Java集合之ArrayList源碼分析


一、ArrayList簡介

  ArrayList是可以動態增長和縮減的索引序列,它是基於數組實現的List類。

  該類封裝了一個動態再分配的Object[]數組,每一個類對象都有一個capacity屬性,表示它們所封裝的Object[]數組的長度,當向ArrayList中添加元素時,該屬性值會自動增加。如果想ArrayList中添加大量元素,可使用ensureCapacity方法一次性增加capacity,可以減少增加重分配的次數提高性能。

  ArrayList的用法和Vector向類似,但是Vector是一個較老的集合,具有很多缺點,不建議使用。另外,ArrayList和Vector的區別是:ArrayList是線程不安全的,當多條線程訪問同一個ArrayList集合時,程序需要手動保證該集合的同步性,而Vector則是線程安全的。

  ArrayList與Collection關系如下圖:

  

二、ArrayList源碼分析

  下面就ArrayList的源代碼進行簡單的分析:

public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
private static final long serialVersionUID = 8683452581122892189L;
//默認的初始容量為10
private static final int DEFAULT_CAPACITY = 10;
private static final Object[] EMPTY_ELEMENTDATA = {};
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
transient Object[] elementData;
// ArrayList中實際數據的數量
private int size;
public ArrayList(int initialCapacity) //帶初始容量大小的構造函數
{
if (initialCapacity > 0) //初始容量大於0,實例化數組
{
this.elementData = new Object[initialCapacity];
}
else if (initialCapacity == 0) //初始化等於0,將空數組賦給elementData
{
this.elementData = EMPTY_ELEMENTDATA;
}
else //初始容量小於,拋異常
{
throw new IllegalArgumentException("Illegal Capacity: "+ initialCapacity);
}
}
public ArrayList() //無參構造函數,默認容量為10
{
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
public ArrayList(Collection<? extends E> c) //創建一個包含collection的ArrayList
{
elementData
= c.toArray(); //返回包含c所有元素的數組
if ((size = elementData.length) != 0)
{
if (elementData.getClass() != Object[].class)
elementData
= Arrays.copyOf(elementData, size, Object[].class);//復制指定數組,使elementData具有指定長度
}
else
{
//c中沒有元素
this.elementData = EMPTY_ELEMENTDATA;
}
}
//將當前容量值設為當前實際元素大小
public void trimToSize()
{
modCount
++;
if (size < elementData.length)
{
elementData
= (size == 0)? EMPTY_ELEMENTDATA:Arrays.copyOf(elementData, size);
}
}

//將集合的capacit增加minCapacity
public void ensureCapacity(int minCapacity)
{
int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)?0:DEFAULT_CAPACITY;
if (minCapacity > minExpand)
{
ensureExplicitCapacity(minCapacity);
}
}
private void ensureCapacityInternal(int minCapacity)
{
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
{
minCapacity
= Math.max(DEFAULT_CAPACITY, minCapacity);
}
ensureExplicitCapacity(minCapacity);
}
private void ensureExplicitCapacity(int minCapacity)
{
modCount
++;
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
private void grow(int minCapacity)
{
int oldCapacity = elementData.length;
     //注意此處擴充capacity的方式是將其向右一位再加上原來的數,實際上是擴充了1.5倍
int newCapacity = oldCapacity + (oldCapacity >> 1);
if (newCapacity - minCapacity < 0)
newCapacity
= minCapacity;
if (newCapacity - MAX_ARRAY_SIZE > 0)
newCapacity
= hugeCapacity(minCapacity);
elementData
= Arrays.copyOf(elementData, newCapacity);
}
private static int hugeCapacity(int minCapacity)
{
if (minCapacity < 0) // overflow
throw new OutOfMemoryError();
return (minCapacity > MAX_ARRAY_SIZE) ?
Integer.MAX_VALUE :
MAX_ARRAY_SIZE;
}
//返回ArrayList的大小
public int size()
{
return size;
}
//判斷ArrayList是否為空
public boolean isEmpty() {
return size == 0;
}
//判斷ArrayList中是否包含Object(o)
public boolean contains(Object o) {
return indexOf(o) >= 0;
}
//正向查找,返回ArrayList中元素Object(o)的索引位置
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;
}
//逆向查找,返回返回ArrayList中元素Object(o)的索引位置
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;
}
//返回此 ArrayList實例的淺拷貝。
public Object clone()
{
try
{
ArrayList
<?> v = (ArrayList<?>) super.clone();
v.elementData
= Arrays.copyOf(elementData, size);
v.modCount
= 0;
return v;
}
catch (CloneNotSupportedException e) {
// this shouldn't happen, since we are Cloneable
throw new InternalError(e);
}
}
//返回一個包含ArrayList中所有元素的數組
public Object[] toArray() {
return Arrays.copyOf(elementData, size);
}
@SuppressWarnings(
"unchecked")
public <T> T[] toArray(T[] a) {
if (a.length < size)
return (T[]) Arrays.copyOf(elementData, size, a.getClass());
System.arraycopy(elementData,
0, a, 0, size);
if (a.length > size)
a[size]
= null;
return a;
}
@SuppressWarnings(
"unchecked")
E elementData(
int index) {
return (E) elementData[index];
}

