LinkedListQueue.java

public class LinkedListQueue<E> {
  private class Node {
    Node next;
    E data;
    public Node(E data, Node next) {
      this.data = data;
      this.next = next;
    }
  }
  
  private Node head, tail;
  
  public LinkedListQueue() {
    head = null;
    tail = null;
  }
  
  public boolean empty() {
    return head == null;
  }
  public void enqueue(E element) {
    if (tail != null) {
      tail.next = new Node(element, null);
      tail = tail.next;
    } else {
      tail = new Node(element, null);
      head = tail;
    }
  }
  public E dequeue() {
    if (empty()) {
      throw new IllegalStateException("Queue is empty");
    }
    E temp = head.data;
    head = head.next;
    return temp;
  }
  public E peek() {
    if(empty()) {
      throw new IllegalStateExceptio("Queue is empty");
    } else {
      return head.data;
  }
}