Implementing Doubly Linked list

I am having trouble fixing four debug issues
namely the constructor, and iterator methods

Here is my code

import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;
import java.util.ListIterator;
import java.util.NoSuchElementException

public class Proj61773 {

	public static void main(String[] args) throws java.io.IOException {
		DoublyLinkedList<String> list = new DoublyLinkedList<String>();

		list.addHead("Test1");
		list.add(1, "Test2");
		list.add(2, "Test3");
		list.add(3, "Test4");
		list.add(4, "Test5");
		list.add("Test6");
		list.add("Test7");

		System.out.println(list);

		list.remove(list.size());
		System.out.println(list);

		DoublyLinkedList<Integer> list2 = new DoublyLinkedList<Integer>();
		list2.addHead(new Integer(1));
		list2.addHead(new Integer(2));
		list2.addTail(new Integer(9));
		list2.addHead(new Integer(3));
		list2.addTail(new Integer(11));
		list2.add(2, new Integer(0));
		System.out.println(list2);
		list2.remove(list2.size());
		System.out.println(list2);
		list2.remove(list2);
		System.out.println(list2);
	}

}

// Doubly LinkedList class
class DoublyLinkedList<E> implements java.util.List<E> {
	private int currentSize = 0;
	private DoublyLinkedListNode<E> head = new DoublyLinkedListNode<E>(null);
	private DoublyLinkedListNode<E> tail = new DoublyLinkedListNode<E>(null);

	// constructor
	public DoublyLinkedList() {
		head.setPrev(null);

		head.setNext(tail);
		tail.setPrev(head);
		tail.setNext(null);
	}

	// constructor
	public DoublyLinkedList(DoublyLinkedListNode<E> head) {
		this.head = head;
		this.tail = head;

		currentSize = 1;
	}

	// constructor
	public DoublyLinkedList(Collection<? extends E> c) {
		this();
		
		Iterator<? extends E> itr = c.iterator();
		while(itr.hasNext()) {
			E element = (E) itr.next();
			add(element);
		}
		//addAll(c);
	}

	public boolean isEmpty() {
		return currentSize == 0;
	}

	public int getSize() {
		return currentSize;
	}

	@Override
	public int size() {
		return currentSize;
	}

    // WORKING for defect # 6
	public E get(int index) {
		if (index < 0 || index > currentSize) {
			throw new IndexOutOfBoundsException();
		} else {
			DoublyLinkedListNode<E> cursor = head;
			for (int i = 0; i <= index; i++) {
				cursor = cursor.getNext();
			}
			return (E) cursor.value;
		}
	}


	private E getPos(int index) {
		if (index < 0 || index > currentSize) {
			throw new IndexOutOfBoundsException();
		} else {
			DoublyLinkedListNode<E> cursor = head;
			for (int i = 0; i <= index; i++) {
				cursor = cursor.getNext();
			}
			return (E) cursor;
		}
	}
	
	@Override
	public int indexOf(Object value) {
		int index = 0;
		DoublyLinkedListNode<E> current = head.next;

		while (current != tail) {
			//if (current.value.equals(current)) {
			if (current.getValue().equals(value)) {
				return index;
			}
			index++;
			current = current.next;
		}
		return -1;
	}

	@Override
	public boolean contains(Object value) {
		return indexOf(value) != -1;
	}

	@Override
	public void add(int index, E element) {
		DoublyLinkedListNode<E> cursor = extracted(index);
		DoublyLinkedListNode<E> temp = new DoublyLinkedListNode<E>(element);
		temp.setPrev(cursor);
		temp.setNext(cursor.getNext());
		cursor.getNext().setPrev(temp);
		cursor.setNext(temp);
		currentSize++;
	}

	private DoublyLinkedListNode<E> extracted(int index) {
		return (DoublyLinkedListNode<E>) getPos(index-1);
	}

	// method to add from head
	public void addHead(E value) {
		DoublyLinkedListNode<E> cursor = head;
		DoublyLinkedListNode<E> temp = new DoublyLinkedListNode<E>(value);
		temp.setPrev(cursor);
		temp.setNext(cursor.getNext());
		cursor.getNext().setPrev(temp);
		cursor.setNext(temp);
		currentSize++;
	}

	// method to add from tail
	public void addTail(E value) {
		DoublyLinkedListNode<E> cursor = tail.getPrev();
		DoublyLinkedListNode<E> temp = new DoublyLinkedListNode<E>(value);
		temp.setPrev(cursor);
		temp.setNext(cursor.getNext());
		cursor.getNext().setPrev(temp);
		cursor.setNext(temp);
		currentSize++;
	}