//返回至指定索引的值
public E get(int index)
{
rangeCheck(index);
//檢查給定的索引值是否越界
return elementData(index);
}

//將指定索引上的值替換為新值,並返回舊值
public E set(int index, E element)
{
rangeCheck(index);
E oldValue
= elementData(index);
elementData[index]
= element;
return oldValue;
}

//將指定的元素添加到此列表的尾部
public boolean add(E e)
{
ensureCapacityInternal(size
+ 1);
elementData[size
++] = e;
return true;
}

// 將element添加到ArrayList的指定位置
public void add(int index, E element) {
rangeCheckForAdd(index);
ensureCapacityInternal(size
+ 1);

//從指定源數組中復制一個數組,復制從指定的位置開始,到目標數組的指定位置結束。
//arraycopy(被復制的數組, 從第幾個元素開始復制, 要復制到的數組, 從第幾個元素開始粘貼, 一共需要復制的元素個數)
//即在數組elementData從index位置開始,復制到index+1位置,共復制size-index個元素
System.arraycopy(elementData, index, elementData, index + 1,size - index);
elementData[index]
= element;
size
++;
}

//刪除ArrayList指定位置的元素
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; //將原數組最后一個位置置為null
return oldValue;
}

//移除ArrayList中首次出現的指定元素(如果存在)。
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;
}

//快速刪除指定位置的元素
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;
}

//清空ArrayList,將全部的元素設為null
public void clear()
{
modCount
++;
for (int i = 0; i < size; i++)
elementData[i]
= null;
size
= 0;
}

//按照c的迭代器所返回的元素順序,將c中的所有元素添加到此列表的尾部
public boolean addAll(Collection<? extends E> c) {
Object[] a
= c.toArray();
int numNew = a.length;
ensureCapacityInternal(size
+ numNew); // Increments modCount
System.arraycopy(a, 0, elementData, size, numNew);
size
+= numNew;
return numNew != 0;
}

//從指定位置index開始,將指定c中的所有元素插入到此列表中
public boolean addAll(int index, Collection<? extends E> c) {
rangeCheckForAdd(index);
Object[] a
= c.toArray();
int numNew = a.length;
ensureCapacityInternal(size
+ numNew); // Increments modCount
int numMoved = size - index;
if (numMoved > 0)
//先將ArrayList中從index開始的numMoved個元素移動到起始位置為index+numNew的后面去
System.arraycopy(elementData, index, elementData, index + numNew, numMoved);
//再將c中的numNew個元素復制到起始位置為index的存儲空間中去
System.arraycopy(a, 0, elementData, index, numNew);
size
+= numNew;
return numNew != 0;
}

