Java Queue Implementation

What is Queue

Code Example of Java Queue

class Queue<Item> {
    private int size;

    private Node first;
    private Node last;
    private class Node {
        Item item;
        Node next;
    }

    public Queue() {
        size = 0;
        first = null;
        last = null;
    }

    public boolean isEmpty() {
        return first == null;
    }

    public void enqueue(Item item) {
        Node oldLast = last;

        last = new Node();
        last.item = item;
        last.next = null;

        if (isEmpty()) {
            first = last;
        } else {
            oldLast.next = last;
        }

        size++;
    }

    public Item dequeue() {
        Item item = first.item;
        first = first.next;

        if (isEmpty()) {
            last = null;
        }

        size--;

        return item;
    }

    public int size() {
        return size;
    }

    public static void main(String[] args) {
        Queue<String> queue = new Queue<String>();
        queue.enqueue("a");
        queue.enqueue("b");
        queue.enqueue("c");

        System.out.println(queue.size());
        System.out.println(queue.dequeue());
        System.out.println(queue.dequeue());
        System.out.println(queue.dequeue());
        System.out.println(queue.size());
    }
}