LinkedList删除方法

    public E remove() {
        return removeFirst(); //删除第一个元素
    }

    public E removeFirst() {
        final Node<E> f = first;
        if (f == null)
            throw new NoSuchElementException();
        return unlinkFirst(f);
    }

    private E unlinkFirst(Node<E> f) {
        // assert f == first && f != null;
        final E element = f.item;
        final Node<E> next = f.next;
        f.item = null;
        f.next = null; // help GC   1
        first = next;                                2        
        if (next == null)
            last = null;                        
        else
            next.prev = null;                3
        size--;
        modCount++;
        return element;
    }

    public E removeLast() {
        final Node<E> l = last;
        if (l == null)
            throw new NoSuchElementException();
        return unlinkLast(l);
    }

    private E unlinkLast(Node<E> l) {
        // assert l == last && l != null;
        final E element = l.item;
        final Node<E> prev = l.prev;
        l.item = null;
        l.prev = null; // help GC   1
        last = prev;                              2
        if (prev == null)
            first = null;
        else
            prev.next = null;
        size--;
        modCount++;
        return element;
    }

    public E remove(int index) {
        checkElementIndex(index);
        return unlink(node(index));
    }

    E unlink(Node<E> x) {
        // assert x != null;
        final E element = x.item;
        final Node<E> next = x.next;
        final Node<E> prev = x.prev;
        if (prev == null) {   // 如果X为头指针
            first = next;            // 让first指针指向next
        } else {
            prev.next = next;  
            x.prev = null;
        }
        if (next == null) {  // 如果X为尾指针
            last = prev;        // 让last指针指向prev
        } else {
            next.prev = prev;
            x.next = null;
        }
        x.item = null;
        size--;
        modCount++;
        return element;
    }

    public boolean remove(Object o) {
        if (o == null) {
            for (Node<E> x = first; x != null; x = x.next) {
                if (x.item == null) {
                    unlink(x);
                    return true;
                }
            }
        } else {
            for (Node<E> x = first; x != null; x = x.next) {
                if (o.equals(x.item)) {
                    unlink(x);
                    return true;
                }
            }
        }
        return false;
    }

    public void clear() {
        // Clearing all of the links between nodes is "unnecessary", but:
        // - helps a generational GC if the discarded nodes inhabit
        //   more than one generation
        // - is sure to free memory even if there is a reachable Iterator
        // 从头到尾断开所有的引用关系
        for (Node<E> x = first; x != null; ) {
            Node<E> next = x.next;
            x.item = null;
            x.next = null;
            x.prev = null;
            x = next;
        }
        first = last = null;
        size = 0;
        modCount++;
    }