ArrayList的扩容机制

实质

ArrayList的底层实现就是数组

其动态变化的长度是由其扩容机制所组成的，在jdk1.8中，扩容代码如下：

/**
 * Increases the capacity to ensure that it can hold at least the
 * number of elements specified by the minimum capacity argument.
 *
 * @param minCapacity the desired minimum capacity
 */
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);
}

可以看出，新的容量是原来的1.5倍.

/**
 * Default initial capacity.
 */
private static final int DEFAULT_CAPACITY = 10;

默认长度是10