Working of Queue
Queue operations work as follows:
- two pointers FRONT and REAR
- FRONT track the first element of the queue
- REAR track the last element of the queue
- initially, set value of FRONT and REAR to -1
Enqueue Operation
- check if the queue is full
- for the first element, set the value of FRONT to 0
- increase the REAR index by 1
- add the new element in the position pointed to by REAR
Dequeue Operation
- check if the queue is empty
- return the value pointed by FRONT
- increase the FRONT index by 1
- for the last element, reset the values of FRONT and REAR to -1
java program of Queue in data structure
// Queue implementation in Java
public class Queue {
int SIZE = 5;
int items[] = new int[SIZE];
int front, rear;
Queue() {
front = -1;
rear = -1;
}
boolean isFull() {
if (front == 0 && rear == SIZE - 1) {
return true;
}
return false;
}
boolean isEmpty() {
if (front == -1)
return true;
else
return false;
}
void enQueue(int element) {
if (isFull()) {
System.out.println("Queue is full");
} else {
if (front == -1)
front = 0;
rear++;
items[rear] = element;
System.out.println("Inserted " + element);
}
}
int deQueue() {
int element;
if (isEmpty()) {
System.out.println("Queue is empty");
return (-1);
} else {
element = items[front];
if (front >= rear) {
front = -1;
rear = -1;
} /* Q has only one element, so we reset the queue after deleting it. */
else {
front++;
}
System.out.println("Deleted -> " + element);
return (element);
}
}
void display() {
/* Function to display elements of Queue */
int i;
if (isEmpty()) {
System.out.println("Empty Queue");
} else {
System.out.println("\nFront index-> " + front);
System.out.println("Items -> ");
for (i = front; i <= rear; i++)
System.out.print(items[i] + " ");
System.out.println("\nRear index-> " + rear);
}
}
public static void main(String[] args) {
Queue q = new Queue();
// deQueue is not possible on empty queue
q.deQueue();
// enQueue 5 elements
q.enQueue(1);
q.enQueue(2);
q.enQueue(3);
q.enQueue(4);
q.enQueue(5);
// 6th element can't be added to because the queue is full
q.enQueue(6);
q.display();
// deQueue removes element entered first i.e. 1
q.deQueue();
// Now we have just 4 elements
q.display();
}
}