Queue Interface

Queue Interface
A Queue is designed to hold a list of "to-dos," or things to be processed in some way. Although other orders are possible, queues are typically thought of as FIFO (first-in, first-out). Queues support all of the standard Collection methods and they also add methods to add and subtract elements and review queue elements.

PriorityQueue This class is new with Java 5. Since the LinkedList class has been enhanced to implement the Queue interface, basic queues can be handled with a LinkedList. The purpose of a PriorityQueue is to create a "priority-in, priority out" queue as opposed to a typical FIFO queue. A PriorityQueue's elements are ordered either by natural ordering (in which case the elements that are sorted first will be accessed first) or according to a Comparator. In either case, the elements' ordering represents their relative priority.

Java Implementation of Circular Queue

public class CircularQueue {

int[] cirQueue;

int front;

int rear;

// int size;
public CircularQueue(int size) {
cirQueue = new int[size];
front = rear = -1;
}

public static void main(String[] args) {
CircularQueue myQueue = new CircularQueue(5);
myQueue.insert(1);
myQueue.insert(2);
myQueue.insert(3);
myQueue.insert(4);
myQueue.insert(5);
myQueue.delete();
myQueue.insert(6);
myQueue.display();

}

public void delete() {
if (front == rear ) {
System.out.println("Can not delete from empty Queue");
return;
}else{
if(front==cirQueue.length)
front=0;
else
front++;
}
}

public void insert(int value) {
if ((rear + 1 == front) || (front == 0 && rear == cirQueue.length - 1)) {
System.out.println("Cant add ... Queue is full ");
return;
} else {
if (rear == cirQueue.length - 1) {
rear = 0;
}else{
rear++;
}
cirQueue[rear] = value;
}
if(front==-1)
front=0;
}

public void display(){
if(front > rear){
for(int i = front; i < cirQueue.length ; i++){
System.out.println(cirQueue[i]);
}
for(int i = 0 ; i < rear ; i++){
System.out.println(cirQueue[i]);
}
}else{
for(int i = front ; i < rear ; i++){
System.out.println(cirQueue[i]);
}
}
}
}