ArrayList源码剖析

先贴上源代码

使用的JDK版本：1.7.0_13

import java.util.Arrays;
import java.util.Collection;
import java.util.ConcurrentModificationException;
import java.util.Iterator;
import java.util.List;
import java.util.ListIterator;
import java.util.NoSuchElementException;
import java.util.RandomAccess;

/*
* Copyright (c) 1997, 2010, Oracle and/or its affiliates. All rights reserved.
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*/

public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
private static final long serialVersionUID = 8683452581122892189L;

//保存了当前对象的数据
private transient Object[] elementData;

private int size;

//由此可以看出，ArrayList实际上是已数组的形式保存的
public ArrayList(int initialCapacity) {
super();
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal Capacity: "+initialCapacity);
this.elementData = new Object[initialCapacity];
}

//默认大小为10
public ArrayList() {
this(10);
}

public ArrayList(Collection<? extends E> c) {
elementData = c.toArray();
si
4000
ze = elementData.length;
// c.toArray might (incorrectly) not return Object[] (see 6260652)
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData, size, Object[].class);
}

//把数组大小调整为和size大小一致，可以节省空间
public void trimToSize() {
modCount++;
int oldCapacity = elementData.length;
if (size < oldCapacity) {
elementData = Arrays.copyOf(elementData, size);
}
}

/**
* Increases the capacity of this <tt>ArrayList</tt> instance, if
* necessary, to ensure that it can hold at least the number of elements
* specified by the minimum capacity argument.
*
* @param   minCapacity   the desired minimum capacity
*/
public void ensureCapacity(int minCapacity) {
if (minCapacity > 0)
ensureCapacityInternal(minCapacity);
}

//在加入新对象时，需要先调用该方法，判断是否需要扩容
private void ensureCapacityInternal(int minCapacity) {
modCount++;
//如果下一个元素超过数组范围
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}

//最大大小
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

//扩容，这里是重点！！
private void grow(int minCapacity) {
int oldCapacity = elementData.length;
//每次扩容大小为当前大小的一半例如：当前：100，扩容后为150
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:
//复制当前数组数据到新数组数据
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;
}

public int size() {
return size;
}

public boolean isEmpty() {
return size == 0;
}

public boolean contains(Object o) {
return indexOf(o) >= 0;
}

//顺序查找元素o，找不到返回-1
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;
}

//倒序找最后一个元素o
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;
}

//快照，这里是浅复制
public Object clone() {
try {
@SuppressWarnings("unchecked")
ArrayList<E> v = (ArrayList<E>) 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();
}
}

//转成数组
public Object[] toArray() {
return Arrays.copyOf(elementData, size);
}

@SuppressWarnings("unchecked")
public <T> T[] toArray(T[] a) {
if (a.length < size)
// Make a new array of a's runtime type, but my contents:
return (T[]) Arrays.copyOf(elementData, size, a.getClass());
System.arraycopy(elementData, 0, a, 0, size);
if (a.length > size)
a[size] = null;
return a;
}

// Positional Access Operations

@SuppressWarnings("unchecked")
E elementData(int index) {
return (E) elementData[index];
}

//取值
public E get(int index) {
rangeCheck(index);
return elementData(index);
}

//放置值
public E set(int index, E element) {
rangeCheck(index);
E oldValue = elementData(index);
elementData[index] = element;
return oldValue;
}

//放入值
public boolean add(E e) {
//先确保是否能放入进去
ensureCapacityInternal(size + 1);  // Increments modCount!!
elementData[size++] = e;
return true;
}

//在某个位置插入值
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++;
}

//移除某个位置的元素时，需要将后面的元素向前移动，调用了System.arraycopy函数，效率不是很高
public E remove(int index) {
rangeCheck(index);
modCount++;
E oldValue = elementData(index);

int numMoved = size - index - 1;
if (numMoved > 0)
//这是一个很重要的函数
System.arraycopy(elementData, index+1, elementData, index,numMoved);
elementData[--size] = null; // Let gc do its work
return oldValue;
}

