前言
為什么會(huì)想寫這東西呢？這是我們都非常常用的數(shù)據(jù)結(jié)構(gòu)，然而平時(shí)除了添加，遍歷操作，很少使用其他功能，即使是這樣，我們也存在萬(wàn)般理由搞清楚他們的 API，這對(duì)于我們?cè)诰幊虝r(shí)如何選擇二者具有良好的指導(dǎo)意義。所以，我覺(jué)得好好看看這二者的區(qū)別，之前一直以為只是 ArrayList 和 LinkedList 只是數(shù)組和鏈表的區(qū)別，我已經(jīng)大錯(cuò)特錯(cuò)了， Java8 的實(shí)現(xiàn)遠(yuǎn)遠(yuǎn)不止于此，LinkedList 還有雙端隊(duì)列的功能，之前一直沒(méi)注意到，最近發(fā)現(xiàn) ArrayDeque，卻沒(méi)有看到 LinkedDeque，恍然想起 LinkedList 實(shí)現(xiàn)了 Deque 接口，這才恍然大悟～～
ArrayList & LinkedList
他們的區(qū)別從實(shí)現(xiàn)的接口上也可以看出來(lái)， LinkedList 多實(shí)現(xiàn)了一個(gè)接口，下次面試官再問(wèn)到，我們可以談?wù)勥@個(gè)問(wèn)題，面試官會(huì)另眼相看的，面試官就喜歡深入研究的
public class ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable
public class LinkedList<E>
extends AbstractSequentialList<E>
implements List<E>, Deque<E>, Cloneable, java.io.Serializable
ArrayList 原理
初始大小為 10，transient Object[] elementData 用來(lái)存儲(chǔ)元素
/**
* Default initial capacity.
*/
private static final int DEFAULT_CAPACITY = 10;
/**
* Shared empty array instance used for empty instances.
*/
private static final Object[] EMPTY_ELEMENTDATA = {};
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
transient Object[] elementData; // non-private to simplify nested class access
private int size;
public ArrayList(int initialCapacity) {
if (initialCapacity > 0) {
this.elementData = new Object[initialCapacity];
} else if (initialCapacity == 0) {
this.elementData = EMPTY_ELEMENTDATA;
} else {
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
}
}
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
add 方法，首先檢查容量，檢查的時(shí)候首先在是否是空的，獲取一個(gè)最小容量，才去執(zhí)行擴(kuò)容，為 elementData 賦值
public boolean add(E e) {
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}
private void ensureCapacityInternal(int minCapacity) {
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
}
ensureExplicitCapacity(minCapacity);
}
private void ensureExplicitCapacity(int minCapacity) {
modCount++;
// overflow-conscious code
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}
擴(kuò)容策略，每次增長(zhǎng) 1/2
private void grow(int minCapacity) {
// overflow-conscious code
int oldCapacity = elementData.length;
int newCapacity = oldCapacity + (oldCapacity >> 1);
if (newCapacity - minCapacity < 0)
newCapacity = minCapacity;
if (newCapacity - MAX_ARRAY_SIZE > 0)
newCapacity = hugeCapacity(minCapacity);
// minCapacity is usually close to size, so this is a win:
elementData = Arrays.copyOf(elementData, newCapacity);
}
如果你想在數(shù)組中確定某一個(gè)元素的話，那么需要遍歷，有兩種遍歷方式，可根據(jù)場(chǎng)景自行選擇
public int indexOf(Object o) {
if (o == null) {
for (int i = 0; i < size; i++)
if (elementData[i]==null)
return i;
} else {
for (int i = 0; i < size; i++)
if (o.equals(elementData[i]))
return i;
}
return -1;
}
public int lastIndexOf(Object o) {
if (o == null) {
for (int i = size-1; i >= 0; i--)
if (elementData[i]==null)
return i;
} else {
for (int i = size-1; i >= 0; i--)
if (o.equals(elementData[i]))
return i;
}
return -1;
}
toArray 通常需要進(jìn)行類型強(qiáng)轉(zhuǎn)
public Object[] toArray() {
return Arrays.copyOf(elementData, size);
}
獲取某個(gè)索引的元素，我們看到也是強(qiáng)轉(zhuǎn)類型
E elementData(int index) {
return (E) elementData[index];
}
/**
* Returns the element at the specified position in this list.
