package  a6;

public final class ListHead {

    public ListHead next;
    public ListHead prev;
    public Object data;

    public ListHead() {
        init();
    }

    public final void del() {
        next.prev = prev;
        prev.next = next;
        init();
    }

    public final void delInit() {
        del();
    }

    public final void addTail(ListHead pnew) {
        ListHead tmp_prev = prev;
        prev = pnew;
        pnew.next = this;
        pnew.prev = tmp_prev;
        tmp_prev.next = pnew;
    }

    public final Object firstEntry() {
        return next.data;
    }

    public final void replace(ListHead pnew) {
        pnew.next = next;
        pnew.next.prev = pnew;
        pnew.prev = prev;
        pnew.prev.next = pnew;
        pnew.data = data;
    }

    public final void replaceInit(ListHead pnew) {
        replace(pnew);
        init();
    }

    public final boolean empty() {
        return next == this;
    }

    public final void init() {
        next = this;
        prev = this;
    }

}