【Java集合類】LinkedList原始碼分析(jdk1.8)
阿新 • • 發佈:2018-12-15
ArrayList和LinkedList是List介面的兩種實現,具有相同的查詢、插入、刪除操作,只是底層的實現方式不一樣。LinkedList是以雙向連結串列形式實現的集合類。
其增刪操作由於不需要移動底層陣列資料,只需要修改連結串列節點指標,所以效率較高。但是隨機訪問時的定位操作效率較低,需要遍歷連結串列節點。(ArrayList與之相反)
資料結構
底層基於雙向連結串列實現的。並且元素允許 null 的存在。
其原始碼如下:
//沒有空頭節點的雙向連結串列 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; } } //表的長度 transient int size = 0; /** * Pointer to first node.表的頭節點指標 * Invariant: (first == null && last == null) || * (first.prev == null && first.item != null) */ transient Node<E> first; /** * Pointer to last node.表的尾節點指標 * Invariant: (first == null && last == null) || * (last.next == null && last.item != null) */ transient Node<E> last;
初始化
由於連結串列,初始化時不需要指定表的長度。
/** * Constructs an empty list. */ public LinkedList() { } /** * Constructs a list containing the elements of the specified * collection, in the order they are returned by the collection's * iterator. * * @param c the collection whose elements are to be placed into this list * @throws NullPointerException if the specified collection is null */ public LinkedList(Collection<? extends E> c) { this(); addAll(c); }
擴容
由於連結串列,擴容時只需要新建節點。
查詢操作
1、通過索引查詢元素(隨機訪問):
注意node(int index)
函式:通過索引找到對應的節點。(後面很多方法都會用的)
public E get(int index) { checkElementIndex(index); return node(index).item; } /** * Returns the (non-null) Node at the specified element index. */ //時間複雜度為n/2 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; } }
2、直接查詢元素:
public int indexOf(Object o) {
int index = 0;
if (o == null) {
for (Node<E> x = first; x != null; x = x.next) {
if (x.item == null)
return index;
index++;
}
} else {
for (Node<E> x = first; x != null; x = x.next) {
if (o.equals(x.item))
return index;
index++;
}
}
return -1;
}
public int lastIndexOf(Object o) {
int index = size;
if (o == null) {
for (Node<E> x = last; x != null; x = x.prev) {
index--;
if (x.item == null)
return index;
}
} else {
for (Node<E> x = last; x != null; x = x.prev) {
index--;
if (o.equals(x.item))
return index;
}
}
return -1;
}
插入操作
在介紹插入操作前,首先介紹根據插入連結串列位置的不同,幾種基本的插入連結串列方式:
- (1)元素作為頭節點插入連結串列:
/**
* Links e as first element.
*/
private void linkFirst(E e) {
final Node<E> f = first;
final Node<E> newNode = new Node<>(null, e, f);
first = newNode;
if (f == null)
last = newNode;
else
f.prev = newNode;
size++;
modCount++;
}
public void addFirst(E e) {
linkFirst(e);
}
- (2)元素作為尾節點插入連結串列:
/**
* 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++;
}
public void addLast(E e) {
linkLast(e);
}
- (3)元素作為節點e的前驅節點插入連結串列(相當於元素作為連結串列的中間節點):
/**
* Inserts element e before non-null Node succ.
*/
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++;
}
1、插入單個元素
public boolean add(E e) {
linkLast(e);
return true;
}
public void add(int index, E element) {
checkPositionIndex(index);
if (index == size)
linkLast(element);
else
//在node(index)節點之前插入
linkBefore(element, node(index));
}
2、插入集合:
public boolean addAll(Collection<? extends E> c) {
return addAll(size, c);
}
//插入成第index個節點(= 即index節點後移 = 即在index節點之前插入)(index從0開始)
public boolean addAll(int index, Collection<? extends E> c) {
checkPositionIndex(index);
Object[] a = c.toArray();
int numNew = a.length;
if (numNew == 0)
return false;
//開始遍歷之前的前驅節點,最終的後繼節點
Node<E> pred, succ;
//特殊情況:當索引index=size時,通過node(index)找不到節點,超過範圍
//(那為什麼不把索引設定為index-1,即插入到index-1節點後?那插入到0節點之前也是特殊情況)
if (index == size) {
succ = null;
pred = last;
} else {
//找到第index個節點
succ = node(index);
pred = succ.prev;
}
for (Object o : a) {
@SuppressWarnings("unchecked") E e = (E) o;
//後繼待定,指向下個遍歷的節點
Node<E> newNode = new Node<>(pred, e, null);
if (pred == null)
first = newNode;
else
//新節點左邊的所有連結成功
pred.next = newNode;
pred = newNode;
}
//連線最後一個節點pred右邊的所有連結
if (succ == null) {
last = pred;
} else {
pred.next = succ;
succ.prev = pred;
}
size += numNew;
modCount++;
return true;
}
刪除操作
首先介紹根據刪除節點在連結串列位置的不同,幾種基本的刪除節點方式:
- (1)刪除頭節點:
/**
* Unlinks non-null first node 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
first = next;
if (next == null)
last = null;
else
next.prev = null;
size--;
modCount++;
return element;
}
public E removeFirst() {
final Node<E> f = first;
if (f == null)
throw new NoSuchElementException();
return unlinkFirst(f);
}
- (2)刪除尾節點:
/**
* Unlinks non-null last node 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
last = prev;
if (prev == null)
first = null;
else
prev.next = null;
size--;
modCount++;
return element;
}
public E removeLast() {
final Node<E> l = last;
if (l == null)
throw new NoSuchElementException();
return unlinkLast(l);
}
- (3)刪除中間節點:
/**
* Unlinks non-null node 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;
}
x.item = null;
size--;
modCount++;
return element;
}
1、通過索引刪除元素:
public E remove(int index) {
checkElementIndex(index);
return unlink(node(index));
}
2、刪除指定元素:
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;
}
佇列操作
主要操作:
//取佇列頭節點,但不刪除
//若first為空時,不丟擲錯誤
public E peek() {
final Node<E> f = first;
return (f == null) ? null : f.item;
}
//若first為空時,丟擲錯誤
public E element() {
return getFirst();
}
//出佇列(頭刪)
public E poll() {
final Node<E> f = first;
return (f == null) ? null : unlinkFirst(f);
}
//入佇列(尾插)
public boolean offer(E e) {
return add(e);
}
棧操作
主要操作:
//入棧,頭插
public void push(E e) {
addFirst(e);
}
//出棧,頭刪
public E pop() {
return removeFirst();
}
轉化成陣列
public Object[] toArray() {
Object[] result = new Object[size];
int i = 0;
for (Node<E> x = first; x != null; x = x.next)
result[i++] = x.item;
return result;
}
@SuppressWarnings("unchecked")
public <T> T[] toArray(T[] a) {
if (a.length < size)
//為什麼用到反射?泛型類在執行時無法new例項化,只能在執行時得到class物件
a = (T[])java.lang.reflect.Array.newInstance(
a.getClass().getComponentType(), size);
int i = 0;
Object[] result = a;
for (Node<E> x = first; x != null; x = x.next)
result[i++] = x.item;
if (a.length > size)
a[size] = null;
return a;
}