//通过对象来删除
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);//这里index已经确定，不需要再进行边界检查了
return true;
}
}
return false;
}

//快速删除，不再检查边界，并且不返回删除的值
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; // Let gc do its work
}

//全部置为null
public void clear() {
modCount++;
for (int i = 0; i < size; i++)
elementData[i] = null;
size = 0;
}

//此处先判断当前个数加c个数之和是否超标，超过则扩容，之和直接进行数组复制
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;
}

//在某个位置插入一个Collection
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 - inde
12757
x;
if (numMoved > 0)
System.arraycopy(elementData, index, elementData, index + numNew,
numMoved);

System.arraycopy(a, 0, elementData, index, numNew);
size += numNew;
return numNew != 0;
}

//范围删除
protected void removeRange(int fromIndex, int toIndex) {
modCount++;
int numMoved = size - toIndex;
System.arraycopy(elementData, toIndex, elementData, fromIndex,
numMoved);
int newSize = size - (toIndex-fromIndex);
while (size != newSize)//需要将后面多出来的所有数据全部置为null
elementData[--size] = null;
}

//判断是否超标
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;
}

public boolean removeAll(Collection<?> c) {
return batchRemove(c, false);
}

public boolean retainAll(Collection<?> c) {
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)
elementData[w++] = elementData[r];
} finally {
// Preserve behavioral compatibility with AbstractCollection,
// even if c.contains() throws.
if (r != size) {
System.arraycopy(elementData, r,
elementData, w,
size - r);
w += size - r;
}
if (w != size) {
for (int i = w; i < size; i++)
elementData[i] = null;
modCount += size - w;
size = w;
modified = true;
}
}
return modified;
}

/**
* Save the state of the <tt>ArrayList</tt> instance to a stream (that
* is, serialize it).
*
* @serialData The length of the array backing the <tt>ArrayList</tt>
*             instance is emitted (int), followed by all of its elements
*             (each an <tt>Object</tt>) in the proper order.
*/
private void writeObject(java.io.ObjectOutputStream s)
throws java.io.IOException{
// Write out element count, and any hidden stuff
int expectedModCount = modCount;
s.defaultWriteObject();

// Write out array length
s.writeInt(elementData.length);

// Write out all elements in the proper order.
for (int i=0; i<size; i++)
s.writeObject(elementData[i]);

if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}

}

/**
* Reconstitute the <tt>ArrayList</tt> instance from a stream (that is,
* deserialize it).
*/
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
// Read in size, and any hidden stuff
s.defaultReadObject();

// Read in array length and allocate array
int arrayLength = s.readInt();
Object[] a = elementData = new Object[arrayLength];

// Read in all elements in the proper order.
for (int i=0; i<size; i++)
a[i] = s.readObject();
}

public ListIterator<E> listIterator(int index) {
if (index < 0 || index > size)
throw new IndexOutOfBoundsException("Index: "+index);
return new ListItr(index);
}

public ListIterator<E> listIterator() {
return new ListItr(0);
}

public Iterator<E> iterator() {
return new Itr();
}

//定义ArrayList的游标
private class Itr implements Iterator<E> {
//指向当前元素
int cursor;       // index of next element to return
//指向刚刚返回出去的那个元素
int lastRet = -1; // index of last element returned; -1 if no such
//这个变量很有意思，通过expectedModCount标记，当游标生成后，在遍历的过程中
//禁止再对ArrayList结构进行操作，例如添加、删除元素等，否则会报：ConcurrentModificationException
int expectedModCount = modCount;

public boolean hasNext() {
return cursor != size;
}

@SuppressWarnings("unchecked")
public E next() {
//检查该List是否在游标生成后被就改过
checkForComodification();
int i = cursor;
if (i >= size)
throw new NoSuchElementException();
Object[] elementData = ArrayList.this.elementData;
if (i >= elementData.length)//又通过长度做了一次上面的检查
throw new ConcurrentModificationException();
cursor = i + 1;
return (E) elementData[lastRet = i];
}

public void remove() {
if (lastRet < 0)
throw new IllegalStateException();
checkForComodification();

try {
ArrayList.this.remove(lastRet);
cursor = lastRet;
lastRet = -1;
expectedModCount = modCount;
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}

final void checkForComodification() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
}
}

