一、先看下ArrayList的构造方法源码

public ArrayList(int initialCapacity) {
    super();
    if (initialCapacity < 0)
         throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
    this.elementData = new Object[initialCapacity];
}

public ArrayList() {
    this(10);
}

由上面的源码可知，当我们新建一个ArrayList对象的时候：

1、如果我们不指定ArrayList的长度，会默认分配长度为10。

2、ArrayList的底层是一个对象数组。

二、添加元素源码

public boolean add(E e) {
       ensureCapacityInternal(size + 1);  // Increments modCount!!
       elementData[size++] = e;
       return true;
}

private void ensureCapacityInternal(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);
}

由添加元素源码可知：

1、首先判断容量是否已经满了，如果满了就会扩容，否则的话就将元素放到对象数组里。