/**
   * Appends the specified element to the end of this list.
   *
   * @param e element to be appended to this list
   * @return <tt>true</tt> (as specified by {@link Collection#add})
   */
  public boolean add(E e) {
    ensureCapacityInternal(size + 1);  // 進行擴容校驗
    elementData[size++] = e;      // 將值添加到數組后面，并將 size+1
    return true;
  }



  /**
   * The array buffer into which the elements of the ArrayList are stored.
   * The capacity of the ArrayList is the length of this array buffer. Any
   * empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
   * will be expanded to DEFAULT_CAPACITY when the first element is added.
   */
  transient Object[] elementData; // non-private to simplify nested class access
  
  private void ensureCapacityInternal(int minCapacity) {
    ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));  // elementData 數組
  }



  /**
   * Default initial capacity.
   */
  private static final int DEFAULT_CAPACITY = 10;
  
  /**
   * Shared empty array instance used for default sized empty instances. We
   * distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when
   * first element is added.
   */
  private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

  // 返回最大的 index
  private static int calculateCapacity(Object[] elementData, int minCapacity) {
    if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {  // 與空數組實例對比
      return Math.max(DEFAULT_CAPACITY, minCapacity);
    }
    return minCapacity;
  }



  private void ensureExplicitCapacity(int minCapacity) {
    modCount++;

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

/**
   * 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);
  }

/**
   * Inserts the specified element at the specified position in this
   * list. Shifts the element currently at that position (if any) and
   * any subsequent elements to the right (adds one to their indices).
   *
   * @param index index at which the specified element is to be inserted
   * @param element element to be inserted
   * @throws IndexOutOfBoundsException {@inheritDoc}
   */
  public void add(int index, E element) {
    rangeCheckForAdd(index);  // 校驗index是否超過當前定義的數組大小范圍，超過則拋出 IndexOutOfBoundsException

    ensureCapacityInternal(size + 1); // Increments modCount!!
    System.arraycopy(elementData, index, elementData, index + 1,
             size - index);   // 復制，向后移動
    elementData[index] = element;
    size++;
  }
  

  /**
   * A version of rangeCheck used by add and addAll.
   */
  private void rangeCheckForAdd(int index) {
    if (index > size || index < 0)
      throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
  }