Swapping elements in a double-linked list

List.java

class List {
    int key;
    List next, prev;
}

swap.java

void swap(List a, List b) {
    if (a == b)
        return;

    if (a.next == b) { // right next to each other
        a.next = b.next;
        b.prev = a.prev;

        if (a.next != null)
            a.next.prev = a;

        if (b.prev != null)
            b.prev.next = b;


        b.next = a;
        a.prev = b;
    } else {
        List p = b.prev;
        List n = b.next;

        b.prev = a.prev;
        b.next = a.next;

        a.prev = p;
        a.next = n;

        if (b.next != null)
            b.next.prev = b;
        if (b.prev != null)
            b.prev.next = b;

        if (a.next != null)
            a.next.prev = a;
        if (a.prev != null)
            a.prev.next = a;

    }
}

Was stuck for a while. This solution helped a lot. Thank you!

Muchisimas gracias, me ayudo mucho