JDK1.7——ArrayList扩容机制

ArrayList概述：

ArrayList是基于数组实现的，是一个动态数组，其容量能自动增长。

ArrayList不是线程安全的，只能用在单线程环境下。

实现了Serializable接口，因此它支持序列化，能够通过序列化传输；

实现了RandomAccess接口，支持快速随机访问，实际上就是通过下标序号进行快速访问；

实现了Cloneable接口，能被克隆。

ArrayList的构造方法：

private transient Object[] elementData;
private static final Object[] EMPTY_ELEMENTDATA = {};
private static final int DEFAULT_CAPACITY = 10;//默认长度是10

public ArrayList() {
super();
this.elementData = EMPTY_ELEMENTDATA;
}
public ArrayList(int initialCapacity) {
super();
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
this.elementData = new Object[initialCapacity];
}
public ArrayList(Collection<? extends E> c) {
elementData = c.toArray();
size = elementData.length;
// c.toArray might (incorrectly) not return Object[] (see 6260652)
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData, size, Object[].class);
}

确保内部容量：

private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
//在java.lang.Integer类中常量MIN_VALUE、MAX_VALUE如下：
public static final int   MIN_VALUE = 0x80000000;//整型取值区间下界：－2147483648
public static final int   MAX_VALUE = 0x7fffffff;//整型取值区间上界：2147483647
//在java.util.AbstractList中modCount定义如下：
protected transient int modCount = 0;

public boolean add(E e) {
//确保内部容量(通过判断，如果够则不进行操作；容量不够就扩容来确保内部容量)
ensureCapacityInternal(size + 1);  // Increments modCount!!
elementData[size++] = e;
return true;
}

private void ensureCapacityInternal(int minCapacity) {
if (elementData == EMPTY_ELEMENTDATA) {//第一次运行将最小容量设置为10
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
}

ensureExplicitCapacity(minCapacity);
}

private void ensureExplicitCapacity(int minCapacity) {
modCount++;//存在以AbstractList中，用于存放结构改变的次数。

// 当插入元素位置大于数组最大容量，则扩容。
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}

ArrayList的具体实现代码（部分，但已经能说明原理）：

package test;

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

public class ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
private static final Object[] EMPTY_ELEMENTDATA = {};
private transient Object[] elementData;
private int size;
private static final int DEFAULT_CAPACITY = 10;
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
public ArrayList(int initialCapacity) {
super();
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
this.elementData = new Object[initialCapacity];
}

public ArrayList() {
super();
this.elementData = EMPTY_ELEMENTDATA;
}

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

public boolean add(E e) {
ensureCapacityInternal(size + 1);  // Increments modCount!!
elementData[size++] = e;
return true;
}

private void ensureCapacityInternal(int minCapacity) {
if (elementData == 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);
}

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);
}

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 String toString() {
Iterator<E> it = iterator();
if (! it.hasNext())
return "[]";

StringBuilder sb = new StringBuilder();
sb.append('[');
for (;;) {
E e = it.next();
sb.append(e == this ? "(this Collection)" : e);
if (! it.hasNext())
return sb.append(']').toString();
sb.append(',');
}
}

private class Itr implements Iterator<E> {

int cursor = 0;

int lastRet = -1;

int expectedModCount = modCount;

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

public E next() {
checkForComodification();
try {
int i = cursor;
E next = get(i);
lastRet = i;
cursor = i + 1;
return next;
} catch (IndexOutOfBoundsException e) {
checkForComodification();
throw new NoSuchElementException();
}
}

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

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

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

@Override
public E get(int index) {
rangeCheck(index);

return elementData(index);
}

private void rangeCheck(int index) {
if (index >= size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}

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

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

@Override
public int size() {
return size;
}

}