	@Override
	public boolean add(E element) {
		addTail(element);
		return true;
	}

	@Override
	public E remove(int index) {
		if (index == 0) {
			throw new IndexOutOfBoundsException();
		} else {
			DoublyLinkedListNode<E> result = extracted(index);
			result.getNext().setPrev(result.getPrev());
			result.getPrev().setNext(result.getNext());
			currentSize--;
			return result.getValue();
		}
	}

	@Override
	public boolean remove(Object obj) {
		
		if (obj == null) {
			for (DoublyLinkedListNode<E> x = head; x != null; x = x.next) {
				if (x.value == null) {
					unlink(x);
					return true;
				}
			}
		} else {
			for (DoublyLinkedListNode<E> x = head; x != null; x = x.next) {
				if (obj.equals(x.value)) {
					unlink(x);
					return true;
				}
			}
		}
		return false;
	}

	
	
	private E unlink (DoublyLinkedListNode<E> x) {
		final E element = x.value;
		final DoublyLinkedListNode<E> next = x.next;
		final DoublyLinkedListNode<E> prev = x.prev;

		if (prev == null) {
			head = next;
		} else {
			prev.next = next;
			x.prev = null;
		}

		if (next == null) {
			tail = prev;
		} else {
			next.prev = prev;
			x.next = null;
		}

		x.value = null;
		currentSize--;
		return element;
	}
	
	
	
	
	@Override
	public void clear() {
		head.next = tail;
		tail.prev = head;
		currentSize = 0;
	}

	//@Override
	public Iterator<E> iterator() {
		return new DoublyLinkedIterator();
	}

	
	
	/**
	 * Returns the (non-null) Node at the specified element index.
	 */
	private DoublyLinkedListNode<E> node(int index) {
		if (index < (currentSize >> 1)) {
			DoublyLinkedListNode<E> x = head;
			for (int i = 0; i < index; i++)
				x = x.next;
			return x;
		} else {
			DoublyLinkedListNode<E> x = tail;
			for (int i = currentSize - 1; i > index; i--)
				x = x.prev;
			return x;
		}
	}
	
	
	
	@Override
	public boolean addAll(Collection<? extends E> c) {
		return addAll(currentSize, c);
	}

	@Override
	public boolean addAll(int index, Collection<? extends E> c) {
		Object[] a = c.toArray();
		int numNew = a.length;
		if (numNew == 0) {
			return false;
		}

		DoublyLinkedListNode<E> pred, succ;
		if (index == currentSize) {
			succ = null;
			pred = tail;
		} else {
			succ = node(index);
			pred = succ.prev;
		}

		for (Object o : a) {
			@SuppressWarnings("unchecked") E e = (E) o;
			DoublyLinkedListNode<E> newNode = new DoublyLinkedListNode<E> ( pred, e, null);
			if (pred == null) {
				head = newNode;
			} else {
				pred.next = newNode;
			}
			pred = newNode;
		}

		if (succ == null) {
			tail = pred;
		} else {
			pred.next = succ;
			succ.prev = pred;
		}
		currentSize += numNew;
		return true;
	}


	
	
	
	
	@Override
	public String toString() {
		StringBuilder sb = new StringBuilder();
	    DoublyLinkedListNode<E> current = head;
	    
	    if (current == null) {
	    	return "[]";
	    }

    	sb.append("[");
	    while (current != null) {
	    	if (current.value!=null) {
		    	sb.append(current.value).append(", ");
	    	}
	    	current = current.getNext();
	    }

	    int strLength = sb.length();  
		sb.delete(strLength-2, strLength);
		sb.append(']');
	    return sb.toString();
	}

	
	
	//// DoublyLinkedIterator class
	private class DoublyLinkedIterator implements java.util.Iterator<E> {
		private DoublyLinkedListNode<E> currentNode;
		private DoublyLinkedListNode<E> nextNode;
		private int nextIndex = 0;
		
		private DoublyLinkedIterator() {
			currentNode = head;

		}

		
		public boolean hasNext() {
			return (currentNode != null );
		}

		
		public E next() {
			if (hasNext()) {
				currentNode = currentNode.next;
				E result = currentNode.value;
				//System.out.println(" current.value: "+ current.value);
				//System.out.println(" result: "+result);
				nextIndex++;
				return result;
			} else {
				throw new java.util.NoSuchElementException();
			}
		}

		
		public void remove() {
			 if (currentNode == null) {
                 throw new java.util.NoSuchElementException();
			 }
			 //DoublyLinkedListNode<E> lastNext = currentNode.next;
			 //System.out.println("**** currentNode - "+currentNode.value);
			 //unlink(currentNode);
			 DoublyLinkedList.this.remove(currentNode);
			 //if (nextNode == currentNode) {
			//	 nextNode = lastNext;
			 //} else {
              //  nextIndex--;
			 //}
			 //currentNode = null;
             currentSize++;		
		}
	}

