Why does java.util.EmptyStackException get thrown in this code?

HelloWorld · #1 (**permalink**) 07-23-2007, 10:52 PM

Here's the first code that I had:

PHP Code:

                      for (int i = 0; i < array.length; i++) {
                        Object o1 = s.pop();
                        Object o2 = q.dequeue();
                        String num1 = (String) o1;
                        String num2 = (String) o2;
                        if (!num1.equals(num2)) {
                            result = "FALSE";
                        } else {
                            result = "TRUE";
                        }
                    }

I have no clue why does this get thrown:

Code:

Exception in thread "main" java.util.EmptyStackException
    at java.util.Stack.peek(Unknown Source)
    at java.util.Stack.pop(Unknown Source)
    at MidtermPracticeMain.<init>(MidtermPracticeMain.java:48)
    at MidtermPracticeMain.main(MidtermPracticeMain.java:104)

I changed it to this one:

PHP Code:

                      for (int i = 0; i < s.size()-1; i++) {
                        if (!s.pop().equals(q.dequeue())) {
                            result = "FALSE";
                        } else {
                            result = "TRUE";
                        }
                    }

Exception doesn't get thrown, (but I still got logic error lol..)
My question is, how can they be different? What's wrong with the first one compared to the second one? Aren't they the same?

sigh... obviously not, but I still can't see the difference...

rpgfan3233 · #2 (**permalink**) 07-23-2007, 11:35 PM

Maybe it has something to do with:

PHP Code:

  for (int i = 0; i < array.length; i++) {

vs

PHP Code:

  for (int i = 0; i < s.size()-1; i++) {

? Without knowing what q, s, array, etc. are, it is somewhat difficult to help you. Please post a bit more code, so that we might better assist you.

Thanks,
rpgfan3233

HelloWorld · #3 (**permalink**) 07-23-2007, 11:38 PM

Thanx rpgfan3233
Yes, that's where the problem starts, it throws me exception if I changed it back to array.length

But, array.length should be the same with s.size() though because I created all of them within this for loop:

PHP Code:

                      StringTokenizer st = new StringTokenizer(line.trim(), " ");
                    int cnt = 0;
                    int length = 0;
                    while (st.hasMoreTokens()) {
                        String num = st.nextToken();
                        s.push(num);
                        q.enqueue(num);
                        v.add(num);
                        array[cnt] = num;
                        ll.add(num);
                        cnt++;
                        length++;
                    }

I edited this already and all works perfectly fine with the length counter lol... logic works correctly, but still wondering why isn't that the same thing..

rpgfan3233 · #4 (**permalink**) 07-24-2007, 12:07 AM

Instead of testing against the size of the stack, why not use a while loop and test whether it is empty. I personally think it would be much safer than the for loop, though I honestly still don't see the difference with that code:

PHP Code:

  while (!s.empty()) {
    Object o1 = s.pop();
    Object o2 = q.dequeue();
    String num1 = (String) o1;
    String num2 = (String) o2;
    if (!num1.equals(num2)) {
        result = "FALSE";
    } else {
        result = "TRUE";
    }
}

I honestly can't see any problem from the code you've presented.

HelloWorld · #5 (**permalink**) 07-24-2007, 12:10 AM

Hey thanx a lot!!
I don't realize that I have this method in stack

PHP Code:

  s.empty()

Will try it out later. Now I gtg bed and do some math lol..

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