【補(bǔ)充說明：】
【modCount】
在父類AbstractList中定義了一個(gè)int型的屬性：modCount，記錄了ArrayList結(jié)構(gòu)性變化的次數(shù)。
protected transient int modCount = 0; * 在ArrayList的所有涉及結(jié)構(gòu)變化的方法中都增加modCount的值，包括：add()、remove()、addAll()、removeRange()及clear()方法。這些方法每調(diào)用一次，modCount的值就加1。
注：add()及addAll()方法的modCount的值是在其中調(diào)用的ensureCapacity()方法中增加的。
【調(diào)用add()方法時(shí)，調(diào)用的函數(shù)：】
程序調(diào)用add，實(shí)際上還會(huì)進(jìn)行一系列調(diào)用，可能會(huì)調(diào)用到grow，grow可能會(huì)調(diào)用hugeCapacity。
add ——>ensureCapacityInternal ——>ensureExplicitCapacity - - ->grow - - ->hugeCapacity
添加 確保內(nèi)部容量(是否擴(kuò)容) 確保明確的容量 擴(kuò)容函數(shù) 指定新容量
實(shí)例看圖
【總結(jié)】：
增：僅是將這個(gè)元素添加到末尾。操作快速
刪：由于需要移動(dòng)插入位置后的元素，并且涉及到數(shù)組的復(fù)制。操作較慢
改：直接對(duì)指定位置元素進(jìn)行修改，不涉及元素的挪動(dòng)和數(shù)組賦值。操作快速
查：直接返回指定下標(biāo)的數(shù)組元素。操作快速
ArrayList有其特殊的應(yīng)用場景，與LinkedList相對(duì)應(yīng)。其優(yōu)點(diǎn)是隨機(jī)讀取，
缺點(diǎn)是插入元素時(shí)需要移動(dòng)大量元素，效率不太高。[查找修改快而插入刪除慢的特點(diǎn)]
ArrayList實(shí)例01(數(shù)組).png
ArrayList實(shí)例02(數(shù)組).png
【ArrayList的一些屬性：】
public class ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
/**
* 版本號(hào)
*/
private static final long serialVersionUID = 8683452581122892189L;
/**
* Default initial capacity.
* 默認(rèn)初始容量
*/
private static final int DEFAULT_CAPACITY = 10;
/**
* Shared empty array instance used for empty instances.
* 用于空實(shí)例的共享空數(shù)組實(shí)例 （空對(duì)象數(shù)組）
*/
private static final Object[] EMPTY_ELEMENTDATA = {};
/**
* Shared empty array instance used for default sized empty instances(實(shí)例). We
* distinguish(區(qū)分) this from EMPTY_ELEMENTDATA to know how much to inflate(膨脹) when
* first element(元素) is added.
* 用于默認(rèn)大小的空實(shí)例的共享空數(shù)組實(shí)例。我們將此與EMPTY_ELEMENTDATA區(qū)分開來，以了解何時(shí)
* 膨脹多少第一個(gè)元素被添加（默認(rèn)長度的空對(duì)象數(shù)組）
* DEFAULTCAPACITY_EMPTY_ELEMENTDATA
默認(rèn)長度 空 元素?cái)?shù)據(jù)
*/
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
/**
* The array buffer into which the elements of the ArrayList are stored(存儲(chǔ)).
* The capacity(容量) of the ArrayList is the length of this array buffer. Any
* empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
* will be expanded to DEFAULT_CAPACITY when the first element is added.
*
* ArrayList的元素存儲(chǔ)在其中的數(shù)組緩沖區(qū)。ArrayList的容量是此數(shù)組緩沖區(qū)的長度。
* 任何使用elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA為空的ArrayList
* 將在添加第一個(gè)元素時(shí)擴(kuò)展到DEFAULT_CAPACITY。（元素?cái)?shù)組）
*/
transient Object[] elementData; // non-private to simplify nested class access
/**
* The size of the ArrayList (the number of elements it contains).
* 實(shí)際元素大小，默認(rèn)為0
* @serial
*/
private int size;
【構(gòu)造函數(shù)：】
/**
* Constructs an empty list with the specified(指定) initial capacity.
* 構(gòu)造具有指定初始容量的空列表
*
* @param initialCapacity the initial capacity of the list
* @throws IllegalArgumentException if the specified initial capacity
* is negative
*/
public ArrayList(int initialCapacity) {
//初始容量大于0
if (initialCapacity > 0) {
//初始化元素?cái)?shù)組
this.elementData = new Object[initialCapacity];
//初始容量為0
} else if (initialCapacity == 0) {
//元素?cái)?shù)組 = （空數(shù)組實(shí)例/空對(duì)象數(shù)組）
this.elementData = EMPTY_ELEMENTDATA;
} else {
//初始容量小于0，拋出異常
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
}
}
/**
* Constructs an empty list with an initial capacity(容量) of ten.
