Null pointer Exception while reversing a Queue using stack

There was a problem in your dequeue method: 1. you were making front=0 and were trying to access front-1 which was -1 as a result the exception was thrown.

2. Removed stack.get as it was not required.

Corrected working code.

public class QueueImpl {
        private int capacity;
        int queueArr[];
        int front = 0;
        int rear = -1;
        int currentSize = 0;

        QueueImpl(int queueSize) {
            this.capacity = queueSize;
            queueArr = new int[this.capacity];
        }

        public void enqueue(int data) {
            if (isQueueFull()) {
                System.out.println("Overflow");
                return;
            } else {
                rear = rear + 1;
                if (rear == capacity - 1) {
                    rear = 0;
                }
                queueArr[rear] = data;
                currentSize++;
                System.out.println("Element " + data + " is pushed to Queue !");
            }

        }

        public int dequeue() {
            int element=-1;
            if (isQueueEmpty()) {
                System.out.println("UnderFlow");
            } else {
                element = queueArr[front];
                front=front+1;
                if (front == capacity - 1) {
                    System.out.println("Pop operation done ! removed: "
                            + queueArr[front - 1]);
                    front = 0;
                } else {
                    System.out.println("Pop operation done ! removed: "
                            + queueArr[front - 1]);
                }
                currentSize--;
            }
            return element;

        }

        private boolean isQueueEmpty() {
            boolean status = false;
            if (currentSize == 0) {
                status = true;
            }
            return status;
        }

        private boolean isQueueFull() {
            boolean status = false;
            if (currentSize == capacity) {
                status = true;
            }
            return status;
        }

        public static void main(String arg[]) {
            QueueImpl queueImpl = new QueueImpl(5);
            queueImpl.enqueue(5);
            queueImpl.enqueue(2);
            queueImpl.enqueue(9);
            queueImpl.enqueue(1);

            queueImpl.printQueue(queueImpl);
            queueImpl.reverse(queueImpl);
            queueImpl.printQueue(queueImpl);
        }

        private void printQueue(QueueImpl queueImpl) {
            System.out.println(queueImpl.toString());
        }

        @Override
        public String toString() {
            return "Queue [front=" + front + ", rear=" + rear + ", size="
                    + currentSize + ", queue=" + Arrays.toString(queueArr) + "]";
        }

        private QueueImpl reverse(QueueImpl queueImpl)
                throws ArrayIndexOutOfBoundsException {
            Stack<Integer> stack = new Stack<Integer>();
            while (!queueImpl.isQueueEmpty()) {
                stack.push(queueImpl.dequeue());
            }
            while (!stack.isEmpty()) {
                queueImpl.enqueue(stack.pop());
            }
            return queueImpl;
        }

    }