ArrayList扩容源码分析

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
    private static final long serialVersionUID = 8683452581122892189L;
    
    private static final int DEFAULT_CAPACITY = 10;//默认初始化容量

    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};//默认空容量

    private static final Object[] EMPTY_ELEMENTDATA = {};//空实例

    transient Object[] elementData;//数组缓冲，将在添加第一个元素时扩展至默认容量

    private int size;//数组大小
}

//指定容量
public ArrayList(int initialCapacity) {
        if (initialCapacity > 0) {
            this.elementData = new Object[initialCapacity];
        } else if (initialCapacity == 0) {
            this.elementData = EMPTY_ELEMENTDATA;
        } else {
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        }
    }

//未指定容量
public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }

//添加元素
public boolean add(E e) {
        ensureCapacityInternal(size + 1);
        elementData[size++] = e;
        return true;
    }

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

private static int calculateCapacity(Object[] elementData, int minCapacity) {
        //判断是否第一次添加元素，是则取10与size+1间最大值
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            return Math.max(DEFAULT_CAPACITY, minCapacity);
        }
        return minCapacity;
    }

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

        //如果最小需要空间比elementData的内存空间要大，扩容
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }

//核心扩容方法
private void grow(int minCapacity) {
        //获取原集合长度 
        int oldCapacity = elementData.length;
        //集合长度扩容为原长度的1.5倍
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        //如果扩容后仍小于添加集合的长度,新建集合长度为添加元素集合大小
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        //若预设值大于默认的最大值检查是否溢出 
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        //调用Arrays.copyOf方法将elementData数组指向新的长度为扩容后长度的内存空间
        elementData = Arrays.copyOf(elementData, newCapacity);
    }    

//检查溢出
private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) // overflow
            throw new OutOfMemoryError();
        return (minCapacity > MAX_ARRAY_SIZE) ?
            Integer.MAX_VALUE :
            MAX_ARRAY_SIZE;
    }