ArrayList源码

//属性
//默认初始大小
private static final int DEFAULT_CAPACITY = 10;
//空数组用这个
private static final Object[] EMPTY_ELEMENTDATA = {};
//扩展数组时用来和EMPTY_ELEMETDAT
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
//添加第一个元素时将会扩大多DEFAULT_CAPACITY
transient Object[] elementData;
//数组大小
private int size;

构造函数：

    public ArrayList(int initialCapacity) {
        // 初始容量大于0，数组大小就是这么多
        if (initialCapacity > 0) {
            this.elementData = new Object[initialCapacity];
        } else if (initialCapacity == 0) {
            // 如果为0，为已经设置的EMPTY_ELEMENTDATA
            this.elementData = EMPTY_ELEMENTDATA;
        } else {
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        }
    }
 
    /**
     * Constructs an empty list with an initial capacity of ten.
     */
    public ArrayList() {
        // 与EMPTY_ELEMENTDATA区分开
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }

扩容机制：

// 确保容量足够
public void ensureCapacity(int minCapacity) {
    int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
        // any size if not default element table
        ? 0
        // larger than default for default empty table. It's already
        // supposed to be at default size.
        : DEFAULT_CAPACITY;
 
    if (minCapacity > minExpand) {
        ensureExplicitCapacity(minCapacity);
    }
}

如果elementData不是DEFAULTCAPACITY_EMPTY_ELEMENTDATA，就给minExpand赋值0，否则就是10；如果minCapacity > minExpand，进行精确确保容量

private void ensureExplicitCapacity(int minCapacity) {
    //修改次数
    modCount++;
 
    // overflow-conscious code
    if (minCapacity - elementData.length > 0)
        grow(minCapacity);
}

如果最小容量比数组长度大就扩容

// 尝试分配最大的数组大小，防止OutOfMemoryError
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
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);
}
private static int hugeCapacity(int minCapacity) {
    if (minCapacity < 0) // overflow
        throw new OutOfMemoryError();
    return (minCapacity > MAX_ARRAY_SIZE) ?
        Integer.MAX_VALUE :
        MAX_ARRAY_SIZE;
}

可以看到newCapacity的大小是oldCapacity的1.5倍，如果newCapacity还是小于minCapacity，newCapacity = minCapacity；如果newCapacity已经比最大容量大了，就调用hugeCapacity方法，就直接把整数最大值赋给newCapacity；最后一步调用copyOf复制新数组