	// Node class
	private class DoublyLinkedListNode<E> {
		private E value;
		private DoublyLinkedListNode<E> next;
		private DoublyLinkedListNode<E> prev;

		public DoublyLinkedListNode(E element) {
			this.value = element;
		}

		public DoublyLinkedListNode(DoublyLinkedListNode<E> n1, E element, DoublyLinkedListNode<E> n2) {
			this.value = element;
			setPrev(prev);
			setNext(next);
		}

		public void setValue(E element) {
			this.value = element;
		}

		public E getValue() {
			return this.value;
		}

		public void setPrev(DoublyLinkedListNode<E> prev) {
			this.prev = prev;
		}

		public void setNext(DoublyLinkedListNode<E> next) {
			this.next = next;

		}

		public DoublyLinkedListNode<E> getNext() {
			return this.next;
		}

		public DoublyLinkedListNode<E> getPrev() {
			return this.prev;
		}
	}


	@Override
	public boolean containsAll(Collection<?> c) {
		// TODO Auto-generated method stub
		throw new UnsupportedOperationException("not implemented");
	}

	@Override
	public int lastIndexOf(Object o) {
		// TODO Auto-generated method stub
		throw new UnsupportedOperationException("not implemented");
	}

	@Override
	public boolean removeAll(Collection<?> c) {
		// TODO Auto-generated method stub

		throw new UnsupportedOperationException("not implemented");
	}

	@Override
	public boolean retainAll(Collection<?> c) {
		// TODO Auto-generated method stub

		throw new UnsupportedOperationException("not implemented");
	}

	@Override
	public E set(int index, E element) {
		throw new UnsupportedOperationException("not implemented");
	}

	@Override
	public List<E> subList(int fromIndex, int toIndex) {
		// TODO Auto-generated method stub

		throw new UnsupportedOperationException("not implemented");
	}

	@Override
	public Object[] toArray() {
		// TODO Auto-generated method stub

		throw new UnsupportedOperationException("not implemented");
	}

	@Override
	public <T> T[] toArray(T[] a) {
		// TODO Auto-generated method stub

		throw new UnsupportedOperationException("not implemented");
	}


	@Override
	public ListIterator<E> listIterator() {
		// TODO Auto-generated method stub
		 throw new UnsupportedOperationException("not implemented");
		//return new DoublyLinkedListItr(currentSize);
	}

	@Override
	public ListIterator<E> listIterator(int index) {
		// TODO Auto-generated method stub
		return new DoublyLinkedListItr(index);
	}

	// EXTRA CREDIT
	private class DoublyLinkedListItr implements ListIterator<E> {
		private DoublyLinkedListNode<E> lastReturned = head;
		private DoublyLinkedListNode<E> next;
		private int nextIndex;
		private int expectedModCount = currentSize;

		public DoublyLinkedListItr(int index) {
			if (index < 0 || index > currentSize)
				throw new IndexOutOfBoundsException("Index: " + index
						+ ", Size: " + currentSize);
			if (index < (currentSize >> 1)) {
				next = head.getNext();
				for (nextIndex = 0; nextIndex < index; nextIndex++)
					next = next.getNext();
			} else {
				next = head;
				for (nextIndex = currentSize; nextIndex > index; nextIndex--) {
					next = next.getPrev();
				}
			}
		}

		public DoublyLinkedListItr() {

		}

		public boolean hasNext() {
			return nextIndex != currentSize;
		}

		public E next() {
			checkForComodification();
			if (nextIndex == currentSize) {
				throw new NoSuchElementException();
			}
			lastReturned = next;
			next = next.getNext();
			nextIndex++;
			System.out.println(" lastReturned.getValue():  "+lastReturned.getValue());
			return lastReturned.getValue();
		}

		public boolean hasPrevious() {
			return nextIndex != 0;
		}

		public E previous() {
			if (nextIndex == 0)
				throw new NoSuchElementException();

			lastReturned = next = next.prev;
			nextIndex--;
			checkForComodification();
			return lastReturned.getValue();
		}

		public int nextIndex() {
			return nextIndex;
		}

		public int previousIndex() {
			return nextIndex - 1;
		}

		public void remove() {
			checkForComodification();
			DoublyLinkedListNode<E> lastNext = lastReturned.next;
			try {

				DoublyLinkedList.this.remove(lastReturned);
			} catch (NoSuchElementException e) {
				throw new IllegalStateException();
			}
			if (next == lastReturned)
				next = lastNext;
			else
				nextIndex--;
			lastReturned = head;
			expectedModCount++;
		}

		public void set(E e) {
			if (lastReturned == head)
				throw new IllegalStateException();
			checkForComodification();
			lastReturned.value = e;
		}

		public void add(E e) {
			checkForComodification();
			lastReturned = head;
			DoublyLinkedListNode<E> newNode = new DoublyLinkedListNode<E>(next, e, next.getPrev());
			newNode.prev.next = newNode;
			newNode.next.prev = newNode;
			currentSize++;
			nextIndex++;
			expectedModCount++;
		}

		final void checkForComodification() {
			if (currentSize != expectedModCount)
				throw new ConcurrentModificationException();
		}
	}

