Create a Queue Implemented by a Linked List in Java

class Link {/* ww w .j  a  v a  2s.  c  o  m*/
  public int iData;
  public Link next;
  public Link(int id) {
    iData = id;
  }

  public String toString() {
    return "{" + iData + "} ";
  }
}

class LinkList {
  private Link first;

  public LinkList() {
    first = null;
  }

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

  public void insertFirst(int dd) {
    Link newLink = new Link(dd);
    newLink.next = first;
    first = newLink;
  }

  public int deleteFirst() {
    Link temp = first;
    first = first.next;
    return temp.iData;
  }

  public String toString() {
    String str = "";
    Link current = first;
    while (current != null) {
      str += current.toString();
      current = current.next;
    }
    return str;
  }
}

class FirstLastList {
  private Link first;

  private Link last;

  public FirstLastList() {
    first = null;
    last = null;
  }

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

  public void insertFirst(int dd) {
    Link newLink = new Link(dd);
    if (isEmpty())
      last = newLink;
    newLink.next = first;
    first = newLink;
  }

  public void insertLast(int dd) {
    Link newLink = new Link(dd);

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

  public int deleteFirst() {
    int temp = first.iData;
    if (first.next == null)
      last = null;
    first = first.next;
    return temp;
  }

  public String toString() {
    String str = "";
    Link current = first;
    while (current != null) {
      str += current.toString();
      current = current.next;
    }
    return str;
  }
}

class LinkQueue {
  private FirstLastList theList;

  public LinkQueue() {
    theList = new FirstLastList();
  }

  public boolean isEmpty() {
    return theList.isEmpty();
  }

  public void insert(int j) {
    theList.insertLast(j);
  }

  public double remove() {
    return theList.deleteFirst();
  }

  public String toString() {
    return theList.toString();
  }
} 

public class Main {
  public static void main(String[] args) {
    LinkQueue theQueue = new LinkQueue();
    theQueue.insert(20);
    theQueue.insert(40);
    System.out.println(theQueue);

    theQueue.insert(60);
    theQueue.insert(80);

    System.out.println(theQueue);

    theQueue.remove();
    theQueue.remove();

    System.out.println(theQueue);
  }
}