ADT Queue without using Java libraries

Question

I am currently preparing for my upcoming exam and there was one tast from an older exam, where we have been given an ADTQueue

public interface ADTQueue<T> { public void enq(T element);
public void deq();
public T front();
public boolean empty(); } 

we now have to implement the class Queue as well as an inner class with the constructor ListElement(T element) and some methods in it...

I have made an implementation, this is my following code:

public class Queue<T> implements ADTQueue<T> {
private ListElement<T> head;
private ListElement<T> tail;

public Queue(){
    head=null;
    tail=null;
}

public void enq (T element){
    if (empty()){
        head= new ListElement(element);
        tail= new ListElement(element);
    }

    tail.nextElement=new ListElement(element);
    tail=tail.nextElement;
}

public void deq(){
    if (empty()){
        throw new Exception();
    }

    T firstElement=front();
    head=head.nextElement;
    if (head==null){
        tail = null;
    }
}

public T front(){
    if(empty()){
        return null;
    }
    return head.element;
}
public boolean empty(){
    return (head==null);
}

public class ListElement<T>{
    private T element = null;
    private ListElement<T> nextElement = null;

    public ListElement(T element) {
        this.element = element;
    }

    public T getElement() {
        return element;
    }

    public ListElement<T> getNextElement() {
        return nextElement;
    }

    public void setNextElement(ListElement<T> nextElement) {
        this.nextElement = nextElement;
    }



}

I would like to know, if it is correct what I did and if I could have done it better. Also, how would it look like, if I wanted to do the same but with double linked list? I know, I need also a get- and setPreviousElement, but I am not sure, what is going to change in the enqueue and dequeue methods... I would be really happy, if you guys could give some advice thx in advance

Answer 1

1) Suggestion for better Return Types:

• public boolean enqueue(T element)

  should be boolean, instead of Void.

• public T dequeue();

  should be T, instead of Void.

2) Link - a single Object in a linked list.

Lines 3+4:

Upon an arrival of a new element, you create 2 Different Links with the same Data (T), instead of 1 Link (that also the head & tail points to this Link).

Lines 5+6: I think you forgot the wrap the rest of the code with else{...}.

Before changes:

1 public void enq (T element){
2 if (empty()){
3    head= new ListElement(element);
4    tail= new ListElement(element);
  }

5 tail.nextElement=new ListElement(element);
6 tail=tail.nextElement;

}

After suggested changes:

1 public void enq (T element){
2 if (empty()){
3    head= new ListElement(element);
4    tail= head;
5  }else{

6   tail.nextElement=new ListElement(element);
7   tail=tail.nextElement;
  }

}