}

Here is the test drive for my code

// test #8 (equals) won't work if they don't have an iterator!

   import java.util.List;
import java.util.ArrayList;
import java.util.LinkedList;
import java.util.ConcurrentModificationException;
import java.util.Iterator;
import java.util.Collection;
//   import lists.DoublyLinkedList;

   public class GradeDLlist {
   
      private static boolean passedAll = true;
   
      public static void test(int num, boolean r, String actual) {
         if (!r) {
            passedAll = false;
            System.out.println(num+": "+ "fail - incorrect result: " + actual);
         }
      }
   
    ////////////////////////////////////////////
   
      public static void main(String [] args) {
         List<Integer> theirs = new ArrayList<Integer>();
         List<Integer> mine = new DoublyLinkedList<Integer>();
      
         int num = 0;
      
         try{
            ++num; test(num,theirs.isEmpty()==mine.isEmpty(),mine.isEmpty()+" cp #1");
         } catch (Exception e) { test(num,false,e+" cp #1"); }
      
         try{
            ++num; test(num,theirs.size()==mine.size(),mine.size()+" cp #1");
         } catch (Exception e) { test(num,false,e+" cp #1"); }
      
         theirs = new ArrayList<Integer>();
         mine = new DoublyLinkedList<Integer>();
         try{
            ++num;
            for (int i = 50; i <= 60; i++) {
               theirs.add(new Integer(i)); mine.add(new Integer(i));
            }
            test(num,theirs.size()==mine.size(),"size() test");
         } catch (Exception e) { test(num,false,e+" cp #1"); }
         
      
      //	try{
      //    ++num; test(num,theirs.equals(null)==mine.equals(null),"l.equals(null) test");
      //} catch (Exception e) { test(num,false,e+" cp #1"); }
      
         /*
        try{
            ++num; test(num,
               new ArrayList().equals(new DoublyLinkedList<Integer>())==new DoublyLinkedList<Integer>().equals(new ArrayList()),
               "equals() test");
         } catch (Exception e) { test(num,false,e+""); }
         */      
         try{
            ++num;
            for (int i = 0; i<=5; i++) {
               Integer obj = new Integer(i*2);
               theirs.add(obj); mine.add(obj);
            }
            test(num,theirs.isEmpty()==mine.isEmpty(),mine.isEmpty()+" cp #1");

         } catch (Exception e) { test(num,false,e+" cp #2"); }
      
         try{
            ++num; test(num,theirs.size()==mine.size(),mine.size()+" cp #3");
         } catch (Exception e) { test(num,false,e+" cp #4"); }
      
         boolean passed = true; int findex=-1;
         try{
            ++num;
            for (int i = 0; i<theirs.size() && passed; i++) 
               if (!theirs.get(i).equals(mine.get(i))) {
            	   passed = false; 
                   findex = i;
               }
            test(num,passed,"on index #"+findex);
         } catch (Exception e) { test(num,false,e+" cp #5");}
      
         try{
            ++num;
           	theirs.add(2,new Integer(-1000));
            mine.add(2,new Integer(-1000));
            passed = true;

            for (int i = 0; i<theirs.size(); i++) 
               if (!theirs.get(i).equals(mine.get(i))) {
                  passed = false; 
                  break;
               }
            test(num,passed,"add(i,object) test");
         } catch (Exception e) { test(num,false,e+" cp #6"); }
      
         ++num;
         try{
            mine.get(3452);
            passed = false;
         } catch (IndexOutOfBoundsException e) { 
               passed = true;
         } catch (Exception e) { 
               passed = false;
         }
         test(num,passed,"IndexOutOfBoundsException not thrown");
      
         ++num;
         try {
            mine.get(-3);
            passed = false;
         } catch (IndexOutOfBoundsException e) {
               passed = true;
         } catch (Exception e) { 
               passed = false;
         }
         test(num,passed,"IndexOutOfBoundsException not thrown");
      
