总结

无参构造创建size=0的空对象数组，等到add时创建DEFAULT_CAPACITY=10大小的对象数组
当用完对象数组时扩容（size+1>length时），size（list中元素个数），length（Object数组长度）
扩容是以elementData.length*1.5方式深拷贝elementData = Arrays.copyOf(elementData, newCapacity) 扩容
为了避免容量不够时，深拷贝扩容对性能的浪费，使用有参构造给出合适的初始容量（initialCapacity）

源码

构造函数

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{	
	
	private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
	
	/**
	 * 无参构造
	 * 创建空Object对象数组，size=0
	 * 等到用add方法时，创建DEFAULT_CAPACITY=10大小的对象数组
	 * 当容量不够时(size+1>elementData.length)，以elementData.length*1.5方式深拷贝elementData = Arrays.copyOf(elementData, newCapacity) 扩容
	 */
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }
	
	private static final Object[] EMPTY_ELEMENTDATA = {};
	
	/**
	 * 有参构造
	 */
	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);
        }
    }
}

add相关方法

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{	
	/**
     * add方法相关
     */
    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }
	
	/**
	 * 确保容量够了之后会把index之后的元素整体往后移动1步
	 */
    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++;
    }
    
    private void ensureCapacityInternal(int minCapacity) {
        ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
    }
	
	private static int calculateCapacity(Object[] elementData, int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            return Math.max(DEFAULT_CAPACITY, minCapacity);
        }
        return minCapacity;
    }
    
	private void ensureExplicitCapacity(int minCapacity) {
        modCount++;

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

	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);
    }
}