CODEWITHSUNDEEP
javastack
Poetical Scientist
Poetical Scientist

Reputation: 39

Java Stack throwing EmptyStackException after push

Below is just one little chunk of my code, I am trying to understand why if I add in the line of code that is commented out it throws an EmptyStackException. I need to add the item at the top of oneStack to the top of twoStack if there are two null values in a row (generated by calling items.getNextItem()). Any insight into why this breaks? Or how I can get the top value of oneStack to also be the top value of twoStack at that point?

I have tried putting that line of code in an if, assigning a variable for the value of oneStack.peek(), but none of that has helped either. It is almost as if that one commented out line is emptying the whole stack (??).

Main point: if in that commented out line of code I swap oneStack.peek() with any other value, it works just fine. So why does it not work with oneStack.peek()?

oneStack.push(firstItem);
twoStack.push(firstItem);
nextItem = items.getNextItem();
oneStack.push(nextItem);
twoStack.push(nextItem);
while (!done) {
        if (oneStack.peek() == null) {
            oneStack.pop();
            oneStack.pop();
            twoStack.pop();
            twoStack.push(oneStack.peek()); // the commented out line below causes this line to throw an EmptyStackException if uncommented.
            newItem = items.getNextItem();
            oneStack.push(nextItem);
            if (oneStack.peek() == null) {
                oneStack.pop();
                twoStack.pop();
                //twoStack.push(oneStack.peek()); // if I uncomment this it breaks, but this needs to happen for twoStack to be correct
            } else {
                twoStack.push(nextItem);
            }
        } else if (oneStack.peek() == targetItem) {
            done = true;
        } else {
            nextItem = items.getNextItem();
            oneStack.push(nextItem);
            twoStack.push(nextItem);
        }

This is the way that items are being generated:

item1, item2, item3, item4, item5, null, null, item6, item7

In the end, this is what is left of the stacks: oneStack: item1, item2, item6, item7

twoStack: item1, item2, item3, item4, item5, item4, item6, item7 (item3 is missing)

twoStack should be: item1, item2, item3, item4, item5, item4, item3, item6, item7

Upvotes: 0

Views: 408

Answers (3)

Ousmane D.
Ousmane D.

Reputation: 56423

I recommend before attempting to pop anything off the stack or peek, you first check that it is not empty. At least it would prevent the exception.

Upvotes: 1

bichito
bichito

Reputation: 1426

Prior to loop:

oneStack = item1, item1 (if item1 is in items) or item2
twoStack = item1, item1 (if item1 is in items) or item2 let's assume item2

Size is two entering loop.

First, second, third, fourth times around:

oneStack = item1, item2 item3 item4 item5 null
twoStack = item1, item2 item3 item4 item5 null

Fifth time

oneStack = item1, item2 item3 item4 
twoStack = item1, item2 item3 item4 item5 item4

Sixth time

oneStack = item1, item2 item3 item4 null
twoStack = item1, item2 item3 item4 item5 item4 null

Seventh time around:

oneStack = item1, item2 item3 
twoStack = item1, item2 item3 item4 item5 item4 item3

eighth and nineth times around

oneStack = item1, item2 item3 item6 item7
twoStack = item1, item2 item3 item4 item5 item4 item3 item6 item7

As you can see it is unclear what the contents of items are at the beginning. I would print to console each pop and the whole stacks after each round. Also print the contents of the stacks and items before entering the loop.

Upvotes: 0

odin
odin

Reputation: 338

Presumably your stack is empty after popping off the last thing, then peeking throws an exception because there is nothing in the stack left to peek.

Upvotes: 0

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