         Integer iobj = new Integer(4);
         try{
            ++num; test(num, theirs.contains(iobj)==mine.contains(iobj), mine.contains(iobj)+" cp #7");
         } catch (Exception e) { test(num,false,e+" cp #8"); }
      
         iobj = new Integer(5);
         try{
            ++num; test(num,theirs.contains(iobj)==mine.contains(iobj),"contains() test - "+mine.contains(iobj));
         } catch (Exception e) { test(num,false,e+" cp #9"); }
      
         try{
            ++num; test(num,theirs.indexOf(null)==mine.indexOf(null),mine.indexOf(null)+" cp #10");
         } catch (Exception e) { test(num,false,e+" cp #11"); }
      
         iobj = new Integer(4);
         try{
            ++num;
            theirs.add(iobj); mine.add(iobj);
            
            test(num,theirs.indexOf(iobj)==mine.indexOf(iobj),"indexOf() test - "+mine.indexOf(iobj) );
         } catch (Exception e) { test(num,false,e+" cp #12"); }
      
         iobj = new Integer(5);
         try{
            ++num; test(num,theirs.indexOf(iobj)==mine.indexOf(iobj),"indexOf() test - "+mine.indexOf(iobj));
         } catch (Exception e) { test(num,false,e+" cp #13"); }
      
         iobj = new Integer(0); boolean br;
         try{
            ++num; test(num,theirs.remove(iobj)==(br=mine.remove(iobj)),br+" cp #14");
         } catch (Exception e) { test(num,false,e+" cp #15"); }
      
         passed = true; findex=-1;
         try{
            ++num;
            for (int i = 0; i<=4 && passed; i++) 
               if (!theirs.get(i).equals(mine.get(i))) {
                  passed = false; findex = i;
               }
            test(num,passed,"on index #"+findex);
         } catch (Exception e) { test(num,false,e+" cp #16"); }
      
         iobj = new Integer(4);
         try{
            ++num; test(num,theirs.remove(iobj)==(br=mine.remove(iobj)),br+" cp #17");
         } catch (Exception e) { test(num,false,e+" cp #18"); }
      
         passed = true; findex=-1;
         try{
            ++num;
            for (int i = 0; i<=3 && passed; i++) 
               if (!theirs.get(i).equals(mine.get(i))) {
                  passed = false; findex = i;
               }
            test(num,passed,"get() test - on index #"+findex);
         } catch (Exception e) { test(num,false,e+" cp #19"); }
      
         iobj = new Integer(-98344);
         try{
            ++num; test(num,theirs.remove(iobj)==(br=mine.remove(iobj)),br+" cp #20");
         } catch (Exception e) { test(num,false,e+" cp #21"); }
      
         passed = true; findex=-1;
         try{
            ++num;
            for (int i = 0; i<=3 && passed; i++) 
               if (!theirs.get(i).equals(mine.get(i))) {
                  passed = false; findex = i;
               }
            test(num,passed,"on index #"+findex);
         } catch (Exception e) { test(num,false,e+" cp #22"); }
      
         //         try{
         //   ++num; test(num,theirs.remove(null)==(br=mine.remove(null)),br+" cp #1");
         //} catch (Exception e) { test(num,false,e+" cp #1"); }
      
         try{
             ++num;
             Collection<Integer> c = new ArrayList<Integer>();
             for (int i = 0; i < 1000; i++) c.add(i);
             mine = new DoublyLinkedList<Integer>(c);
             theirs = new LinkedList<Integer>(c);
        	 System.out.println("**** mine size: "+mine.size());
        	 System.out.println("**** theirs size "+theirs.size());
        	 System.out.println("**** mine: "+mine.toString());
        	 System.out.println("**** their "+theirs.toString());
             
             test(num,mine.equals(theirs),"DoublyLinkedList(Collection) constructor test"); 
         } catch (Exception e) { test(num,false,e+" cp #23"); }

         try{
             ++num;
             mine = new DoublyLinkedList<Integer>();
             theirs = new ArrayList<Integer>();
             for (int i = 0; i < 100; i++) {
                 mine.add(i); theirs.add(i);
             }
             Iterator<Integer> myItr = mine.iterator(),
                 theirItr = theirs.iterator();
             passed = true;

             while(theirItr.hasNext()) {
                 if(!(myItr.next().equals(theirItr.next()))) {
                     passed = false; break;
                 }
             }
             test(num,passed,"iterator hasNext(), next() test"); 
         } catch (Exception e) { test(num,false,e+" cp #24"); }

         try {
             ++num;
             mine = new DoublyLinkedList<Integer>();
             theirs = new ArrayList<Integer>();
             for (int i = 0; i < 10; i++) {
                 mine.add(i); theirs.add(i);
             }
             Iterator<Integer> myItr = mine.iterator(),
                 theirItr = theirs.iterator();
             
