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Implementing A Queue With An Array

I'm working on coding an array implementation of a queue in Java.  The implementation should have the following features:

  • The array has a head which marks the item least recently inserted.
  • The array has a tail which marks* the item most recently inserted.
  • Enqueue() adds an item to the tail of the array.
  • Dequeue() removes an item from the head of the array.
  • The array will resize() dynamically as it gets too small or too large.
  • Size() returns the size of the queue from head to tail.
  • IsEmpty() returns whether or not the queue contains anything at all.
You start out by initializing an empty array.  It looks like this:


[0] null (head / tail)

So at the beginning, the head is set to zero.  I guess the tail can also be zero.  Then you enqueue.

[0] "To" (head / tail)

So the first time you try to enqueue, the head is null.  Fill the head.  Now you want the tail to move up.  

Here's the deal with the tail (*) -- in normal cases, you want to put whatever you enqueue into the tail.  So the tail is associated with the end of the queue, as I conceive it, but actually it marks where we will insert the next item.

So the general process is this: insert at the tail, move the tail up.

What about dequeue?  Here you want to return what's at the head, empty the head, then move the head up.

But that leads to an interesting situation:

[1] "To" (Head)
[2] "Be"
[3] "Or"
[4] null (Tail)

**dequeue**

[1] null
[2] "Be" (Head)
[3] "Or"
[4] null (Tail)

So now say that I enqueue.  What happens to the tail?  Well, if I increase it, the tail will be out of bounds.  So one option would be to resize the array.  But I still have space for a new insert at the beginning of the array, where I did my last dequeue.  The trick here, I think, is to increment the tail like this:

tail = (tail + 1) % array.length

So when the tail gets past the end, you send it back to the beginning.  That leads to some complications I'm still trying to work out and will possibly discuss in my next post.

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