LinearLinkedList

[Lil_Aziz1]

I recently started reading about Linked Lists and I have a couple of questions. Before I start asking, let me post the ListNode class:

package linkedLists;

public class ListNode {
	private Object value;
	private ListNode next;
	
	public ListNode(Object value, ListNode next) {
		this.value = value;
		this.next = next;
	}
	
	public Object getValue() {
		return value;
	}
	
	public ListNode getNext() {
		return next;
	}
	
	public void setValue(Object value) {
		this.value = value;
	}
	
	public void setNext(ListNode next) {
		this.next = next;
	}
}

Pretty straightforward. Next, there is the LinearLinkedList class:

package linkedLists;

public class LinearLinkedList {
	
	ListNode firstNode;
	
	public LinearLinkedList() {
		firstNode = null;
	}
	
	public boolean isEmpty() {
		return firstNode == null;
	}
	
	public ListNode getFirstNode() {
		return firstNode;
	}
	
	public void setFirstNode(ListNode node) {
		firstNode = node;
	}
	
	public void addFirst(Object o) {
		if (isEmpty())
			firstNode = new ListNode(o, null);
		else {
			//[I]implementation shown below[/I]
		}
	}
	
	public void addLast(Object o) {
		if (isEmpty())
			firstNode = new ListNode (o, null);
		else {
			//[I]implementation shown below[/I]
		}
	}
       
        [I]...other codes will be implemented later. (not in this thread)[/I]
}

Question 1: addFirst()
The book implements the missing code like this:

firstNode = new ListNode(o, firstNode);

When I did it, I did another way:

			ListNode first = new ListNode(o, null);
			first.setNext(firstNode.getNext());
			firstNode = first;

The question is, are those two similar?

Question 2: addLast()
The book implements the missing code like this:

ListNode current = firstNode;
while (current.getNext() != null)
     current = current.getNext();
current.setNext(new ListNode(o, null));

Mine on the other hand, gets a null pointer access (yellow underline in eclipse. The underlined word is "current" in "current.setNext(new ListNode(o, null));"

while ((current = current.getNext()) != null);
[U]current[/U].setNext(new ListNode(o, null));
firstNode = current;

Why am I getting the null pointer access? Also, why doesn't the book's code have the following:

firstNode = current;

Don't I need to reference firstNode to current because current is a local variable?

Thanks in advance!!

[Norm]

Its better to ALWAYS use {} with loops and if statements.
On What condition do you fall out of the while loop? What value does current have then?

[Lil_Aziz1]

I fall out of the while loop when current.getNext() is equal to null.

EDIT: OHH. so the book's while loop is one iteration behind me, which is where it's suppose to be. Mine goes one iteration too far; thus, it's value is null!

Thanks! Rep+! What about the first question?

[Norm]

while (current.getNext() != null)
current = current.getNext();

vs

while (current.getNext() != null) {
current = current.getNext();
}

Some day you'll add a line after the while and everything will go to s.

[Lil_Aziz1]

I don't know if you noticed, but there is a semicolon after the while loop:

while ((current = current.getNext()) != null);
current.setNext(new ListNode(o, null));
firstNode = current;

I intentionally put it the semicolon so all it does is iterate through the firstNode and make current get to the last element.

I learned that the following two snippets are equivalent.

while ((current = current.getNext()) != null);

while ((current = current.getNext()) != null) {
}

Although I tried it with the {} and nothing changed. I'm still getting the null pointer access.

[Norm]

The first loop does NOT have a ; after the condition:

The book implements the missing code like this:
Code:
ListNode current = firstNode;
while (current.getNext() != null)
current = current.getNext();
current.setNext(new ListNode(o, null));
Mine on the other hand, gets a null pointer access (yellow underline in eclipse. The underlined word is "current" in "current.setNext(new ListNode(o, null));"
Code:
while ((current = current.getNext()) != null);
current.setNext(new ListNode(o, null));
firstNode = current;

[Lil_Aziz1]

Right. But aren't the {} implied if there is only one line of code to execute? I double checked the book and this is exactly how they had it so it's not a typo on my part.

[Norm]

I'm not talking about legal statements.
I'm talking about safe code.

[Lil_Aziz1]

Oh okay.

Going back on topic, I think I got almost all my questions answered except this part:

firstNode = current;

Don't I need to reference firstNode to current because current is a local variable?
Why don't we have to do that?

[JosAH]

while ((current = current.getNext()) != null);
[U]current[/U].setNext(new ListNode(o, null));
firstNode = current;

With code like that you're taking one step too many and you're falling of your list. The last element of your list is the element which doesn't have a next element (read that again ;-) so you have to change your loop like this:

for (current= firstElemt; current != null && current.getNext() != null; current= current.getNext());

The test current != null is necessary in case the list is empty. While the current element has a next element it 'hops' to the next element, otherwise the loop terminates and current points to the last element in the list.

kind regards,

Jos