Queue is a kind of Abstract data structure that holds items in same order. Queue follow FIFO mechanism for operations like insertion and deletion of items.

Note :

- One end is always used to insert data called enqueue.

- Other end is always used to delete data called dequeue.

- So Adding an item to the rear and deleting an item from front of queue.

Time complexity always constant that is O(1).

Implement queue using array :

public class QueueOperations  {

    private int storageCapacity;
    private int[] queueArr;
    private int rear = -1;
    private int front = 0;
    private int size = 0;

    QueueOperations(int capacity){
        storageCapacity = capacity;
        queueArr = new int[storageCapacity];
    }

    public void addItem(int item){

        if (size == storageCapacity){
            Log.e(" overflow ::" ," queue overflow condition .");
        }else {
            rear++;
            if (rear == size - 1) {
                rear = 0;
                front = 0;
            }
            queueArr[rear] = item;
            size++;
        }
    }

    public void removeItem(){

        if (size == 0){
            Log.e(" underflow ::" ," queue underflow condition .");
        }else{

            front++;
            if (front == rear){
                front = 0;
                rear = 0;
            }
            size--;
        }
    }

    public void peek(){

        if (size == -1){
            Log.e(" queue empty"," queue empty");
        }else{
            Log.e(" queue size"," :: " + size);
            Log.e(" queue front at "," :: " + front);
            Log.e(" queue rear at"," :: " + rear);
            Log.e(" ----------------------- ","--------------- ");

            for (int i =front ; i<=rear;i++){
                Log.e(" queue data"," :: " + queueArr[i]);

            }
        }
    }

}