* 構(gòu)造一個(gè)初始容量為10的空列表
*/
public ArrayList() {
//無參構(gòu)造函數(shù)，設(shè)置元素?cái)?shù)組為空，長度為10
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
/**
* Constructs a list containing the elements of the specified
* collection, in the order they are returned by the collection's
* iterator.
* 構(gòu)造一個(gè)包含指定集合元素的列表，其順序由集合的迭代器返回。
* @param c the collection whose elements are to be placed into this list
* @throws NullPointerException if the specified collection is null
* 集合參數(shù)構(gòu)造函數(shù)
*/
public ArrayList(Collection<? extends E> c) {
//轉(zhuǎn)換為數(shù)組
elementData = c.toArray();
//參數(shù)為非空集合
if ((size = elementData.length) != 0) {
// c.toArray might (incorrectly) not return Object[] (see 6260652)
//是否成功轉(zhuǎn)換為Object類型數(shù)組
if (elementData.getClass() != Object[].class)
//不為Object數(shù)組，進(jìn)行復(fù)制
elementData = Arrays.copyOf(elementData, size, Object[].class);
} else {
// replace(替換) with empty array. 替換為空數(shù)組
this.elementData = EMPTY_ELEMENTDATA;
}
}
【擴(kuò)容相關(guān)下函數(shù)：】
//按照函數(shù)名意為：確保內(nèi)部容量 函數(shù) / 擴(kuò)容函數(shù)
private void ensureCapacityInternal(int minCapacity) {
//判斷元素?cái)?shù)組是否為空數(shù)組 （DEFAULTCAPACITY_EMPTY_ELEMENTDATA 默認(rèn)長度的空對(duì)象數(shù)組））
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
//取較大值（默認(rèn)初始容量，傳入的最小容量）
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
}
//調(diào)用【確保明確的容量】 函數(shù)；數(shù)組已經(jīng)初始化過就執(zhí)行這一步
ensureExplicitCapacity(minCapacity);
}
//按照函數(shù)名意為：確保明確的容量 函數(shù)
private void ensureExplicitCapacity(int minCapacity) {
//結(jié)構(gòu)性 修改時(shí)+1
modCount++;
// overflow-conscious code
//當(dāng)傳入的最小容量 減去 元素?cái)?shù)組的長度 大于0時(shí)，需要進(jìn)行擴(kuò)容
if (minCapacity - elementData.length > 0)
//調(diào)用 【grow函數(shù)】對(duì)數(shù)組進(jìn)行擴(kuò)容
grow(minCapacity);
}
/**
* The maximum size of array to allocate.
* Some VMs reserve some header words in an array.
* Attempts to allocate larger arrays may result in
* OutOfMemoryError: Requested array size exceeds VM limit
* 最大數(shù)組容量(集合最大容量)
*/
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
/**
* Increases the capacity to ensure that it can hold at least the
* number of elements specified by the minimum capacity argument.
* 增加容量以確保它至少可以容納由參數(shù)指定的元素?cái)?shù)。
*
* @param minCapacity the desired minimum capacity
* 擴(kuò)容函數(shù)
*/
private void grow(int minCapacity) {
// overflow-conscious code
//獲取數(shù)組的舊容量
int oldCapacity = elementData.length;
//新容量為舊容量的1.5倍
int newCapacity = oldCapacity + (oldCapacity >> 1);
if (newCapacity - minCapacity < 0)
//當(dāng)新容量 減去 傳入的最小容量 小于0時(shí)，將傳入的最小容量賦值給 新容量
newCapacity = minCapacity;
if (newCapacity - MAX_ARRAY_SIZE > 0)
//如果 新容量 減去 最大數(shù)組容量 大于0，
newCapacity = hugeCapacity(minCapacity); //指定新容量
// minCapacity is usually close to size, so this is a win:
//Arrays.copyOf功能是實(shí)現(xiàn)數(shù)組的復(fù)制，返回復(fù)制后的數(shù)組。參數(shù)是被復(fù)制的數(shù)組和復(fù)制的長度：
//拷貝擴(kuò)容
elementData = Arrays.copyOf(elementData, newCapacity);
}
/*
* 指定新容量
*/
private static int hugeCapacity(int minCapacity) {
if (minCapacity < 0) // overflow
throw new OutOfMemoryError();
return (minCapacity > MAX_ARRAY_SIZE) ?
Integer.MAX_VALUE :
MAX_ARRAY_SIZE;
}
【增刪改查函數(shù)：】
/**
* Returns the element at the specified position(位置) in this list.
