View difference between Paste ID: vrk5qcB1 and RCkp2n4i

 import java.util.Iterator;
 public class DoubleLinkedList<ELEMENT> implements ILinkedList<ELEMENT> {
     //region Node Class
 
     protected class Node<ELEMENT> {
         protected ELEMENT item;
         protected Node<ELEMENT> next;
         protected Node<ELEMENT> prev;
 
         protected Node() {
             this(null, null, null);
         }
         protected Node(ELEMENT item) {
             this(item, null, null);
         }
         protected Node(ELEMENT item, Node<ELEMENT> next) {
             this(item, next, null);
         }
         protected Node(ELEMENT item, Node<ELEMENT> next, Node<ELEMENT> prev) {
             this.item = item;
             this.next = next;
             this.prev = prev;
         }
 
         @Override
         public String toString() {
             return this.item.toString();
         }
     }
     //endregion
 
     //region Attributes
 
     protected Node<ELEMENT> head;
     protected int count;
     protected Node<ELEMENT> tail;
     //endregion
 
     //region Constructors
 
     public DoubleLinkedList() {
         this.head = null;
         this.count = 0;
         this.tail = null;
     }
     //endregion
 
     //region Linked List Methods
 
     // Returns the number of elements in this list.
     public int size() {
         return this.count;
     }
 
     public void addFirstRookieVersion(ELEMENT item) {
         if (this.size() <= 0) {
             this.head = this.tail = new Node<ELEMENT>(item, null, null);
             ++this.count;
         }
         else {
             Node<ELEMENT> temp = new Node<ELEMENT>(item, null, null);
             temp.next = this.head;
             this.head.prev = temp;
             this.head = temp;
             ++this.count;
         }
     }
     // Inserts the specified element at the beginning of this list.
     public void addFirst(ELEMENT item) {
         Node<ELEMENT> temp = new Node<ELEMENT>(item, this.head, null);
         if (this.size() <= 0) {
             this.tail = temp;
         }
         else {
             this.head.prev = temp;
         }
         this.head = temp;
         ++this.count;
     }
 
     public void addLastRookieVersion(ELEMENT item) {
         if (this.size() <= 0) {
             this.head = this.tail = new Node<ELEMENT>(item, null, null);
             ++this.count;
         }
         else {
             Node<ELEMENT> temp = new Node<ELEMENT>(item, null, null);
             temp.prev = this.tail;
             this.tail.next = temp;
             this.tail = temp;
             ++this.count;
         }
     }
     // Appends the specified element to the end of this list.
     public void addLast(ELEMENT item) {
         Node<ELEMENT> temp = new Node<ELEMENT>(item, null, this.tail);
         if (this.size() <= 0) {
             this.head = temp;
         }
         else {
             this.tail.next = temp;
         }
         this.tail = temp;
         ++this.count;
     }
 
     // Removes and returns the first element from this list.
     public ELEMENT removeFirst() {
         if (this.size() <= 0) {
             throw new RuntimeException("La lista está vacía...");
         }
         ELEMENT item = this.head.item;
         this.head = this.head.next;
         if (this.head == null) {
             this.tail = null;
         }
         else {
             this.head.prev = null;
         }
         --this.count;
         return item;
     }
 
     // Removes and returns the last element from this list.
     public ELEMENT removeLast() {
         if (this.size() <= 0) {
             throw new RuntimeException("La lista está vacía...");
         }
         ELEMENT item = this.tail.item;
         if (this.head.next == null) {
             this.head = this.tail = null;
         } else {
             this.tail = this.tail.prev;
             this.tail.next = null;
         }
         --this.count;
         return item;
     }
     //endregion
 
     //region Object Methods
 
     @Override
     public String toString() {
 
         if (this.size() <= 0) {
             return "";
         }
 
         // from https://docs.oracle.com/en/java/javase/15/docs/api/java.base/java/lang/StringBuilder.html
         StringBuilder sb = new StringBuilder();
 
         sb.append("[" + this.head.item.toString());
         for (Node<ELEMENT> skip = this.head.next; skip != null; skip = skip.next) {
             sb.append(",\n " + skip.item.toString());
         }
         sb.append("]");
 
         return sb.toString();
     }
     //endregion
 
     //region Iterable Methods
     @Override
     public Iterator<ELEMENT> iterator() {
         return new DoubleLinkedListIterator(this.head);
     }
 
     public class DoubleLinkedListIterator implements Iterator<ELEMENT> {
         private Node<ELEMENT> current;
 
         public DoubleLinkedListIterator(Node<ELEMENT> current) {
             this.current = current;
         }
 
         @Override
         public boolean hasNext() {
             return this.current != null;
         }
 
         @Override
         public ELEMENT next() {
             if (!this.hasNext()) {
                 throw new RuntimeException("La lista está vacía...");
             }
             ELEMENT item = this.current.item;
             this.current = this.current.next;
             return item;
         }
     }
 
     public Iterator<ELEMENT> iteratorBack() {
         return new DoubleLinkedListIteratorBack(this.tail);
     }
 
     public class DoubleLinkedListIteratorBack implements Iterator<ELEMENT> {
         private Node<ELEMENT> current;
 
         public DoubleLinkedListIteratorBack(Node<ELEMENT> current) {
             this.current = current;
         }
 
         @Override
         public boolean hasNext() {
             return this.current != null;
         }
 
         @Override
         public ELEMENT next() {
             if (!this.hasNext()) {
                 throw new RuntimeException("La lista está vacía...");
             }
             ELEMENT item = this.current.item;
             this.current = this.current.prev;
             return item;
         }
 
     }
 
     //endregion
 
 
 }