Doubly Link List code

Question

Hi,
I have got the following code for doubly link list from a book:
public Link deleteFirst ( ) {
Link temp=First;
If(first.next==null)
last = null;
else
first.next.previous=null;
first=first.next;
return temp;
In the above if condition, when we one element in the list, I feel that first should also point to null. If I code it in this way,is it correct?
if(first.next==null)
last=null;
first= null;
return temp;
else {
first.next.previous=null;
first=first.next;
return temp;
Is the modified code correct?
Zulfi.

Answer 1

Your modification gives the correct result, but so does the original even though it might not be immediately obvious. Your version is definitely easier to read :)
Here's the original broken in parts:// temp now points to first
Link temp=first;
// if there is only one element
if (first.next == null) {
    // the last element should be null because it's removed
    last = null;
} else {
    // else break the link from the second element to the first
    first.next.previous=null;
// update "first" to the second element in the list.
// NOTICE: if the list has only one element and it's properly
// constructed this will set "first" to null!
first=first.next;
// return the removed element
return temp;Actually neither implementations are perfect; they don't check if you are trying to remove the first element of an empty list...