Reflection is usually a waste of effort, you'd be better off using
normal Java or going the whole hog to a dynamically-typed language.
Reflection loses static type safety, and doesn't gain the apparent
advantages of dynamic languages either.

Anyway,

You can see whether a Method is var args, using the rather cryptic
method:
http://java.sun.com/j2se/1.5.0/docs/api/java/lang/reflect/Method.html#isVarArgs()

So if (that's true) method.invoke(new Object[]{arrayList.toArray()});
else method.invoke(arrayList.toArray());