             
             while(theirItr.hasNext()) {
                 myItr.next(); 
                 myItr.remove(); 
                 theirItr.next(); 
                 theirItr.remove();
             }
             test(num,mine.size()==theirs.size(),"iterator remove() test"); 
         } catch (Exception e) { test(num,false,e+" cp #25"); }

         try{
             ++num;
             mine = new DoublyLinkedList<Integer>();
             theirs = new ArrayList<Integer>();
             mine.add(1); theirs.add(1);
             Iterator<Integer> myItr = mine.iterator(),
                theirItr = theirs.iterator();
             try {
                 myItr.remove(); 
                 test(num,false,"iterator IllegalStateException test 1 ");
             } catch(IllegalStateException e1) {
                 try{
                     theirItr.remove();
                     test(num,false,"iterator IllegalStateException test 2");
                 } catch(IllegalStateException e2) {
                 }
             }
         } catch (Exception e) { test(num,false,e+" cp #26"); }


         try {
             ++num;
             mine = new DoublyLinkedList<Integer>();
             theirs = new ArrayList<Integer>();
             mine.add(1); theirs.add(1);
             Iterator<Integer> myItr = mine.iterator(),
                theirItr = theirs.iterator();
             mine.add(2); theirs.add(2);
             try {
                 myItr.next(); 
                 test(num,false,"iterator ConcurrentModificationException test #27");
             } catch(ConcurrentModificationException e1) {
                 try{
                     theirItr.next();
                     test(num,false,"iterator ConcurrentModificationException test #28");
                 } catch(ConcurrentModificationException e2) {
                 }
             }
         } catch (Exception e) { test(num,false,e+" cp #29"); }

         try {
             ++num;
             mine = new DoublyLinkedList<Integer>();
             theirs = new ArrayList<Integer>();
             for (int i = 1; i <= 5; i++) {
                 mine.add(i); theirs.add(i);
             }
             
             test(num,mine.toString().equals(theirs.toString()),"toString test");
         } catch (Exception e) { test(num,false,e+" cp #30"); }

