSP, but with new start and count
String nextDisplayURL = encodeRedirectURL("thispage.jsp?start=" + start + "&count=" + count + "&item=" + product);
%>
<a href="<%=nextDisplayURL%>">Next Page</a>
How would you do a previous page URL? Also, I need to
place the "previous", "next" and the different page
number values at the top of the JSP page, as in the
following url:
http://www.eddiebauer.com/eb/cat_default.asp?nv=2|21472
9&tid=&c=&sc=&cm_cg=&lp=n2f
How do I do this? I figure this might be a problem
since I am processing the code in the middle of the
page, and then I need to have these variables right at
the top. Any suggestions?One way is to make new variable names, 'nextStart', 'previousStart', 'nextCount', 'previousCount'.
Calculate these at the top, based on start and count, but leave the ints you use for start and count unchanged. You may want to store the count in the session rather than pass it along, if this is the case. Then you just worry about the start, or page number (start would be (page number - 1) * count. This would be better because the count for the last page will be less than the count on other pages, If we put the count in session we will remember that for previous pages...
I think with the details I provided in this and previous post, you should be able to write the necessary code.
>
Thanks once again for all of your help, I really do
appreciate the time and effort.
Take care.
Sincerely;
Fayyaz