/**

Default initial capacity.初始化容量为10
*/
private static final int DEFAULT_CAPACITY = 10;
private static final Object[] EMPTY_ELEMENTDATA = {};
/**Shared empty array instance used for default sized empty instances. Wedistinguish this from EMPTY_ELEMENTDATA to know how much to inflate whenfirst 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. Anyempty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATAwill 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;
//最大数组大小
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
//默认实例为一个空数组
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
//添加元素
public boolean add(E e) {
//确定容量
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}

private void ensureCapacityInternal(int minCapacity) {
ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
}

private void ensureExplicitCapacity(int minCapacity) {
modCount++;

// overflow-conscious code//超过扩容if (minCapacity - elementData.length > 0) grow(minCapacity);

}
private void grow(int minCapacity) {
// overflow-conscious code
int oldCapacity = elementData.length;
//扩容方式 旧容量+旧容量右移一位
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);
}
//循环清除
public void clear() {
modCount++;

// clear to let GC do its workfor (int i = 0; i < size; i++) elementData[i] = null;size = 0;

}
//删除 --很巧妙的删除
public E remove(int index) {
rangeCheck(index);

modCount++;E oldValue = elementData(index);//获得需要移动的位数int numMoved = size - index - 1;if (numMoved > 0) //进行复制，最后一位会保留下来 System.arraycopy(elementData, index+1, elementData, index, numMoved);//数组大小减少一位，最后一位赋值为空 elementData[--size] = null; // clear to let GC do its workreturn oldValue;

}