ArrayList概述：

ArrayList是实现List接口的，大小可变数组的实现。实现了所有可选列表操作，并允许包括 null 在内的所有元素。除了实现 List 接口外，此类还提供一些方法来操作内部用来存储列表的数组的大小

ArrayList的实现：

/**     * The array buffer into which the elements of the ArrayList are stored.     * The capacity of the ArrayList is the length of this array buffer.     */    private transient Object[] elementData;    /**     * The size of the ArrayList (the number of elements it contains).     *     * @serial     */    private int size;

elementData存储ArrayList中的元素，size表示包含元素的数量。让我们来看下ArrayList内部数组是如何自我Copy的。

Add方法:

/**     * Appends the specified element to the end of this list.     *     * @param e element to be appended to this list     * @return true (as specified by {
    @link Collection#add})     */    public boolean add(E e) {        ensureCapacityInternal(size + 1);  // Increments modCount!!        elementData[size++] = e;        return true;    }    /**     * Inserts the specified element at the specified position in this     * list. Shifts the element currently at that position (if any) and     * any subsequent elements to the right (adds one to their indices).     *     * @param index index at which the specified element is to be inserted     * @param element element to be inserted     * @throws IndexOutOfBoundsException {
    @inheritDoc}     */    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方法：

/**     * Removes the element at the specified position in this list.     * Shifts any subsequent elements to the left (subtracts one from their     * indices).     *     * @param index the index of the element to be removed     * @return the element that was removed from the list     * @throws IndexOutOfBoundsException {
    @inheritDoc}     */    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; // Let gc do its work        return oldValue;    }    /**     * Removes the first occurrence of the specified element from this list,     * if it is present.  If the list does not contain the element, it is     * unchanged.  More formally, removes the element with the lowest index     * i such that     * (o==null ? get(i)==null : o.equals(get(i)))     * (if such an element exists).  Returns true if this list     * contained the specified element (or equivalently, if this list     * changed as a result of the call).     *     * @param o element to be removed from this list, if present     * @return true if this list contained the specified element     */    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);                    return true;                }        }        return false;    }

从以上可以看出执行的过程就是一个数组的复制（System.arraycopy）

System.arraycopy：

首先我们来看看System.arraycopy的声明：

public static native void arraycopy(Object src,  int  srcPos,                                        Object dest, int destPos,                                        int length);

src:源数组

srcPos:源数组中的起始位置

dest:目标数组

destPos:目标数组中的起始位置

length:要复制数量

该方法被标记了native，调用了系统的C/C++代码，在JDK中是看不到的。在复制大量数组元素时用该方法，以取得更高的效率。