java: using a variable's value as an object name (not the eval() way)

Question

Ok, so coming from a background of mostly perl, and mostly writing dirty little apps to automate my tasks, I've read the pages about the evils of eval(), and I always use a hash (in perl). I'm currently working on a little project (mostly for me and a couple of other technical people at work), for creating "canned response" e-mails. To allow for additions, subtractions, edits, etc., I'd like to essentially describe the response form(s) in XML, and have my app parse the XML and create the response forms at runtime. I want to use Java (to integrate it into an existing Java tool that I created), and boiled down to a trivial example, what I'm trying to do is take some XML like:

<Form Name="first" Title="Title!">
  <Label Name="before">Your Request:</Label>
  <Textbox Name="input"/>
  <Label Name="after">has been completed.</Label>
  <Output>%before%%input%%after%</Output>
</Form>
<Form Name="second">
...

and from parsing that, I want to create a JFrame named first, which contains a JLabel named before with the obvious text, then a textbox, then another JLabel... you get the idea (I eventually want to use the output tag to control exactly how the response is formatted).

I can parse the XML, and get the element name and such, but I don't know how to instantiate the Objects with a name that is the value of a variable, effectively:

JFrame $(thisNode.getAttributes().getNamedItem("Name").getNodeValue()) = new JFrame(thisNode.getAttributes().getNamedItem("Title").getNodeValue());

I've read basically the whole first page of google results on java reflection, but I haven't come across anyone doing quite what I'm looking for (at least not that I could tell). Having basically zero experience with reflection, I'm curious if this is something that can be accomplished using it, or if I should take the same approach as I would in Perl, and create a HashMap or HashTable of Objects, and tie them to a entry in a Hash of JFrames. Or, I'm open to ideas that don't fall into those two categories. The Hash is sort of my stand-by answer, because I've done it in Perl plenty of times, and I'm sure I can make it work in Java, but if there's a feature (like reflection) that's made to do this task, then why not do it the way it was intended to be done?

Answer 1

What you're asking isn't possible in Java. It doesn't work that way and these sorts of tricks, which are common in dynamic languages, aren't the Java way. You can certainly do:

JFrame frame = JFrameBuilder.buildFromTemplate("frame.xml");

where you create a JFrameBuilder class that reads the XML and creates an object from it but the variable name can't be dynamic. You have to remember that there are two steps in Java.

1. Java source files are compiled into bytecode;
2. The bytecode is read by a Java interpreter (JVM) and executed.

What you want is essentially asking to execute code in step (1). Now annotations can do things in a compile step (like adding interfaces, implementing methods and so on) but local variable naming is not one of those things.

Answer 2

You could (not necessarily that you should) generate Java source based on your XML, compile the generated code, and finally, execute the compiled code. This could be more efficient if you saved the generated .class files and reused them instead of parsing the XML every time the program is run (it can check the timestamp on the XML and only generate and compile if it's been modified since the last code generation).