add()
add()方法有两个版本,一个是add(E e),该方法在LinkedList的末尾插入元素,因为有last指向链表末尾,在末尾插入元素的花费是常数时间。只需要简单修改几个相关引用即可;另一个是add(int index, E element),该方法是在指定下表处插入元素,需要先通过线性查找找到具体位置,然后修改相关引用完成插入操作。
/**
* Appends the specified element to the end of this list.
*
* <p>This method is equivalent to {@link #addLast}.
*
* @param e element to be appended to this list
* @return {@code true} (as specified by {@link Collection#add})
*/
public boolean add(E e) {
linkLast(e);
return true;
}
/**
* Links e as last element.
*/
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++;
}
add(int index, E element), 当index==size时,等同于add(E e); 如果不是,则分两步: 1.先根据index找到要插入的位置,即node(index)方法;2.修改引用,完成插入操作。
/**
* Inserts the specified element at the specified position in this list.
* Shifts the element currently at that position (if any) and any
* subsequent elements to the right (adds one to their indices).
*
* @param index index at which the specified element is to be inserted
* @param element element to be inserted
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public void add(int index, E element) {
checkPositionIndex(index);
if (index == size)
linkLast(element);
else
linkBefore(element, node(index));
}
上面代码中的node(int index)函数有一点小小的trick,因为链表双向的,可以从开始往后找,也可以从结尾往前找,具体朝那个方向找取决于条件index < (size >> 1),也即是index是靠近前端还是后端。从这里也可以看出,linkedList通过index检索元素的效率没有arrayList高。
/**
* Returns the (non-null) Node at the specified element index.
*/
Node<E> node(int index) {
// assert isElementIndex(index);
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;
}
}
------
原文链接:https://pdai.tech/md/java/collection/java-collection-LinkedList.html
|