// 序列化版本UID
private static final long
        serialVersionUID = 8683452581122892189L;

/**
 * 默认的初始容量
 */
private static final int
        DEFAULT_CAPACITY = 10;

/**
 * 用于空实例的共享空数组实例
 * new ArrayList(0);
 */
private static final Object[]
        EMPTY_ELEMENTDATA = {};

/**
 * 用于提供默认大小的实例的共享空数组实例
 * new ArrayList();
 */
private static final Object[]
        DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

/**
 * 存储ArrayList元素的数组缓冲区
 * ArrayList的容量，是数组的长度
 * 
 * non-private to simplify nested class access
 */
transient Object[] elementData;

/**
 * ArrayList中元素的数量
 */
private int size;

/**
 * 带一个初始容量参数的构造方法
 *
 * @param  initialCapacity  初始容量
 * @throws  如果初始容量非法就抛出
 *          IllegalArgumentException
 */
public ArrayList(int initialCapacity) {
    if (initialCapacity > 0) {
        this.elementData =
                new Object[initialCapacity];
    } else if (initialCapacity == 0) {
        this.elementData = EMPTY_ELEMENTDATA;
    } else {
        throw new IllegalArgumentException(
                "Illegal Capacity: "+ initialCapacity);
    }
}

/**
 * 无参构造方法 将elementData 赋值为
 *   DEFAULTCAPACITY_EMPTY_ELEMENTDATA
 */
public ArrayList() {
    this.elementData =
            DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}

/**
 * 带一个集合参数的构造方法
 *
 * @param c 集合，代表集合中的元素会被放到list中
 * @throws 如果集合为空，抛出NullPointerException
 */
public ArrayList(Collection<? extends E> c) {
    elementData = c.toArray();
    // 如果 size != 0
    if ((size = elementData.length) != 0) {
        // c.toArray 可能不正确的，不返回 Object[]
        // https://bugs.openjdk.java.net/browse/JDK-6260652
        if (elementData.getClass() != Object[].class)
            elementData = Arrays.copyOf(
                    elementData, size, Object[].class);
    } else {
        // size == 0
        // 将EMPTY_ELEMENTDATA 赋值给 elementData
        this.elementData = EMPTY_ELEMENTDATA;
    }
}

public static void main(String[] args) {
  
    // 方式一：java.util.ArrayList
    List<String> list = new ArrayList<>(Arrays.asList("list"));
    
    System.out.println(list.getClass());// class java.util.ArrayList
    Object[] listArray = list.toArray();
    System.out.println(listArray.getClass());  // class [Ljava.lang.Object;
    listArray[0] = new Object();

    System.out.println("----------------------");

    // 方式二：java.util.Arrays 的私有内部类ArrayList
    List<String> asList = Arrays.asList("asList");
   
    System.out.println(asList.getClass());  // class java.util.Arrays$ArrayList
    Object[] asListArray = asList.toArray(); // class [Ljava.lang.String;    
    System.out.println(asListArray.getClass());    
    asListArray[0] = new Object(); // ⚠️ java.lang.ArrayStoreException
}

public Object[] toArray() {
    // ArrayLisy中 elementData是这样定义的
    // transient Object[] elementData;
    return Arrays.copyOf(elementData, size);
}

public static <T> T[] copyOf(T[] original, int newLength) {
    // original.getClass() 是 class [Ljava.lang.Object
    return (T[]) copyOf(original, newLength, original.getClass());
}

public static <T,U> T[] copyOf(U[] original, 
          int newLength, Class<? extends T[]> newType) {
    @SuppressWarnings("unchecked")
    /**
     * 如果newType是Object[] copy 数组 类型就是 Object 
     * 否则就是 newType 类型
     */
    T[] copy = ((Object)newType == (Object)Object[].class)
        ? (T[]) new Object[newLength]
        : (T[]) Array.newInstance(newType.getComponentType(), newLength);
    System.arraycopy(original, 0, copy, 0,
                     Math.min(original.length, newLength));
    return copy;
}

private static class ArrayList<E> extends AbstractList<E>
        implements RandomAccess, java.io.Serializable {

    // 存储元素的数组
    private final E[] a;

    ArrayList(E[] array) {
        // 直接把接收的数组 赋值 给 a
        a = Objects.requireNonNull(array);
    }

    /**
     * obj 为空抛出异常
     * 不为空 返回 obj
     */
    public static <T> T requireNonNull(T obj) {
        if (obj == null)
            throw new NullPointerException();
        return obj;
    }

    @Override
    public Object[] toArray() {
        // 返回 a 的克隆对象
        return a.clone();
    }

}

public static <T> List<T> asList(T... a) {
    return new ArrayList<>(a);
}

/**
 * 在列表最后添加指定元素
 *
 * @param e 要添加的指定元素
 * @return true
 */
public boolean add(E e) {
    // ⚠️ 增加 modCount ！！
    ensureCapacityInternal(size + 1); 
    elementData[size++] = e;
    return true;
}

/**
 * 在指定位置添加指定元素
 * 如果指定位置已经有元素，就将该元素和随后的元素移动到右面一位
 *
 * @param index 待插入元素的下标
 * @param element 待插入的元素
 * @throws 可能抛出 IndexOutOfBoundsException
 */
public void add(int index, E element) {
    rangeCheckForAdd(index);


    // 增加 modCount ！！
    ensureCapacityInternal(size + 1);
    System.arraycopy(elementData, index, elementData, index + 1,
                     size - index);
    elementData[index] = element;
    size++;
}

