addFront(T x){
    if(isFull())    throw IllegalStateException // queue is full

    if(headIndex == 0){
        headIndex = A.length-1
    }
    else{
        headIndex--
    }

    A[headIndex] = x
    size++
}

void addBack(T x){
    if(isFull())    throw IllegalStateException // queue is full

    tailIndex = (tailIndex+1) % A.length;
    A[tailIndex] = x
    size++
}

T removeFront(){
    if(isEmpty())    throw NoSuchElementException // empty queue

    headValue = A[headIndex]
    A[headIndex] = null
    headIndex = (headIndex+1) % A.length
    size--
    return headValue
}

T removeBack(){
    if(isEmpty())    throw NoSuchElementException // empty queue

    tailValue = A[tailIndex]
    A[tailIndex] = null
    if(tailIndex == 0){
        tailIndex = A.length-1
    }
    else{
        tailIndex--
    }
    size--
    return tailValue
}

getFront(){
    if(isEmpty())    throw NoSuchElementException
    return A[headIndex]
}

getBack(x){
    if(isEmpty())    throw NoSuchElementException
    else{
        return A[tailIndex]
    }
}

isFull(){
    return (size == capacity)
}

isEmpty(){
    return (size == 0)
}