int numMoved = size - index - 1; if (numMoved > 0) System.arraycopy(elementData, index+1, elementData, index, numMoved); elementData[--size] = null; // clear to let GC do its work
if (index < (size >> 1)) { Node<E> x = first; for (int i = 0; i < index; i++) x = x.next; return x; } else { Node<E> x = last; for (int i = size - 1; i > index; i--) x = x.prev; return x; } }
增加和删除快
LinkedList 新增元素的方法有两种,一种是将添加的元素加到队尾,首先是将 last 元素置换到临时变量中,生成一个新的 Node 节点对象,然后将 last 引用指向新节点对象,之前的 last 对象的前指针指向新节点对象。另外一种是添加元素到任意位置,如果我们是将元素添加到任意两个元素的中间位置,添加元素操作只会改变前后元素的前后指针,指针将会指向添加的新元素。底层源码如下:
publicvoid add(intindex, E element) { checkPositionIndex(index);
if (index == size) linkLast(element); else linkBefore(element, node(index)); }
void linkLast(E e) { final Node<E> l = last; final Node<E> newNode = new Node<>(l, e, null); last = newNode; if (l == null) first = newNode; else l.next = newNode; size++; modCount++; }
void linkBefore(E e, Node<E> succ) { // assert succ != null; final Node<E> pred = succ.prev; final Node<E> newNode = new Node<>(pred, e, succ); succ.prev = newNode; if (pred == null) first = newNode; else pred.next = newNode; size++; modCount++; }
LinkedList 删除元素的操作中,我们首先要通过循环找到要删除的元素,如果要删除的位置处于 List 的前半段,就从前往后找;若其位置处于后半段,就从后往前找。
if (index < (size >> 1)) { Node<E> x = first; for (int i = 0; i < index; i++) x = x.next; return x; } else { Node<E> x = last; for (int i = size - 1; i > index; i--) x = x.prev; return x; } }
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) { first = next; } else { prev.next = next; x.prev = null; }
if (next == null) { last = prev; } else { next.prev = prev; x.next = null; }