/**
* An optimized version of AbstractList.ListItr
*/
private class ListItr extends Itr implements ListIterator<E> {
ListItr(int index) {
super();
cursor = index;
}

public boolean hasPrevious() {
return cursor != 0;
}

public int nextIndex() {
return cursor;
}

public int previousIndex() {
return cursor - 1;
}

@SuppressWarnings("unchecked")
public E previous() {
checkForComodification();
int i = cursor - 1;
if (i < 0)
throw new NoSuchElementException();
Object[] elementData = ArrayList.this.elementData;
if (i >= elementData.length)
throw new ConcurrentModificationException();
cursor = i;
return (E) elementData[lastRet = i];
}

public void set(E e) {
if (lastRet < 0)
throw new IllegalStateException();
checkForComodification();

try {
ArrayList.this.set(lastRet, e);
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}

public void add(E e) {
checkForComodification();

try {
int i = cursor;
ArrayList.this.add(i, e);
cursor = i + 1;
lastRet = -1;
expectedModCount = modCount;
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}
}

//返回一个子列表
public List<E> subList(int fromIndex, int toIndex) {
//先判断范围
subListRangeCheck(fromIndex, toIndex, size);
return new SubList(this, 0, fromIndex, toIndex);
}

static void subListRangeCheck(int fromIndex, int toIndex, int size) {
if (fromIndex < 0)
throw new IndexOutOfBoundsException("fromIndex = " + fromIndex);
if (toIndex > size)
throw new IndexOutOfBoundsException("toIndex = " + toIndex);
if (fromIndex > toIndex)
throw new IllegalArgumentException("fromIndex(" + fromIndex +
") > toIndex(" + toIndex + ")");
}
/**
* 由SubList的构造可知：SubList构造出来的List指向了父ArrayList，对SubList的所有操作都会反映到
* ArrayList上去，二者是一个共同体，不是独立出来的副本，这点很重要！！
*
*/
private class SubList extends AbstractList<E> implements RandomAccess {
private final AbstractList<E> parent;
private final int parentOffset;
private final int offset;
int size;

SubList(AbstractList<E> parent, int offset, int fromIndex, int toIndex) {
this.parent = parent;
this.parentOffset = fromIndex;
this.offset = offset + fromIndex;
this.size = toIndex - fromIndex;
this.modCount = ArrayList.this.modCount;
}

//可以看到所有的get、set操作都是在操作其父ArrayList，SubList只是记录了偏移量
public E set(int index, E e) {
rangeCheck(index);
checkForComodification();
E oldValue = ArrayList.this.elementData(offset + index);
ArrayList.this.elementData[offset + index] = e;
return oldValue;
}

public E get(int index) {
rangeCheck(index);
checkForComodification();
return ArrayList.this.elementData(offset + index);
}

public int size() {
checkForComodification();
return this.size;
}

public void add(int index, E e) {
rangeCheckForAdd(index);
checkForComodification();
parent.add(parentOffset + index, e);
this.modCount = parent.modCount;
this.size++;
}

public E remove(int index) {
rangeCheck(index);
checkForComodification();
E result = parent.remove(parentOffset + index);
this.modCount = parent.modCount;
this.size--;
return result;
}

protected void removeRange(int fromIndex, int toIndex) {
checkForComodification();
parent.removeRange(parentOffset + fromIndex,
parentOffset + toIndex);
this.modCount = parent.modCount;
this.size -= toIndex - fromIndex;
}

public boolean addAll(Collection<? extends E> c) {
return addAll(this.size, c);
}

