/**
 * ArrayList 是List接口的动态数组实现，实现了List的所有可选操作，并且允许所有元素，包括null。
 * ArrayList 跟Vector差不多，但它不是线程安全的。
 * ArrayList 的容量会根据列表大小自动调整。在添加大量元素之前，可以使用ensureCapacity 方法来保证列表有足够空间存放元素。
 * ArrayList 不是线程安全的，所以如果多条线程将要对其进行结构性改变时（如添加删除元素），需要使用synchronized 进行同步。
 * 如果不存在这样的对象，则需要使用其同步包装类 Collections.synchronizedList
 *   List list = Collections.synchronizedList(new ArrayList(...));
 *
 * iterator() 方法将会返回一个listIterator，其中的方法是“fail-fast(快速失败的)”，如果在创建了迭代器之后，在用迭代器遍历一个列表时，
 * 如果遍历过程中对集合对象的内容进行了修改（增加、删除、修改），则会抛出Concurrent Modification Exception。
 * 但fail-fast 的行为是无法得到保证的，不可能对是否出现不同步并发修改做出任何硬性保证。
 * 快速失败迭代器会尽最大努力抛出 ConcurrentModificationException。
 * 因此，为提高这类迭代器的正确性而编写一个依赖于此异常的程序是错误的做法：迭代器的快速失败行为应该仅用于检测 bug。
 */

public class ArrayList<E> extends AbstractList<E>
        implements List<E>,RandomAccess,Cloneable,java.io.Serializable
{
    private static final long serialVersionUID = 8683452581122892189L;

    
     * 默认容量
     */
    int DEFAULT_CAPACITY = 10
     * 空实例共享的空数组
     * todo 为什么要区分 EMPTY_ELEMENTDATA 与 DEFAULTCAPACITY_EMPTY_ELEMENTDATA
     final Object[] EMPTY_ELEMENTDATA = {};

    
     * 默认大小的空实例共享的空数组
     final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA =
     * ArrayList的元素存储在其中的数组缓冲区。ArrayList的容量是这个数组缓冲区的长度。
     * 当添加第一个元素时，任何为 DEFAULTCAPACITY_EMPTY_ELEMENTDATA 的空ArrayList的容量
     * 将扩充到默认大小DEFAULT_CAPACITY（10）。
     * 设置为非private 是为了方便内部类进行访问
     *
     * todo 内部动态数组的维护
     transient Object[] elementData;

    
     * 列表中实际存储的元素个数
     *
     * todo size 与 capacity
     int size;

    
     * 构造一个指定初始容量的空列表
     public ArrayList( initialCapacity) {
        if (initialCapacity > 0) {
            this.elementData = new Object[initialCapacity];
        } else if (initialCapacity == 0this.elementData = EMPTY_ELEMENTDATA;
        } else {
            throw new IllegalArgumentException("Illegal Capacity: "+
                    initialCapacity);
        }
    }

    
     * 构造一个默认容量的空列表
     public ArrayList() {
         DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }

    
     * 构造一个包含集合C中所有元素的列表，存储顺序为集合C迭代器遍历的顺序
     public ArrayList(Collection<? extends E> c) {
        elementData = c.toArray();
        if ((size = elementData.length) != 0// c.toArray might (incorrectly) not return Object[] (see 6260652)
            if (elementData.getClass() != Object[].class)
                elementData = Arrays.copyOf(elementData,size,Object[].);
        }  replace with empty array.
             EMPTY_ELEMENTDATA;
        }
    }

    
     * 调整容量大小到列表当前元素个数，以节约存储空间
     void trimToSize() {
        todo modCount的作用
        modCount++;
        if (size < elementData.length) {
            elementData = (size == 0)
                    ? EMPTY_ELEMENTDATA
                    : Arrays.copyOf(elementData,size);
        }
    }

    
     * 增加列表容量以确保它至少能容纳指定数量的元素
     *
     * todo 扩容方式
     void ensureCapacity( minCapacity) {
        最小扩容量
        如果elementData是空数组则最小扩容量为0，否则最小扩容量为10
        int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
                ? 0
                : DEFAULT_CAPACITY;

        如果指定的容量比最小扩容量大，则进行扩容操作
        if (minCapacity > minExpand) {
            ensureExplicitCapacity(minCapacity);
        }
    }

    int calculateCapacity(Object[] elementData,if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            return Math.max(DEFAULT_CAPACITY,minCapacity);
        }
         minCapacity;
    }