/**
 * 计算容量
 */
private static int calculateCapacity(
        Object[] elementData, int minCapacity) {

    if (elementData ==
            DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
        return Math.max(DEFAULT_CAPACITY, minCapacity);
    }
    return minCapacity;
}

private void ensureCapacityInternal(int minCapacity) {
    ensureExplicitCapacity(
            calculateCapacity(elementData, minCapacity)
    );
}

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

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

/**
 * 数组可以分配的最大size
 * 一些虚拟机在数组中预留一些header words
 * 如果尝试分配更大的size，可能导致OutOfMemoryError
 */
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

/**
 * 增加容量，至少保证比minCapacity大
 * @param minCapacity 期望的最小容量
 */
private void grow(int minCapacity) {
    // 有可能溢出的代码
    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);
}

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

/**
 * 移除列表中指定下标位置的元素
 * 将所有的后续元素，向左移动
 *
 * @param 要移除的指定下标
 * @return 返回被移除的元素
 * @throws 下标越界会抛出IndexOutOfBoundsException
 */
public E remove(int index) {
    rangeCheck(index);

    modCount++;
    E oldValue = elementData(index);

    int numMoved = size - index - 1;
    if (numMoved > 0)
            System.arraycopy(elementData, 
                    index+1, elementData, index,  numMoved);
    // 将引用置空，让GC回收
    elementData[--size] = null;

    return oldValue;
}

/**
 * 移除第一个在列表中出现的指定元素
 * 如果存在，移除返回true
 * 否则，返回false
 *
 * @param o 指定元素
 */
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;
}

/**
 * 返回指定元素第一次出现的下标
 * 如果不存在该元素，返回 -1
 * 如果 o ==null 会特殊处理
 */
public int indexOf(Object o) {
    if (o == null) {
        for (int i = 0; i < size; i++)
            if (elementData[i]==null)
                return i;
    } else {
        for (int i = 0; i < size; i++)
            if (o.equals(elementData[i]))
                return i;
    }
    return -1;
}

/**
 * 返回指定位置的元素
 *
 * @param  index 指定元素的位置 
 * @throws index越界会抛出IndexOutOfBoundsException
 */
public E get(int index) {
    rangeCheck(index);

    return elementData(index);
}

/**
 * 将ArrayLisy实例的状态保存到一个流里面
 */
private void writeObject(java.io.ObjectOutputStream s)
    throws java.io.IOException{
    // Write out element count, and any hidden stuff
    int expectedModCount = modCount;
    s.defaultWriteObject();

    // Write out size as capacity for behavioural compatibility with clone()
    s.writeInt(size);

    // 按照顺序写入所有的元素
    for (int i=0; i<size; i++) {
        s.writeObject(elementData[i]);
    }

    if (modCount != expectedModCount) {
        throw new ConcurrentModificationException();
    }
}

/**
 * 根据一个流(参数)重新生成一个ArrayList
 */
private void readObject(java.io.ObjectInputStream s)
    throws java.io.IOException, ClassNotFoundException {
    elementData = EMPTY_ELEMENTDATA;

    // Read in size, and any hidden stuff
    s.defaultReadObject();

    // Read in capacity
    s.readInt();

    if (size > 0) {
        // be like clone(), allocate array based upon size not capacity
        ensureCapacityInternal(size);

        Object[] a = elementData;
        // Read in all elements in the proper order.
        for (int i=0; i<size; i++) {
            a[i] = s.readObject();
        }
    }
}

public List<E> subList(int fromIndex, int toIndex) {
    subListRangeCheck(fromIndex, toIndex, size);
    return new SubList(this, 0, fromIndex, toIndex);
}

private class SubList extends AbstractList<E> implements RandomAccess {
    private final AbstractList<E> parent;
    private final int parentOffset;
    private final int offset;
    int size;

    SubList(AbstractList<E> parent,
            int offset, int fromIndex, int toIndex) {
        this.parent = parent;
        this.parentOffset = fromIndex;
        this.offset = offset + fromIndex;
        this.size = toIndex - fromIndex;
        this.modCount = ArrayList.this.modCount;
    }

    public E set(int index, E e) {
        rangeCheck(index);
        checkForComodification();
        E oldValue = ArrayList.this.elementData(offset + index);
        ArrayList.this.elementData[offset + index] = e;
        return oldValue;
    }

    // 省略代码...
}

public Iterator<E> iterator() {
    return new Itr();
}

// 下一个要返回的元素的下标
int cursor;
// 最后一个要返回元素的下标 没有元素返回 -1
int lastRet = -1;
// 期望的 modCount
int expectedModCount = modCount;

public boolean hasNext() {
    return cursor != size;
}

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];
}

final void checkForComodification() {
    if (modCount != expectedModCount)
        throw new ConcurrentModificationException();
}

public static void main(String[] args) {
    ArrayList arrayList = new ArrayList();
    for (int i = 0; i < 10; i++) {
        arrayList.add(i);
    }
    remove(arrayList);
    System.out.println(arrayList);
}

public static void remove(ArrayList<Integer> list) {
    Iterator<Integer> iterator = list.iterator();
    while (iterator.hasNext()) {
        Integer number = iterator.next();
        if (number % 2 == 0) {
            // 抛出ConcurrentModificationException异常
            list.remove(number);
        }
    }
}

public void remove() {
    if (lastRet < 0)
        throw new IllegalStateException();
    checkForComodification();

    try {
        ArrayList.this.remove(lastRet);
        cursor = lastRet;
        lastRet = -1;
        // 移除之后将modCount 重新赋值给 expectedModCount
        expectedModCount = modCount;
    } catch (IndexOutOfBoundsException ex) {
        throw new ConcurrentModificationException();
    }
}