//刪除fromIndex到toIndex之間的全部元素
protected void removeRange(int fromIndex, int toIndex)
{
modCount
++;
//numMoved為刪除索引后面的元素個數
int numMoved = size - toIndex;
//將刪除索引后面的元素復制到以fromIndex為起始位置的存儲空間中去
System.arraycopy(elementData, toIndex, elementData, fromIndex,numMoved);
int newSize = size - (toIndex-fromIndex);
//將ArrayList后面(toIndex-fromIndex)個元素置為null
for (int i = newSize; i < size; i++)
{
elementData[i]
= null;
}
size
= newSize;
}

//檢查索引是否越界
private void rangeCheck(int index)
{
if (index >= size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
private void rangeCheckForAdd(int index)
{
if (index > size || index < 0)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}

private String outOfBoundsMsg(int index) {
return "Index: "+index+", Size: "+size;
}

//刪除ArrayList中包含在c中的元素
public boolean removeAll(Collection<?> c)
{
Objects.requireNonNull(c);
return batchRemove(c, false);
}

//刪除ArrayList中除包含在c中的元素,和removeAll相反
public boolean retainAll(Collection<?> c)
{
Objects.requireNonNull(c);
//檢查指定對象是否為空
return batchRemove(c, true);
}

private boolean batchRemove(Collection<?> c, boolean complement) {
final Object[] elementData = this.elementData;
int r = 0, w = 0;
boolean modified = false;
try
{
for (; r < size; r++)
if (c.contains(elementData[r]) == complement) //判斷c中是否有elementData[r]元素

elementData[w
++] = elementData[r];
}
finally
{
if (r != size)
{
System.arraycopy(elementData, r, elementData, w, size
- r);
w
+= size - r;
}
if (w != size)
{
// clear to let GC do its work
for (int i = w; i < size; i++)
elementData[i]
= null;
modCount
+= size - w;
size
= w;
modified
= true;
}
}
return modified;
}

//將ArrayList的“容量,所有的元素值”都寫入到輸出流中
private void writeObject(java.io.ObjectOutputStream s) throws java.io.IOException
{
int expectedModCount = modCount;
s.defaultWriteObject();
//寫入數組大小
s.writeInt(size);
//寫入所有數組的元素
for (int i=0; i<size; i++) {
s.writeObject(elementData[i]);
}
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
}

//先將ArrayList的“大小”讀出,然后將“所有的元素值”讀出
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
elementData
= EMPTY_ELEMENTDATA;
s.defaultReadObject();
s.readInt();
// ignored
if (size > 0) {
// be like clone(), allocate array based upon size not capacity
ensureCapacityInternal(size);
Object[] a
= elementData;
// Read in all elements in the proper order.
for (int i=0; i<size; i++) {
a[i]
= s.readObject();
}
}
}

三、ArrayList遍歷方式

  ArrayList支持3種遍歷方式

  1、通過迭代器遍歷:

    Iterator iter = list.iterator();
while (iter.hasNext())
{
System.out.println(iter.next());
}

  2、隨機訪問,通過索引值去遍歷,由於ArrayList實現了RandomAccess接口

    int size = list.size();
for (int i=0; i<size; i++)
{
System.out.println(list.get(i));
}

  3、for循環遍歷:

    for(String str:list)
{
System.out.println(str);
   }

  完整的代碼示例如下:

public class DemoMain 
{
public static void main(String[] args)
{
List
<String> list=new ArrayList<String>();
list.add(
"nihao");
list.add(
"xujian");
list.add(
"wang");
System.out.println(
"--------通過迭代器遍歷---------");
Iterator iter
= list.iterator();
while (iter.hasNext())
{
System.out.println(iter.next());
}

System.out.println(
"--------通過隨機訪問---------");
int size = list.size();
for (int i=0; i<size; i++)
{
System.out.println(list.get(i));
}

System.out.println(
"--------通過for循環訪問---------");
for(String str:list)
{
System.out.println(str);
}
}
}

  運行結果如圖示:

  


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