private transient Object[] elementData;
private static final Object[] EMPTY_ELEMENTDATA = {};

//1.ArrayList默认构造方法
public ArrayList() {  
  super();
  this.elementData = EMPTY_ELEMENTDATA;
}

//JDK1.6 ArrayList
public ArrayList() {
  this(10);
}  

//2.ArrayList带有初始化大小的构造方法
public ArrayList(int initialCapacity) {
  super();
  if (initialCapacity < 0)
    throw new IllegalArgumentException("Illegal Capacity: "+
                      initialCapacity);
  this.elementData = new Object[initialCapacity];
}

//3.可将一个集合作为ArrayList的参数构造成ArrayList
public ArrayList(Collection<? extends E> c) {
  elementData = c.toArray();  //将集合转换为数组
  size = elementData.length;  //集合中的元素大小
  // c.toArray might (incorrectly) not return Object[] (see 6260652) 这里是个bug，参考http://bugs.java.com/bugdatabase/view_bug.do?bug_id=6260652
  if (elementData.getClass() != Object[].class)
    elementData = Arrays.copyOf(elementData, size, Object[].class);
}

package com.algorithm.sort;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

/**
 * bug编号:6260652。toArray有可能不会返回Object[]
 * Created by yulinfeng on 2017/6/26.
 */
public class Test {
  public static void main(String[] args) {
    correctly();
    incorrectly();
  }
  
  /**
   * 返回Object[]
   */
  private static void correctly() {
    List<String> list = new ArrayList<String>();
    list.add("test");
    System.out.println(list.getClass());
    Object[] objArray = list.toArray();
    System.out.println(objArray.getClass());
  }
  /**
   * 不返回Object[]
   */
  private static void incorrectly() {
    List<String> list = Arrays.asList("test");
    System.out.println(list.getClass());
    Object[] objArray = list.toArray();
    System.out.println(objArray.getClass());
  }
}

//ArrayList#add
public boolean add(E e) {
  ensureCapacityInternal(size + 1); //确保容量是否充足
  elementData[size++] = e;  //将元素添加至数组
  return true;
}

//ArrayList#ensureCapacityInternal
private void ensureCapacityInternal(int minCapacity) {
  if (elementData == EMPTY_ELEMENTDATA) {
    minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);   //如果此时还没有初始化列表容量大小，则对其初始化，默认容量为10
  }
  ensureExplicitCapacity(minCapacity); //检查容量是否充足
}

//ArrayList#ensureEcplicitCapacity
private void ensureExplicitCapacity(int minCapacity) {
  modCount++;  //注意此变量
  if (minCapacity - elementData.length > 0)
    grow(minCapacity);  //容量不够则进行扩容
}

protected transient int modCount = 0;

//AbstractList$Itr
int expectedModCount = modCount;

//AbstractList$Itr#checkForComodification
final void checkForComodification() {
  if (modCount != expectedModCount)
    throw new ConcurrentModificationException();
}

//ArrayList#grow
private void grow(int minCapacity) {
  int oldCapacity = elementData.length;
  int newCapacity = oldCapacity + (oldCapacity >> 1);  //扩容策略为，每次新增容量的大小为旧容量的一半。也就是说如果默认容量为10，则第一次扩容大小为10 / 2 = 5，第二次扩容大小为15 / 2 = 7。
  if (newCapacity - minCapacity < 0)
    newCapacity = minCapacity;  //扩容策略扩得太小
  if (newCapacity - MAX_ARRAY_SIZE > 0)  //扩容策略扩得太大，大于最大数组大小时，最多等于Integer.MAX_VALUE
    newCapacity = hugeCapacity(minCapacity);
  
  elementData = Arrays.copyOf(elementData, newCapacity);
}

public E get(int index) {
  rangeCheck(index);  //检查索引是否越界
  return elementData(index);
}

//ArrayList#remove
public E remove(int index) {
  rangeCheck(index);  //检查索引是否越界
  modCount++;  //记录modCount，上面已提及
  E oldValue = elementData(index);  //取出指定索引元素
  int numMoved = size - index - 1;  //移动的元素个数
  if (numMoved > 0)
    System.arraycopy(elementData, index+1, elementData, index, numMoved);
  elementData[--size] = null; //将最后一个数组元素置为null，方便GC

  return oldValue;
}