Null pointer Exception while reversing a Queue using stack - data-structures

Question

I was practicing on how to implement queue using arrays. I have easily implemented how to enqueue and dequeue the elements in queue. But I have encountered an exception while implementing reverse of queue using stacks
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(){
if(isQueueEmpty()){
System.out.println("UnderFlow");
}
else{
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 queueArr[front-1];
}
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.dequeue();
queueImpl.printQueue(queueImpl);
queueImpl.reverse(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 {
int i=0;
Stack<Integer> stack=new Stack<Integer>();
while(!queueImpl.isQueueEmpty()){
stack.push(queueImpl.dequeue());
}
while(!stack.isEmpty()){
stack.get(i);
i++;
}
while(!stack.isEmpty()){
queueImpl.enqueue(stack.pop());
}
return queueImpl;
}
}
The error log is -
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: -1
at com.tcs.QueueUsingAraay.QueueImpl.dequeue(QueueImpl.java:51)
at com.tcs.QueueUsingAraay.QueueImpl.reverse(QueueImpl.java:93)
at com.tcs.QueueUsingAraay.QueueImpl.main(QueueImpl.java:78)

Answer 1

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.
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;
}
}