java基礎集合類——LinkedList 源碼略讀
阿新 • • 發佈:2019-05-08
serializa 鏈表 ngs 功能 ray 方法 == memory ()
1.概覽
LinkedList是java的動態數組另一種實現方式,底層是基於雙向鏈表,而不是數組。
public class LinkedList<E>
extends AbstractSequentialList<E>
implements List<E>, Deque<E>, Cloneable, java.io.Serializable
LinkedList實現了動態數組與雙向隊列兩個接口,提供了兩種方法集合,可以用來實現隊列、棧之類的功能。
2. 成員變量
先來看成員變量
transient int size = 0; transient Node<E> first; transient Node<E> last; private static class Node<E> { E item; Node<E> next; Node<E> prev; Node(Node<E> prev, E element, Node<E> next) { this.item = element; this.next = next; this.prev = prev; } }
鏈表一般就是有個head的節點就能完成對應的工作。LinkedList實現了雙向鏈表,除了head,還有一個last節點和一個size參數,這主要是為了效率考慮,不然查詢一次長度或者尾都得來一次全鏈路叠代,太慢了。Node內部類就不說了,非常簡單的一個節點類。
3. 方法
3.1 構造方法
public LinkedList() { // 此時first=last=null,size=0 } public LinkedList(Collection<? extends E> c) { this(); addAll(c); }
3.2 添加元素
添加一個元素
public boolean add(E e) { linkLast(e); return true; } 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++; }
很簡單的添加邏輯,再來看一下addAll的實現
public boolean addAll(Collection<? extends E> c) {
return addAll(size, c);
}
public boolean addAll(int index, Collection<? extends E> c) {
checkPositionIndex(index);
Object[] a = c.toArray();
int numNew = a.length;
if (numNew == 0)
return false;
Node<E> pred, succ;
if (index == size) {
succ = null;
pred = last;
} else {
succ = node(index);
pred = succ.prev;
}
for (Object o : a) {
@SuppressWarnings("unchecked") E e = (E) o;
Node<E> newNode = new Node<>(pred, e, null);
if (pred == null)
first = newNode;
else
pred.next = newNode;
pred = newNode;
}
if (succ == null) {
last = pred;
} else {
pred.next = succ;
succ.prev = pred;
}
size += numNew;
modCount++;
return true;
}
3.3 刪除元素
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) {
first = next;
} else {
prev.next = next;
x.prev = null;
}
if (next == null) {
last = prev;
} else {
next.prev = prev;
x.next = null;
}
x.item = null;
size--;
modCount++;
return element;
}
3.4 修改元素
public E set(int index, E element) {
checkElementIndex(index);
Node<E> x = node(index);
E oldVal = x.item;
x.item = element;
return oldVal;
}
3.5 檢索元素
public E get(int index) {
checkElementIndex(index);
return node(index).item;
}
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;
}
}
檢索是LinkedList比較值得看的一個方法,java的實現很簡單,先判斷index是大於當前size的一半還是小於,如果是大於則從尾節點往前否則從首結點往後檢索。從代碼上看,雖然雙向鏈表的實現讓性能快了一點,但還是O(n)的耗時,我覺得後續版本的優化可以向HashMap那樣,當判斷LinkedList的size大於一個閾值時可以將雙向鏈接改造為紅黑樹或者跳表,從而實現O(lgn)的性能,當然這樣也對空間消耗更多一點。
3.6 清空元素
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
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++;
}
從代碼上看,LinkedList的clear方法是沒有內存泄漏問題的,註意有個for循環,這裏是為了gc優化。
java基礎集合類——LinkedList 源碼略讀