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My dequeue method currently does not delete the item I wish for it too, instead it deletes the last element from the collection. For example,

If I add in the elements: 1, 2, 3 My toString method will return 1, 2, 3 as expected.

Then when I use my driver to call dequeue, it should dequeue the 0th element, 1 in this case.

Although, the method says "Removed element 1 from the queue" prompting that the T result variable has embodied the correct value, although the method does not work as expected, as when calling the toString method after to print the contents, it will print: 1, 2

Then when I call the enqueue method again, on the same queue, if I enqueue the String 3, it will print this as the new queue: 3, 2, 3

I am confused as to where my logic error is, as I assume it is with the extreme case of the collection being filled, but I still run into errors with my dequeue method when the collection is not at max. I attached my code below.

public class QueueRA<T> implements QueueInterface<T> { protected T[] items; protected int front, back, numItems; @SuppressWarnings("unchecked") public QueueRA() { front = 0; numItems = 0; back = 0; items = (T[]) new Object[3]; } @Override public boolean isEmpty() { return numItems == 0; } @Override public void enqueue(T newItem) throws QueueException { if(numItems == items.length) { resize(); enqueue(newItem); } else { items[back] = newItem; back = (back + 1) % items.length; numItems++; } } @Override public T dequeue() throws QueueException { T result; if(numItems != 0) { result = items[front]; items[front] = null; front = (front + 1) % items.length; numItems--; } else { throw new QueueException("The queue does not contain any elements."); } return result; } @SuppressWarnings("unchecked") @Override public void dequeueAll() { back = 0; front = 0; numItems = 0; items = (T[]) new Object[3]; } @Override public T peek() throws QueueException { T result; if(numItems != 0) { result = items[front]; } else { throw new QueueException("The queue does not contain any elements."); } return result; } /** * */ @SuppressWarnings("unchecked") protected void resize() { T[] newItems = (T[]) new Object[numItems+4]; for(int i = 0; i < numItems; i++) { newItems[i] = items[i]; } this.front = 0; this.back = numItems; this.items = newItems; } /** * */ public String toString() { String toReturn = ""; for(int i = 0; i < numItems; i++) { if( (i+1) == numItems) { toReturn = toReturn.concat(items[i] + " "); } else { toReturn = toReturn.concat(items[i] + ", "); } } return toReturn; } }
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    Obviously your resize is wrong... At the first look you are setting front to 0 but copy array as is(without reordering) Commented Oct 12, 2020 at 23:40
  • @Selvin The resize should not effect the dequeue method though, in the terms of only having the three elements in the queue, correct? I am not really following how fixing the resize would fix the other errors. Sorry for sounding dumb I just have been at this for a while and still can't nip it. Commented Oct 12, 2020 at 23:46

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Your toString() and resize() methods are wrong.

In your example code, you created an arraySize of 3 initially and then added 1, 2, and 3. This occupies all the 3 slots in the array and your numItems is set to 3. The front is set to 0 and back is also set to 0 because after adding 3, your algorithm is:

back = (back+1)%items.length;

back was initially 2. Now it is calculated as:

-> (2+1)%3 -> 3%3 -> 0

So your back is 0 at this moment.

Now, when you call dequeue(), the front pops 1 out. So now your array looks like this:
items[0] -> empty
items[1] -> 2
items[2] -> 3
back = 0
front = 1

So, when you enque next and add 3, your enqueue() method checks that number of items in the system is less than size of the array (2 < 3) and adds 3 to the location pointed by back (which is 0 now) and increments it to 1. So, your array looks like this now:
items[0] -> 3
items[1] -> 2
items[2] -> 3
back = 1
front = 1

Now, in your toString() method, without considering the values of front and back, you start from 0 to end:

for(int i = 0; i < numItems; i++) { . . . }

What you need to be doing is start at i = front. if front < back, that means that you travel lineraly from "front" to "back" and print everything. If front > back then you do two loops:

for(int i=front; i < arr.length; i++) { // Code to print arr[i] } for(int i=0; i < back; i++) { // Code to print arr[i] }

This way, it will print from front to end and then from 0 to back.

Also, your resize method is wrong for the same reason. You cannot copy from 0 to numItems, you need to start the copy at "front" and then progress like how I wrote for the toString(). Doing this will ensure that your queue's order is preserved across multiple resizes.

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Adding to @Arun Subramanian's answer.

Your resize() method was simply copying all of the elements sequentially from items to newItems, then setting front = 0 and back = numItems. Which would have been fine if you had implemented the queue sequentially. However, your implementation of a queue is not sequential, it's a circular / ring implementation. Therefor you have to copy the elements in a circular way.

One way to do this is mentioned in @ArunSubramanian's answer, "If front < back, that means that you travel linearly from front to back and print everything. If front > back then you do two loops." Another way is to use a do-while loop with modular arithmetic, as in the following:

@SuppressWarnings("unchecked") protected void resize() { T[] newItems = (T[]) new Object[numItems + 4]; int i = 0; // index used to copy items to newItems // do-while used in case the queue is full (i.e. front == back) do { newItems[i] = items[front]; // modular arithmetic used to circle back on items front = (front + 1) % items.length; i += 1; } while(front != back); this.front = 0; this.back = numItems; this.items = newItems; }

Similarly your toString() method can be implemented in the following way:

@Override public String toString() { String toReturn = ""; // need this check, otherwise do-while will illegally access items[0] if(!isEmpty()) { int len = items.length; int i = front; // do-while used in case front == back do { String delim = ((i+1) % len == back) ? " " : ", "; toReturn += items[i] + delim; i = (i+1) % len; // modular arithmetic used to circle back } while(i != back); } return toReturn; }
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Thank you for the clarification, I was obviously getting stuck on the circular fashion using mod. Your answer helped me understand how its working/how to implement it