public boolean addAll(int index, Collection<? extends E> c) {
rangeCheckForAdd(index);
int cSize = c.size();
if (cSize==0)
return false;

checkForComodification();
parent.addAll(parentOffset + index, c);
this.modCount = parent.modCount;
this.size += cSize;
return true;
}

public Iterator<E> iterator() {
return listIterator();
}

public ListIterator<E> listIterator(final int index) {
checkForComodification();
rangeCheckForAdd(index);
final int offset = this.offset;

return new ListIterator<E>() {
int cursor = index;
int lastRet = -1;
int expectedModCount = ArrayList.this.modCount;

public boolean hasNext() {
return cursor != SubList.this.size;
}

@SuppressWarnings("unchecked")
public E next() {
checkForComodification();
int i = cursor;
if (i >= SubList.this.size)
throw new NoSuchElementException();
Object[] elementData = ArrayList.this.elementData;
if (offset + i >= elementData.length)
throw new ConcurrentModificationException();
cursor = i + 1;
return (E) elementData[offset + (lastRet = i)];
}

public boolean hasPrevious() {
return cursor != 0;
}

@SuppressWarnings("unchecked")
public E previous() {
checkForComodification();
int i = cursor - 1;
if (i < 0)
throw new NoSuchElementException();
Object[] elementData = ArrayList.this.elementData;
if (offset + i >= elementData.length)
throw new ConcurrentModificationException();
cursor = i;
return (E) elementData[offset + (lastRet = i)];
}

public int nextIndex() {
return cursor;
}

public int previousIndex() {
return cursor - 1;
}

public void remove() {
if (lastRet < 0)
throw new IllegalStateException();
checkForComodification();

try {
SubList.this.remove(lastRet);
cursor = lastRet;
lastRet = -1;
expectedModCount = ArrayList.this.modCount;
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}

public void set(E e) {
if (lastRet < 0)
throw new IllegalStateException();
checkForComodification();

try {
ArrayList.this.set(offset + lastRet, e);
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}

public void add(E e) {
checkForComodification();

try {
int i = cursor;
SubList.this.add(i, e);
cursor = i + 1;
lastRet = -1;
expectedModCount = ArrayList.this.modCount;
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}

final void checkForComodification() {
if (expectedModCount != ArrayList.this.modCount)
throw new ConcurrentModificationException();
}
};
}

public List<E> subList(int fromIndex, int toIndex) {
subListRangeCheck(fromIndex, toIndex, size);
return new SubList(this, offset, fromIndex, toIndex);
}

private void rangeCheck(int index) {
if (index < 0 || index >= this.size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}

private void rangeCheckForAdd(int index) {
if (index < 0 || index > this.size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}

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

private void checkForComodification() {
if (ArrayList.this.modCount != this.modCount)
throw new ConcurrentModificationException();
}
}
}

由上可以得出以下结论：

ArrayList使用数组进行数据保存，即（Object[] elementData），该对象修饰符为： private transient ，是不参与序列化的，序列化时，需调用提供的（ private void writeObject(java.io.ObjectOutputStream s)）方法，数组默认大小为10。

对数据进行Add操作时，ArrayList先进行了容量检查，如果不够保存当前对象，则进行扩容，扩容大小为当前容量的一半，即100扩容到150，再将旧数据复制过来，因此ArrayList有可能造成空间的浪费，在这一点上，ArrayList提供了一个trimToSize()方法，可以将数组长度置为size大小，可以压缩空间。

由于使用数组保存，在中间插入元素或删除元素时，调用了System.arraycopy方法，需要将插入点后面所有元素移位，该方法是一个native方法，直接内存复制，理论上效率还行，但也相比链表（LinkedList）效率还是不高。但是正因为使用了数组保存，get效率比较高，因此ArrayList适合于需要频繁读的情况。

ArrayList的clone方法是浅复制。

ArryList提供的subList（fromIndex,endIndex）方法返回的List和自身是同一个对象，对subList所有操作（增加、删除等）都会反映到ArrayList上，是一个共同体。