Java 源码系列 1——ArrayList

private void grow(int minCapacity) {
    // overflow-conscious code
    int oldCapacity = elementData.length;
    // 每次扩大原有容量的一半
    int newCapacity = oldCapacity + (oldCapacity >> 1);
    // 如果扩大一半后还是无法满足，则使用minCapacity
    if (newCapacity - minCapacity < 0)
        newCapacity = minCapacity;
    // 如果超过最大size，则获取最大容量的数组
    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);
}

/**
 * 添加在尾部
 */
public boolean add(E e) {
    // 检查容量
    ensureCapacityInternal(size + 1);
    // 添加在尾部
    elementData[size++] = e;
    return true;
}
/**
 * 按指定位置添加
 */
public void add(int index, E element) {
    // 检查index
    rangeCheckForAdd(index);
    // 检查容量
    ensureCapacityInternal(size + 1);
    // index后面的元素全部往后移一位
    System.arraycopy(elementData, index, elementData, index + 1,
                        size - index);
    elementData[index] = element;
    size++;
}

public E remove(int index) {
    // 检查index
    rangeCheck(index);
    modCount++;
    E oldValue = elementData(index);
    int numMoved = size - index - 1;
    if (numMoved > 0)
        // index后面的元素全部往前移一位
        System.arraycopy(elementData, index+1, elementData, index,
                            numMoved);
    elementData[--size] = null; // clear to let GC do its work
    return oldValue;
}

public static void main(String[] args) {
    ArrayList<Integer> a = new ArrayList<>(12);
    System.out.println(a.size());
    a.set(3, 3);
}