New to dynamic class loading - could you help me?

Question

Hi all!
Thanks so much for taking a minute to read my post.
I am a rookie at java and programming.
I have a program that needs to load a class (or java file) dynamically during runtime. (At least I think that is what it needs to do.)
I want the user of the program to be able to specify a .java or a .class file either somewhere on the local disk or network, but not necessarily in the same location as the program itself. Once that file is selected, I would like the program to be able to use that file as if it were a .class file created in the working directory when the program was compiled and created.
I have read a lot of the forums, and I think I need to write my own class loader. I tried creating a class loader, and the code is shown below. (I borrowed this code from some of the other forums I've read.)
Here is my problem.
1. Should I be trying to extend the ClassLoader class? Am I on the right track?
2. If I am on the right track, should I require the user of my program to select a previously compiled ".class" file? Or should I ask them to indicate a ".java" file, and try to compile the java file with the program?
3. I'm sure that if the above two questions are answered, the solution to this problem will become obvious. But for clarity, I should mention that with the current code (below), I keep getting a java.lang.NoClassDefFoundError inside the class FileClassLoader at the line:
c = defineClass (classname, buff, 0, buff.length); Obviously, I am not providing the right information to this FileClassLoader in order to create the class. Can you help me out?
Thank you in advance!
I really appreciate it!
Best Regards,
Julia
class SampleClass
  public void sampleMethod()
   final JFileChooser fc = new JFileChooser();
   int returnVal = fc.showOpenDialog(optionPane);
   if (returnVal == JFileChooser.APPROVE_OPTION) {
     File file = fc.getSelectedFile();
     try
        FileClassLoader loader = new FileClassLoader(file);
           Class cl = loader.loadClass(file.getName(),false);
        }catch ( ClassNotFoundException e ) {
       System.out.println("ClassNotFoundException");
     }catch (IllegalAccessException e) {
       System.out.println("IllegalAccess");      
     }catch( InstantiationException e) {
       System.out.println("Can't Instantiate That");
class FileClassLoader extends ClassLoader
     private File file;
     public FileClassLoader (File thisFile)
          this.file = thisFile;
     protected synchronized Class loadClass(String s, boolean bool) throws ClassNotFoundException
          Class c = null;
          try {
               int size = (int)file.length();
               byte buff[] = new byte[size];
               FileInputStream fis = new FileInputStream(file);
               DataInputStream dis = new DataInputStream (fis);
               dis.readFully (buff);
               dis.close();
               String classname = null;
               String filename = file.getName();
               int i = filename.lastIndexOf('.');
               if(i>0 && i<filename.length()-1)
                    classname = filename.substring(0,i);
               c = defineClass (classname, buff, 0, buff.length);
               resolveClass (c);
          } catch (java.io.IOException e) {
               e.printStackTrace();
          return c;
}

Answer 1

Hi!
If you really need to do that, you can give a look at my previous post at:
http://forum.java.sun.com/thread.jsp?forum=4&thread=285674
You can play with the code to suit your needs.
Hope this helps.