         if (passedAll) System.out.println("passed all "+num+" tests");
      }
   
   }

Here is the output of my code

**** mine size: 1000
**** theirs size 1000
**** mine: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 171, 172, 173, 174, 175, 176, 177, 178, 179, 180, 181, 182, 183, 184, 185, 186, 187, 188, 189, 190, 191, 192, 193, 194, 195, 196, 197, 198, 199, 200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 211, 212, 213, 214, 215, 216, 217, 218, 219, 220, 221, 222, 223, 224, 225, 226, 227, 228, 229, 230, 231, 232, 233, 234, 235, 236, 237, 238, 239, 240, 241, 242, 243, 244, 245, 246, 247, 248, 249, 250, 251, 252, 253, 254, 255, 256, 257, 258, 259, 260, 261, 262, 263, 264, 265, 266, 267, 268, 269, 270, 271, 272, 273, 274, 275, 276, 277, 278, 279, 280, 281, 282, 283, 284, 285, 286, 287, 288, 289, 290, 291, 292, 293, 294, 295, 296, 297, 298, 299, 300, 301, 302, 303, 304, 305, 306, 307, 308, 309, 310, 311, 312, 313, 314, 315, 316, 317, 318, 319, 320, 321, 322, 323, 324, 325, 326, 327, 328, 329, 330, 331, 332, 333, 334, 335, 336, 337, 338, 339, 340, 341, 342, 343, 344, 345, 346, 347, 348, 349, 350, 351, 352, 353, 354, 355, 356, 357, 358, 359, 360, 361, 362, 363, 364, 365, 366, 367, 368, 369, 370, 371, 372, 373, 374, 375, 376, 377, 378, 379, 380, 381, 382, 383, 384, 385, 386, 387, 388, 389, 390, 391, 392, 393, 394, 395, 396, 397, 398, 399, 400, 401, 402, 403, 404, 405, 406, 407, 408, 409, 410, 411, 412, 413, 414, 415, 416, 417, 418, 419, 420, 421, 422, 423, 424, 425, 426, 427, 428, 429, 430, 431, 432, 433, 434, 435, 436, 437, 438, 439, 440, 441, 442, 443, 444, 445, 446, 447, 448, 449, 450, 451, 452, 453, 454, 455, 456, 457, 458, 459, 460, 461, 462, 463, 464, 465, 466, 467, 468, 469, 470, 471, 472, 473, 474, 475, 476, 477, 478, 479, 480, 481, 482, 483, 484, 485, 486, 487, 488, 489, 490, 491, 492, 493, 494, 495, 496, 497, 498, 499, 500, 501, 502, 503, 504, 505, 506, 507, 508, 509, 510, 511, 512, 513, 514, 515, 516, 517, 518, 519, 520, 521, 522, 523, 524, 525, 526, 527, 528, 529, 530, 531, 532, 533, 534, 535, 536, 537, 538, 539, 540, 541, 542, 543, 544, 545, 546, 547, 548, 549, 550, 551, 552, 553, 554, 555, 556, 557, 558, 559, 560, 561, 562, 563, 564, 565, 566, 567, 568, 569, 570, 571, 572, 573, 574, 575, 576, 577, 578, 579, 580, 581, 582, 583, 584, 585, 586, 587, 588, 589, 590, 591, 592, 593, 594, 595, 596, 597, 598, 599, 600, 601, 602, 603, 604, 605, 606, 607, 608, 609, 610, 611, 612, 613, 614, 615, 616, 617, 618, 619, 620, 621, 622, 623, 624, 625, 626, 627, 628, 629, 630, 631, 632, 633, 634, 635, 636, 637, 638, 639, 640, 641, 642, 643, 644, 645, 646, 647, 648, 649, 650, 651, 652, 653, 654, 655, 656, 657, 658, 659, 660, 661, 662, 663, 664, 665, 666, 667, 668, 669, 670, 671, 672, 673, 674, 675, 676, 677, 678, 679, 680, 681, 682, 683, 684, 685, 686, 687, 688, 689, 690, 691, 692, 693, 694, 695, 696, 697, 698, 699, 700, 701, 702, 703, 704, 705, 706, 707, 708, 709, 710, 711, 712, 713, 714, 715, 716, 717, 718, 719, 720, 721, 722, 723, 724, 725, 726, 727, 728, 729, 730, 731, 732, 733, 734, 735, 736, 737, 738, 739, 740, 741, 742, 743, 744, 745, 746, 747, 748, 749, 750, 751, 752, 753, 754, 755, 756, 757, 758, 759, 760, 761, 762, 763, 764, 765, 766, 767, 768, 769, 770, 771, 772, 773, 774, 775, 776, 777, 778, 779, 780, 781, 782, 783, 784, 785, 786, 787, 788, 789, 790, 791, 792, 793, 794, 795, 796, 797, 798, 799, 800, 801, 802, 803, 804, 805, 806, 807, 808, 809, 810, 811, 812, 813, 814, 815, 816, 817, 818, 819, 820, 821, 822, 823, 824, 825, 826, 827, 828, 829, 830, 831, 832, 833, 834, 835, 836, 837, 838, 839, 840, 841, 842, 843, 844, 845, 846, 847, 848, 849, 850, 851, 852, 853, 854, 855, 856, 857, 858, 859, 860, 861, 862, 863, 864, 865, 866, 867, 868, 869, 870, 871, 872, 873, 874, 875, 876, 877, 878, 879, 880, 881, 882, 883, 884, 885, 886, 887, 888, 889, 890, 891, 892, 893, 894, 895, 896, 897, 898, 899, 900, 901, 902, 903, 904, 905, 906, 907, 908, 909, 910, 911, 912, 913, 914, 915, 916, 917, 918, 919, 920, 921, 922, 923, 924, 925, 926, 927, 928, 929, 930, 931, 932, 933, 934, 935, 936, 937, 938, 939, 940, 941, 942, 943, 944, 945, 946, 947, 948, 949, 950, 951, 952, 953, 954, 955, 956, 957, 958, 959, 960, 961, 962, 963, 964, 965, 966, 967, 968, 969, 970, 971, 972, 973, 974, 975, 976, 977, 978, 979, 980, 981, 982, 983, 984, 985, 986, 987, 988, 989, 990, 991, 992, 993, 994, 995, 996, 997, 998, 999]
**** their [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 171, 172, 173, 174, 175, 176, 177, 178, 