Node preX = x.prev;
Node nexX = x.next;
Node preY= y.prev;
Node nexY= y.next;

//edge case #1: x or y has nothing.
if(x==null || y == null) return;

//edge case #2: x == y, nothing to swap
if(x==y) return;

//edge case #3: edge case #3: x is right next to y

if (nexX == y)
{
    if(preX!=null) //if x has a previous node
        preX.next = y;

    y.prev = preX;

    if(nexY!=null) //if y has a next nodel
        nexY.prev = x;

    x.next = nexY;
    x.prev = y;
    y.next = x;
}
else if(nexY == x)
{
    if(preY != null) //if y has a previous node
        preY.next = x;

    x.prev = preY;;

    if(nexX != null) //if x has a next node
            nexX.prev = y;

    y.next = nexX;
    y.prev = x;
    x.next = y;
else
{
//swap part
y.prev = preX;
y.next = nexX;

if(nexX != null) //if x has a next node
    nexX.prev = y;

if(preX!=null) //if x has a previous node
    preX.next = y;

x.prev = preY;
x.next = nexY;

if(nexY!=null) //if y has a next nodel
    nexY.prev = x;

if(preY != null) //if y has a previous node
    preY.next = x;
}
//if either x or y was the head, the actual 'head' reference wasn't changed; so update it now.
if(head == x)
    head = y;
else if(head == y)
    head = x;

/* we need the following if tail exists
if(tail == x)
tail = y;
else if(tail == y)
tail = x;
*/