public void addLast(Item item) {
if (item == null) {
throw new java.lang.NullPointerException("Cannot add a null item!");
}
Node oldLast = last;
last = new Node();
last.item = item;
last.next = null;
if (isEmpty()) {
last.pre = null;
first = last;
} else {
oldLast.next = last;
last.pre = oldLast;
}
N++;
}
public void addFirst(Item item) {
if (item == null) {
throw new java.lang.NullPointerException("Cannot add a null item!");
}
Node oldFirst = first;
first = new Node();
first.item = item;
first.pre = null;
if (last == null) { // condition must be this. Cannot be
// "if(isEmpty())"!!! because it can't be empty now
last = first;
} else {
first.next = oldFirst;
oldFirst.pre = first;
}
N++;
}