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);}

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);}

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。