源码来自jdk1.8
ArrayList是对List接口的数组实现，所以可以随机访问数组元素（RandomAccess）。ArrayList最重要的是它的底层实现了一个Resizable array，也就是可以在操作的过程中数组可以根据需要扩大。
public class ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable{
private static final long serialVersionUID = 8683452581122892189L;
/**
* Default initial capacity.
*/
private static final int DEFAULT_CAPACITY = 10;
/**
* Shared empty array instance used for empty instances.
*/
private static final Object[] EMPTY_ELEMENTDATA = {};
/**
* Shared empty array instance used for default sized empty instances. We
* distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when
* first element is added.
*/
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
/**
* The array buffer into which the elements of the ArrayList are stored.
* The capacity 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.
*/
transient Object[] elementData; // non-private to simplify nested class access
/**
* The size of the ArrayList (the number of elements it contains).
* @serial
*/
private int size;
// 这里省略了方法
}
自动扩容
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);
}
grow函数是自动扩容的核心，每次尝试将数组扩大50%，如果不够，就直接为指定大小，如果非常的大，就修正为最大容量，最后使用Arrays.copyOf来产生新的数组。注意调用grow的ensureExplicitCapcity函数中有modCount++，也就是说，只要调用了ensure这个函数（四个add方法），无论你最终有没有调用grow生成一个新的数组，都会增加一次修改次数。
add
没有index参数的add方法将新元素添加到数组末位，有index的会先将index之后的所有元素往后移动一个位置。所以是O(n)
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++;
}
remove
remove会增加修改次数，移动元素，将多余的元素置为null，这样GC会清理这个多余的元素。
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; // clear to let GC do its work
return oldValue;
}