179, 180, 181, 182, 183, 184, 185, 186, 187, 188, 189, 190, 191, 192, 193, 194, 195, 196, 197, 198, 199, 200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 211, 212, 213, 214, 215, 216, 217, 218, 219, 220, 221, 222, 223, 224, 225, 226, 227, 228, 229, 230, 231, 232, 233, 234, 235, 236, 237, 238, 239, 240, 241, 242, 243, 244, 245, 246, 247, 248, 249, 250, 251, 252, 253, 254, 255, 256, 257, 258, 259, 260, 261, 262, 263, 264, 265, 266, 267, 268, 269, 270, 271, 272, 273, 274, 275, 276, 277, 278, 279, 280, 281, 282, 283, 284, 285, 286, 287, 288, 289, 290, 291, 292, 293, 294, 295, 296, 297, 298, 299, 300, 301, 302, 303, 304, 305, 306, 307, 308, 309, 310, 311, 312, 313, 314, 315, 316, 317, 318, 319, 320, 321, 322, 323, 324, 325, 326, 327, 328, 329, 330, 331, 332, 333, 334, 335, 336, 337, 338, 339, 340, 341, 342, 343, 344, 345, 346, 347, 348, 349, 350, 351, 352, 353, 354, 355, 356, 357, 358, 359, 360, 361, 362, 363, 364, 365, 366, 367, 368, 369, 370, 371, 372, 373, 374, 375, 376, 377, 378, 379, 380, 381, 382, 383, 384, 385, 386, 387, 388, 389, 390, 391, 392, 393, 394, 395, 396, 397, 398, 399, 400, 401, 402, 403, 404, 405, 406, 407, 408, 409, 410, 411, 412, 413, 414, 415, 416, 417, 418, 419, 420, 421, 422, 423, 424, 425, 426, 427, 428, 429, 430, 431, 432, 433, 434, 435, 436, 437, 438, 439, 440, 441, 442, 443, 444, 445, 446, 447, 448, 449, 450, 451, 452, 453, 454, 455, 456, 457, 458, 459, 460, 461, 462, 463, 464, 465, 466, 467, 468, 469, 470, 471, 472, 473, 474, 475, 476, 477, 478, 479, 480, 481, 482, 483, 484, 485, 486, 487, 488, 489, 490, 491, 492, 493, 494, 495, 496, 497, 498, 499, 500, 501, 502, 503, 504, 505, 506, 507, 508, 509, 510, 511, 512, 513, 514, 515, 516, 517, 518, 519, 520, 521, 522, 523, 524, 525, 526, 527, 528, 529, 530, 531, 532, 533, 534, 535, 536, 537, 538, 539, 540, 541, 542, 543, 544, 545, 546, 547, 548, 549, 550, 551, 552, 553, 554, 555, 556, 557, 558, 559, 560, 561, 562, 563, 564, 565, 566, 567, 568, 569, 570, 571, 572, 573, 574, 575, 576, 577, 578, 579, 580, 581, 582, 583, 584, 585, 586, 587, 588, 589, 590, 591, 592, 593, 594, 595, 596, 597, 598, 599, 600, 601, 602, 603, 604, 605, 606, 607, 608, 609, 610, 611, 612, 613, 614, 615, 616, 617, 618, 619, 620, 621, 622, 623, 624, 625, 626, 627, 628, 629, 630, 631, 632, 633, 634, 635, 636, 637, 638, 639, 640, 641, 642, 643, 644, 645, 646, 647, 648, 649, 650, 651, 652, 653, 654, 655, 656, 657, 658, 659, 660, 661, 662, 663, 664, 665, 666, 667, 668, 669, 670, 671, 672, 673, 674, 675, 676, 677, 678, 679, 680, 681, 682, 683, 684, 685, 686, 687, 688, 689, 690, 691, 692, 693, 694, 695, 696, 697, 698, 699, 700, 701, 702, 703, 704, 705, 706, 707, 708, 709, 710, 711, 712, 713, 714, 715, 716, 717, 718, 719, 720, 721, 722, 723, 724, 725, 726, 727, 728, 729, 730, 731, 732, 733, 734, 735, 736, 737, 738, 739, 740, 741, 742, 743, 744, 745, 746, 747, 748, 749, 750, 751, 752, 753, 754, 755, 756, 757, 758, 759, 760, 761, 762, 763, 764, 765, 766, 767, 768, 769, 770, 771, 772, 773, 774, 775, 776, 777, 778, 779, 780, 781, 782, 783, 784, 785, 786, 787, 788, 789, 790, 791, 792, 793, 794, 795, 796, 797, 798, 799, 800, 801, 802, 803, 804, 805, 806, 807, 808, 809, 810, 811, 812, 813, 814, 815, 816, 817, 818, 819, 820, 821, 822, 823, 824, 825, 826, 827, 828, 829, 830, 831, 832, 833, 834, 835, 836, 837, 838, 839, 840, 841, 842, 843, 844, 845, 846, 847, 848, 849, 850, 851, 852, 853, 854, 855, 856, 857, 858, 859, 860, 861, 862, 863, 864, 865, 866, 867, 868, 869, 870, 871, 872, 873, 874, 875, 876, 877, 878, 879, 880, 881, 882, 883, 884, 885, 886, 887, 888, 889, 890, 891, 892, 893, 894, 895, 896, 897, 898, 899, 900, 901, 902, 903, 904, 905, 906, 907, 908, 909, 910, 911, 912, 913, 914, 915, 916, 917, 918, 919, 920, 921, 922, 923, 924, 925, 926, 927, 928, 929, 930, 931, 932, 933, 934, 935, 936, 937, 938, 939, 940, 941, 942, 943, 944, 945, 946, 947, 948, 949, 950, 951, 952, 953, 954, 955, 956, 957, 958, 959, 960, 961, 962, 963, 964, 965, 966, 967, 968, 969, 970, 971, 972, 973, 974, 975, 976, 977, 978, 979, 980, 981, 982, 983, 984, 985, 986, 987, 988, 989, 990, 991, 992, 993, 994, 995, 996, 997, 998, 999]
21: fail - incorrect result: DoublyLinkedList(Collection) constructor test
23: fail - incorrect result: iterator remove() test
24: fail - incorrect result: iterator IllegalStateException test 1 
25: fail - incorrect result: iterator ConcurrentModificationException test #27

can anyone isolate and provide any insight on why I am getting those errors and how i can fix them?