Extending functionality from existing custom taglib

Question

Hi!
I'm trying to extend the functionality of an already existing custom taglib. To test this possibility I�ve so far created a small taglib containing two tags. QbeTag.java and QbeSumTag.java.
The QbeTag class handles a database connection and constructs a table showing the result of a query passed in as an attribute in the tag. (The database contains a �dummy table� holding information about persons.)
Now let�s pretend that I don�t have the source code for the QbeTag but need to/ want to extend its functionality. For instance:
In the QbeSumTag I want to summarize all the persons ages and present it in a new text field somewhere else on the page.
To do this I probably need to access the QbeTag class from the QbeSumTag class, or is there any other way to achieve this wanted functionality?
Up to this point:
What I�ve tried is to implement the QbeSumTag so that It extends the QbeTag class. This means that I now can access the doStartTag, doEndTag, and so on methods from the QbeSumTag by typing super.doStartTag().
The problem with this approach is that this way only gives me the possibility to run the methods not altering the process it self and I can�t alter the out stream. A syntax like:
public int doStartTag() throws JspException
return super.doStartTag();
Will only result in a call for the QbeTag�s doStartTag() method to run and do it�s stuff.
Is there any way I can solve this or am I stuck? Maybe one approach would be to nest the QbeSumTag with the QbeTag wich would mean that there will be a dependency between the tow tags.
I believe that this question is quite difficult to answer but it could be the starting point for an interesting discussion. Hasn�t anyone tried to extend the functionality of the Jakarta taglib, say the DB tags or Mail tag.
All thoughts and replies will great fully received.
/Daniel

Answer 1

Well, the answer to this is the same as the answer to extending any third party classes... it depends on how the original class was created.
If the original class was designed with extension in mind, then (hopefully) its realy functionality will be isolated to a series of protected methods (that you would have access to) and private methods (you would not have access to).
You would use the public and protected methods to retrieve the data you need to do your work, or override those ones whose work you want to replace (for instance, if it has a protected displayOutput(Map dataMap) method, you are in luck!).
If everything is done giving you know access to the pieces, then you may be stuck providing a tag that has the original tag nested inside it. Buffer the output from the original tag and parse it, retrieving the info you need, then output your own stuff...