*
* @param index index of the element to return
* @return the element at the specified position in this list
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public E get(int index) {
rangeCheck(index);
return elementData(index);
}
還有 set 方法，覆蓋之前的值，返回舊值
public E set(int index, E element) {
rangeCheck(index);
E oldValue = elementData(index);
elementData[index] = element;
return oldValue;
}
插入，需要移動(dòng)之后的所有元素
public void add(int index, E element) {
rangeCheckForAdd(index);
ensureCapacityInternal(size + 1); // Increments modCount!!
System.arraycopy(elementData, index, elementData, index + 1,
size - index);
elementData[index] = element;
size++;
}
刪除某個(gè)索引的元素，同樣需要移動(dòng)元素
public E remove(int index) {
rangeCheck(index);
modCount++;
E oldValue = elementData(index);
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
return oldValue;
}
刪除某個(gè) object，看到了 fastRemove，只不過(guò)是去除了邊界驗(yàn)證
public boolean remove(Object o) {
if (o == null) {
for (int index = 0; index < size; index++)
if (elementData[index] == null) {
fastRemove(index);
return true;
}
} else {
for (int index = 0; index < size; index++)
if (o.equals(elementData[index])) {
fastRemove(index);
return true;
}
}
return false;
}
LinkedList
由于是鏈表，很多首尾節(jié)點(diǎn)的插入刪除操作，方便了很多
屬性
很簡(jiǎn)單的三個(gè)屬性
transient int size = 0;
/**
* Pointer to first node.
* Invariant: (first == null && last == null) ||
* (first.prev == null && first.item != null)
*/
transient Node<E> first;
/**
* Pointer to last node.
* Invariant: (first == null && last == null) ||
* (last.next == null && last.item != null)
*/
transient Node<E> last;
node 靜態(tài)內(nèi)部類，根據(jù) Effective Java 的描述，這種設(shè)計(jì)是比較好的。
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;
}
}
我們簡(jiǎn)單看一些方法即可，由于實(shí)現(xiàn)了 Deque，大部分操作非常相似
add 在尾部添加元素，無(wú)需邊界檢查
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++;
}
獲取元素，需要注意的是為了加快速度，會(huì)判斷更接近后邊還是前邊，這樣能省一般時(shí)間
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;
}
}
而 indexOf(Object o) 就沒(méi)這個(gè)幸運(yùn)了，必須老老實(shí)實(shí)從頭開(kāi)始
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;
}
一些關(guān)于隊(duì)列的操作，注意方法的返回值，如果出現(xiàn)為空是否會(huì)拋出異常呢，像 element pop remove 會(huì)拋異常，而 peek，poll 則不會(huì)，這些都要注意，否則一不小心就會(huì)引發(fā)慘案?。?！
public E peek() {
final Node<E> f = first;
return (f == null) ? null : f.item;
}
/**
* Retrieves, but does not remove, the head (first element) of this list.
*
* @return the head of this list
* @throws NoSuchElementException if this list is empty
* @since 1.5
*/
public E element() {
return getFirst();
}
public E getFirst() {
final Node<E> f = first;
if (f == null)
throw new NoSuchElementException();
return f.item;
}
/**
* Retrieves and removes the head (first element) of this list.
*
* @return the head of this list, or {@code null} if this list is empty
* @since 1.5
*/
public E poll() {
final Node<E> f = first;
return (f == null) ? null : unlinkFirst(f);
}
public E pop() {
return removeFirst();
}
其它的便不貼代碼了，都很好理解了
小結(jié)
總的來(lái)說(shuō)，對(duì)這兩個(gè)東西的用法更加清晰了，來(lái)龍去脈摸得更準(zhǔn)確了～
