New to Java

Hi there.

I'm having trouble with a final method which I'm trying to implement. I am using the following interface:

public interface ILinkedList {

	public abstract Object getHead();

	public abstract ILinkedList getTail();

	public abstract void setHead(Object head);

	public abstract void setTail(ILinkedList tail);

}

And have the following two classes:

public class LinkedList implements ILinkedList {

	private Object head;
	private ILinkedList tail;

	public Object getHead() {
		return head;
	}

	public ILinkedList getTail() {
		return tail;
	}

	public void setHead(Object head) {
		this.head = head;
	}

	public void setTail(ILinkedList tail) {
		this.tail = tail;
	}

	public LinkedList(Object head, ILinkedList tail) {
		this.head = head;
		this.tail = tail;
	}

}

and

public class ListManipulation {

	/*
	 * Return a string representing the list as a comma-separated list of values
	 * enclosed in [...] . Use the toString() method of the individual list
	 * entries to print the values.
	 * 
	 * An example of an iterative method. The complexity of this method comes
	 * from the fact that a comma-separated list of n objects has n-1 commas
	 * unless n = 0. So there needs to be a special case.
	 */
	public static String convertToString(ILinkedList l) {
		/*
		 * build up the result in a StringBuffer rather than a String for
		 * efficiency
		 */
		StringBuffer s = new StringBuffer("[");
		if (l != null) {
			/*
			 * deal with the first element specially, as there is no comma
			 * before it
			 */
			s.append(l.getHead().toString());
			l = l.getTail();
			/*
			 * Now things are regular and we go into a loop to run down the rest
			 * of the LL printing each item preceded by a comma
			 */
			while (l != null) {
				s.append(", " + l.getHead().toString());
				l = l.getTail();
			}
		}
		/* Finally close the bracket and convert to a String */
		s.append("]");
		return s.toString();
	}

	/*
	 * return a new list consisting of the elements of l1, followed by those of
	 * l2. The result may share nodes with l2 if this is useful
	 * 
	 * implement recursively
	 */
	public static ILinkedList append(ILinkedList l1, ILinkedList l2) {

		while (l2 != null) {
			l1.setTail(l2);
			l2 = l2.getTail();
		}
		// l2 is null and so new list is just l1
		return l1;

	}
}

It is the append method which I'm unsure of. I understand that if I have two linked lists (written in shorthand as) [1,2,3] & [4,5], that I want to take the head of the second list (4) and append it to the tail of the first list (2,3) - I have to implement this recursively. What I have at the moment in the append method, I believe will make the first list [1,4], as I think it will replace the current tail with the head of the second list.

Does anybody have any suggestions as to how I would otherwise implement this to append the second list onto the first? I tried l1.setTail(l2.getHead()); but the interface doesn't allow this (and the interface cannot be changed).

Thank you to anybody who is able to suggest anything :)
Christopher

edit: i've taken out some extraneous code that I realised would just be cluttering up the place, and might be putting off people helping. I hope someone is able to offer some advice :)
I have also posted the same question at another help forum, to see if someone there is able to help. If they do, I shall let you know so that the same things aren't being said more than once.
http://www.java-forums.org/new-java/13550-linked-list-manipulation.html#post44543

Edited by: chrisdb89 on Nov 21, 2008 8:41 AM