* 返回此列表中指定位置的元素。
* @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) {
//檢驗(yàn)索引是否合法
rangeCheck(index);
//返回索引下標(biāo)所對(duì)應(yīng)的值
return elementData(index);
}
/**
* Replaces(替換) the element(元素) at the specified position in this list with
* the specified(指定) element.
* 用指定的元素替換此列表中指定位置的元素
* @param index index of the element to replace
* @param element element to be stored at the specified position
* @return the element previously at the specified position
* @throws IndexOutOfBoundsException {@inheritDoc}
* 【設(shè)定指定下標(biāo)索引的元素值】
*/
public E set(int index, E element) {
//校驗(yàn) 索引是否合法（index 不能 大于 size）
rangeCheck(index);
//舊值
E oldValue = elementData(index);
//賦新值
elementData[index] = element;
//返回舊值
return oldValue;
}
/**
* Appends the specified element to the end of this list.
* 將指定的元素追加到此列表的末尾。
* @param e element to be appended to this list
* @return <tt>true</tt> (as specified by {@link Collection#add})
* 添加元素
*/
public boolean add(E e) {
//調(diào)用【確保內(nèi)部容量】函數(shù)：參數(shù)為(實(shí)際元素大小+1)
//添加之前先檢查是否需要擴(kuò)容，此時(shí)數(shù)組長度最小為size+1
ensureCapacityInternal(size + 1); // Increments modCount!!
//將元素添加到數(shù)組末尾
elementData[size++] = e;
return true;
}
/**
* 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) {
//插入位置范圍檢查
rangeCheckForAdd(index);
//檢查是否需要擴(kuò)容
ensureCapacityInternal(size + 1); // Increments modCount!!
//挪動(dòng)插入位置后面的元素
System.arraycopy(elementData, index, elementData, index + 1,
size - index);
//在要插入的位置賦上新值
elementData[index] = element;
size++;
}
/**
* Removes the element at the specified position in this list.
* Shifts any subsequent elements to the left (subtracts one from their
* indices).
*
* @param index the index of the element to be removed
* @return the element that was removed from the list
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public E remove(int index) {
//檢查索引是否合法
rangeCheck(index);
modCount++;
//得到舊值
E oldValue = elementData(index);
//需要移動(dòng)的元素的個(gè)數(shù)
int numMoved = size - index - 1;
if (numMoved > 0)
/**
* Object src ：源數(shù)組
* int srcPos ：在源數(shù)組中的起始位置
* Object dest：目標(biāo)數(shù)組
* int destPos：在目標(biāo)數(shù)組中的起始位置
* int length ：要復(fù)制的數(shù)組元素的數(shù)量
*/
//void java.lang.System.arraycopy(Object src, int srcPos, Object dest, int destPos, int length)
System.arraycopy(elementData, index+1, elementData, index,numMoved);
//賦值為空，有利于進(jìn)行GC
elementData[--size] = null; // clear to let GC do its work
//返回舊值
return oldValue;
}
/**
* Removes the first occurrence of the specified element from this list,
* if it is present. If the list does not contain the element, it is
* unchanged. More formally, removes the element with the lowest index
* <tt>i</tt> such that
* <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>
* (if such an element exists). Returns <tt>true</tt> if this list
* contained the specified element (or equivalently, if this list
* changed as a result of the call).
*
* @param o element to be removed from this list, if present
* @return <tt>true</tt> if this list contained the specified element
*/
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;
}
/*
* Private remove method that skips bounds checking and does not
* return the value removed.
*/
private void fastRemove(int index) {
modCount++;
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
}
/**
* Removes all of the elements from this list. The list will
* be empty after this call returns.
*/
public void clear() {
modCount++;
// clear to let GC do its work
for (int i = 0; i < size; i++)
elementData[i] = null;
size = 0;
}
/**
* Appends all of the elements in the specified collection to the end of
* this list, in the order that they are returned by the
* specified collection's Iterator. The behavior of this operation is
* undefined if the specified collection is modified while the operation
* is in progress. (This implies that the behavior of this call is
* undefined if the specified collection is this list, and this
* list is nonempty.)
*
* @param c collection containing elements to be added to this list
* @return <tt>true</tt> if this list changed as a result of the call
* @throws NullPointerException if the specified collection is null
*/
public boolean addAll(Collection<? extends E> c) {
Object[] a = c.toArray();
int numNew = a.length;
//檢查是否需要擴(kuò)容
ensureCapacityInternal(size + numNew); // Increments modCount
//挪動(dòng)插入位置后面的元素
System.arraycopy(a, 0, elementData, size, numNew);
size += numNew;
return numNew != 0;
}
