ArrayList实例变量
//ArrayList默认容量
private static final int DEFAULT_CAPACITY = 10;
//默认空的Object数组， 用于定义空的ArrayList
private static final Object[] EMPTY_ELEMENTDATA = {};
//ArrayList存放存放元素的Object数组
private transient Object[] elementData;
//ArrayList中元素的数量
private int size;

public ArrayList() {
  super();
  this.elementData = EMPTY_ELEMENTDATA;
}

public ArrayList(int initialCapacity) {
  super();
  if (initialCapacity < 0)
    throw new IllegalArgumentException("Illegal Capacity: "+
                      initialCapacity);
  this.elementData = new Object[initialCapacity];
}

public ArrayList(Collection<? extends E> c) {
  elementData = c.toArray();
  size = elementData.length;
  // c.toArray might (incorrectly) not return Object[] (see 6260652)
  if (elementData.getClass() != Object[].class)
    elementData = Arrays.copyOf(elementData, size, Object[].class);
}

private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

public boolean add(E e) {
    ensureCapacityInternal(size + 1); // Increments modCount!!
    elementData[size++] = e;
    return true;
  }

private void ensureCapacityInternal(int minCapacity) {
    if (elementData == EMPTY_ELEMENTDATA) {
      minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
    }
    ensureExplicitCapacity(minCapacity);
  }
private void ensureExplicitCapacity(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);
  }

private static int hugeCapacity(int minCapacity) {
    if (minCapacity < 0) // overflow
      throw new OutOfMemoryError();
    return (minCapacity > MAX_ARRAY_SIZE) ?
      Integer.MAX_VALUE :
      MAX_ARRAY_SIZE;
  }

private class Itr implements Iterator<E> {
    int cursor;    // index of next element to return
    int lastRet = -1; // index of last element returned; -1 if no such
    //expectedModCount 是modCount的一个副本
    int expectedModCount = modCount;
    public boolean hasNext() {
      return cursor != size;
    }
    @SuppressWarnings("unchecked")
    public E next() {
      checkForComodification();
      //记录当前位置
      int i = cursor;
      if (i >= size)
        throw new NoSuchElementException();
      Object[] elementData = ArrayList.this.elementData;
      if (i >= elementData.length)
        throw new ConcurrentModificationException();
      //下一个元素的位置
      cursor = i + 1;
      return (E) elementData[lastRet = i];
    }
    //使用迭代器的remove方法
    public void remove() {
      if (lastRet < 0)
        throw new IllegalStateException();
      checkForComodification();
      try {
        //注意内部类调用外部类的方式
        ArrayList.this.remove(lastRet);
        //remove之后需要重新调整各个指针的位置
        cursor = lastRet;
        lastRet = -1;
        expectedModCount = modCount;
      } catch (IndexOutOfBoundsException ex) {
        throw new ConcurrentModificationException();
      }
    }
    final void checkForComodification() {
      if (modCount != expectedModCount)
        throw new ConcurrentModificationException();
    }
  }

private class ListItr extends Itr implements ListIterator<E> {
    ListItr(int index) {
      super();
      cursor = index;
    }
    public boolean hasPrevious() {
      return cursor != 0;
    }
    public int nextIndex() {
      return cursor;
    }
    public int previousIndex() {
      return cursor - 1;
    }
    @SuppressWarnings("unchecked")
    public E previous() {
      checkForComodification();
      //arrayList前一个元素的位置
      int i = cursor - 1;
      if (i < 0)
        throw new NoSuchElementException();
      Object[] elementData = ArrayList.this.elementData;
      if (i >= elementData.length)
        throw new ConcurrentModificationException();
      cursor = i;
      return (E) elementData[lastRet = i];
    }
    //该迭代器中添加了set方法
    public void set(E e) {
      if (lastRet < 0)
        throw new IllegalStateException();
      checkForComodification();
      try {
        ArrayList.this.set(lastRet, e);
      } catch (IndexOutOfBoundsException ex) {
        throw new ConcurrentModificationException();
      }
    }
    //该迭代器添加了add方法
    public void add(E e) {
      checkForComodification();
      try {
        int i = cursor;
        ArrayList.this.add(i, e);
        //重新标记指针位置
        cursor = i + 1;
        lastRet = -1;
        expectedModCount = modCount;
      } catch (IndexOutOfBoundsException ex) {
        throw new ConcurrentModificationException();
      }
    }
  }

