Question

I have the following piece of code. I would like to know what sort of exceptions would be caught inside the catch block in a multithreaded enviorment.

class Goo
{
private Vector objects = new Vector();

public void kempute ()
{
    Iterator iter = objects.iterator();
    while (iter.hasNext()) {
        try {
            Object o = (Object)iter.next();
            System.out.println(o.hashCode());
        }
        catch (Exception ex) {
            //
        }
    }
}

public void addme (Object o)
{
    objects.add(o);
}
}

I really couldnt find any exceptions. Any help is really appreciated.

Answer 1

An exception could be caught at that point if

Object o = (Object)iter.next();

or

System.out.println(o.hashCode());

throw an unchecked exception. Here are a couple of possibilities:

• If you had previously called addObject(null), a call to update() would lead to a NullPointerException when it calls hashCode() on a null reference.

• If you have an instance of Foo shared by two threads, and one calls addObject while the other is calling compute(), then you could get a ConcurrentModificationException.

• If Vector was a custom class and not java.util.Vector, then iter.next() could throw some other unchecked exception … at the whim of the author of said custom class. (OK … unlikely in the light of the OP’s comments.)

• (From aix’s answer) – o.hashCode() could in theory throw any unchecked exception, depending on how o‘s class implements the hashCode() method.

• There may be other possibilities too.

---

If you are merely curious as to what the possible exceptions might be, the above more or less covers it.

If you want to tidy up this code, then it is probably a bad thing to be catching Exception at that point. Each of those cases listed is a sign of a bug here or elsewhere in the application. Rather that catching the exception at that point and (presumably) trying to “make good” the problem, it is better to bomb out with a stacktrace, and then go back and fix the root problem that was being hidden by the catch.