LinkedQueue.java

package com.examplehub.datastructures.queue;

import com.examplehub.datastructures.linkedlist.Node;
import java.util.StringJoiner;

public class LinkedQueue<E> {

  /** The front pointer of LinkedQueue. */
  private Node<E> front;

  /** The rear pointer of LinkedQueue. */
  private Node<E> rear;

  /** The number of elements in LinkedQueue. */
  private int size;

  public LinkedQueue() {
    front = rear = null;
  }

  /**
   * Remove item from the front of LinkedQueue.
   *
   * @return the item at the front of LinkedQueue.
   * @throws IllegalAccessError if LinkedQueue is empty.
   */
  public E dequeue() throws IllegalAccessException {
    if (size == 0) {
      throw new IllegalAccessException();
    }
    E retValue = front.data;
    if ((front = front.next) == null) {
      rear = null;
    }
    size--;
    return retValue;
  }

  /**
   * Added item to the rear of LinkedQueue.
   *
   * @param item the item to be added.
   * @return {@code true} if add successfully.
   */
  public boolean enqueue(E item) {
    Node<E> newNode = new Node<>(item);
    if (size == 0) {
      front = rear = newNode;
    } else {
      rear = rear.next = newNode;
    }
    size++;
    return true;
  }

  /**
   * Test if this queue is empty or not.
   *
   * @return {@code true} if this queue is empty, otherwise {@code false}.
   */
  public boolean empty() {
    return size == 0;
  }

  @Override
  public String toString() {
    StringJoiner joiner = new StringJoiner(" <- ");
    Node<E> temp = front;
    while (temp != null) {
      joiner.add(temp.data.toString());
      temp = temp.next;
    }
    return joiner.toString();
  }

  /**
   * Return the number of elements in this LinkedQueue.
   *
   * @return the number of elements in this LinkedQueue.
   */
  public int size() {
    return size;
  }

  /** Clear all elements in the LinkedQueue. */
  public void clear() {
    front = rear = null;
    size = 0;
  }
}