public boolean add(E e) {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
      Object[] elements = getArray();
      int len = elements.length;
      Object[] newElements = Arrays.copyOf(elements, len + 1);
      newElements[len] = e;
      setArray(newElements);
      return true;
    } finally {
      lock.unlock();
    }
  }

private boolean batchRemove(Collection<?> c, boolean complement) {
    //下面会提到使用final的原因
    final Object[] elementData = this.elementData;
    int r = 0, w = 0;
    boolean modified = false;
    try {
      //遍历List中的元素，进行验证
      for (; r < size; r++)
        if (c.contains(elementData[r]) == complement)
          elementData[w++] = elementData[r];
    } finally {
      //try中如果出现异常，则保证数据一致性执行下面的copy操作
      if (r != size) {
        System.arraycopy(elementData, r,
                 elementData, w,
                 size - r);
        w += size - r;
      }
      //清理无用的元素， 通知GC回收
      if (w != size) {
        // clear to let GC do its work
        for (int i = w; i < size; i++)
          elementData[i] = null;
        modCount += size - w;
        size = w;
        modified = true;
      }
    }
    return modified;
  }

private static class Node<E> {
  E item;
  Node<E> next;
  Node<E> prev;
  Node<E> (Node<E> prev, E element, Node<E> next) {
    this.item = element;
    this.next = next;
    this.prev = prev;
  }
}

transient int size = 0;
transient Node<E> first;
transient Node<E> last;

public LinkedList() {}

public LinkedList(Collection<? extends E> c) {
    this();
    addAll(c);
}

private void linkFirst(E e) {
  final Node<E> f = first;
  final Node<E> newNode = new Node<>(null, e, f);
  first = newNode;
  //判断是否是空链表
  if (f == null)
    last = newNode;
  else
    f.prev = newNode;
  size++;
  modCount++;
  }

  void linkLast(E e) {
    final Node<E> l = last;
    final Node<E> newNode = new Node<>(l, e, null);
    last = newNode;
    if (l == null)
      first = newNode;
    else
      l.next = newNode;
    size++;
    modCount++;
  }

void linkBefore(E e, Node<E> succ) {
    //确定当然结点非空
    final Node<E> pred = succ.prev;
    final Node<E> newNode = new Node<>(pred, e, succ);
    succ.prev = newNode;
    //判断当前结点是否是第一个结点
    if (pred == null)
      first = newNode;
    else
      pred.next = newNode;
    size++;
    modCount++;
  }

  private E unlinkFirst(Node<E> f) {
    // assert f == first && f != null;
    final E element = f.item;
    final Node<E> next = f.next;
    f.item = null;
    f.next = null; // help GC
    first = next;
    if (next == null)
      last = null;
    else
      next.prev = null;
    size--;
    modCount++;
    return element;
  }

  private E unlinkLast(Node<E> l) {
    //保证l==last 并且l != null
    final E element = l.item;
    final Node<E> prev = l.prev;
    l.item = null;
    l.prev = null; // help GC
    last = prev;
    if (prev == null)
      first = null;
    else
      prev.next = null;
    size--;
    modCount++;
    return element;
  }

Node<E> node(int index) {
    //确保index的正确性
    if (index < (size >> 1)) {
      Node<E> x = first;
      for (int i = 0; i < index; i++)
        x = x.next;
      return x;
    } else {
      Node<E> x = last;
      for (int i = size - 1; i > index; i--)
        x = x.prev;
      return x;
    }
  }