public boolean remove(Object o) {
  if (o == null) {
    for (int index = 0; index < size; index++)
      if (elementData[index] == null) {
        fastRemove(index);
        return true;
      }
  } else {
    for (int index = 0; index < size; index++)
      if (o.equals(elementData[index])) {
        fastRemove(index);
        return true;
      }
  }
  return false;
}

/*
 * Private remove method that skips bounds checking and does not
 * return the value removed.
 */
private void fastRemove(int index) {
  modCount++;
  int numMoved = size - index - 1;
  if (numMoved > 0)
    //remove會(huì)導(dǎo)致之后的元素往前移動(dòng)，而下標(biāo)不改變時(shí)就會(huì)出現(xiàn)bug
    System.arraycopy(elementData, index+1, elementData, index,
             numMoved);
  elementData[--size] = null; // clear to let GC do its work
}

final void checkForComodification() {
  if (modCount != expectedModCount)
    throw new ConcurrentModificationException();
}
//調(diào)用next時(shí)會(huì)調(diào)用checkForComodification方法檢查 這兩個(gè)字段
//而fastRemove里面只對(duì)modCount 進(jìn)行了改變 導(dǎo)致拋出異常
public E next() {
  checkForComodification();
  int i = cursor;
  if (i >= size)
    throw new NoSuchElementException();
  Object[] elementData = ArrayList.this.elementData;
  if (i >= elementData.length)
    throw new ConcurrentModificationException();
  cursor = i + 1;
  return (E) elementData[lastRet = i];
}

//內(nèi)部其實(shí)就是迭代器遍歷
default boolean removeIf(Predicate<? super E> filter) {
  Objects.requireNonNull(filter);
  boolean removed = false;
  final Iterator<E> each = iterator();
  while (each.hasNext()) {
    if (filter.test(each.next())) {
      each.remove();
      removed = true;
    }
  }
  return removed;
}