Re: why do I get this runtime error

Joshua Cranmer <>
Fri, 05 Oct 2007 21:16:49 GMT
Aryeh M. Friedman wrote:

When I run this:

[ snip ]

 > I get:

Exception in thread "main" java.lang.ClassCastException: scratch.Main
cannot be cast to scratch.Main
          at scratch.Main.main(

Fun exceptions happen when working with class loaders.

The short answer:
Multiple class loaders don't mix. One can change the default class
loader through some java -D option; I don't remember what it is however.

The long answer:
A class is uniquely determined by its instantiating class loader and its
fully-qualified name.

This is an example of a working loadClass function. I did not write this
myself, and it is based off of a very old version of Java (discussion of
custom class loaders is almost nil), so I do not know which parts are

public final Class<?> loadClass(String name, boolean resolve) throws
     ClassNotFoundException {
     // Don't bother loading Java classes
     if (name.startsWith("java.") || name.startsWith("javax."))
         return super.loadClass(name, resolve);

     // Did we already define it?
     Class result = findLoadedClass(name);
     if (result == null) {
         String internalName = name.replace('.', '/') + ".class";
         InputStream is = getParent().getResourceAsStream(internalName);
         if (is != null) {
             try {
                 // Load the raw bytes.
                 ByteArrayOutputStream bos = new ByteArrayOutputStream();
                 int b;
                 while ((!=-1)
                 byte[] bytes = bos.toByteArray();
                 result = defineClass(name, bytes, 0, bytes.length);
             } catch (IOException ex) {
                 throw new ClassNotFoundException(name +
                     " could not be loaded", ex);

     if (result != null) {
         if (resolve) {
     } else {
         // Let someone else find it.
         result = super.loadClass(name, resolve);
     return result;

Even here, it is still incompatible with system-default loaded classes.
Beware of bugs in the above code; I have only proved it correct, not
tried it. -- Donald E. Knuth

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