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

    void ensureExplicitCapacity( minCapacity) {
        modCount++;

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

    
     * 数组容量最大值（- 8 是因为部分 JVM 需要在数组中存入部分头信息）
     int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8
     * 扩容函数，扩容1.5 倍，扩容后的列表大小在minCapacity与1.5 倍原大小之间取最大值
     void grow(int oldCapacity = elementData.length;
         >> 是右移运算符，作用结果是将原数值的二进制数右移指定位数，转换成十进制的效果
         就是除以2的指定次方，这里右移一位即除以2
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        if (newCapacity - MAX_ARRAY_SIZE > 0 hugeCapacity(minCapacity);
         minCapacity is usually close to size,so this is a win:
        elementData = Arrays.copyOf(elementData,newCapacity);
    }

    int hugeCapacity(if (minCapacity < 0)  overflow
             OutOfMemoryError();
        return (minCapacity > MAX_ARRAY_SIZE) ?
                Integer.MAX_VALUE :
                MAX_ARRAY_SIZE;
    }

    
     * 元素个数
      size() {
         size;
    }

    
     * 是否为空
     boolean isEmpty() {
        return size == 0;
    }

    
     * 是否包含某个元素
      contains(Object o) {
        return indexOf(o) >= 0
     * 查找元素第一次出现的位置
      indexOf(Object o) {
        if (o == nullfor (int i = 0; i < size; i++)
                if (elementData[i]==)
                     i;
        } if (o.equals(elementData[i]))
                     i;
        }
        return -1
     * 查找元素最后一次出现的位置
      lastIndexOf(Object o) {
        int i = size-1; i >= 0; i--
     * 返回一个深克隆对象
      Object clone() {
        try Object 的克隆方法，复制本对象及其内所有基本类型成员和 String 类型成员
             但不会复制引用对象
            ArrayList<?> v = (ArrayList<?>) super.clone();
            v.elementData =;
             v;
        } catch (CloneNotSupportedException e) {
             this shouldn't happen,since we are Cloneable
             InternalError(e);
        }
    }

    
     * 转化成对象数组
      Object[] toArray() {
        
     * 将列表转存到数组a中，如果a的空间足够则直接存放，否则会新建一个数组进行存储
     */
    @SuppressWarnings("unchecked")
    public <T> T[] toArray(T[] a) {
        if (a.length < size)
             Make a new array of a's runtime type,but my contents:
             (T[]) Arrays.copyOf(elementData,a.getClass());
        System.arraycopy(elementData,0,a,0if (a.length > size)
            a[size] =  a;
    }

     位置访问操作


    @SuppressWarnings("unchecked")
    E elementData( index) {
         (E) elementData[index];
    }

    
     * 取序号为index的元素
     public E get( index) {
        rangeCheck(index);

         elementData(index);
    }

    
     * 替换指定位置的元素，并返回原来的元素
     public E set( index,E element) {
        rangeCheck(index);

        E oldValue = elementData(index);
        elementData[index] = element;
         oldValue;
    }

    
     * 添加元素
      add(E e) {
        ensureCapacityInternal(size + 1);   Increments modCount!!
        elementData[size++] = e;
        return true
     * 插入元素
     void add( Increments modCount!!
        System.arraycopy(elementData,index,elementData,index + 1 index);
        elementData[index] = element;
        size++
     * 移除指定序号的元素
     public E remove( index) {
        rangeCheck(index);

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

        int numMoved = size - index - 1if (numMoved > 0)
            System.arraycopy(elementData,index+1null;  clear to let GC do its work

        
     * 移除某个元素
      remove(Object o) {
        int index = 0; index < size; index++if (elementData[index] == ) {
                    fastRemove(index);
                    ;
                }
        }  (o.equals(elementData[index])) {
                    fastRemove(index);
                    ;
                }
        }
        false/*
     * 快速移除
     void fastRemove( index) {
        modCount++)
            调用System.arraycopy 进行数组拷贝
            四个参数分别是：待拷贝的数组，原数组的拷贝起始位置，拷贝到的目标数组，目标数组的起始位置
            这里相当于把原数组的一部分拷贝到另一部分，将数组从index+1以后的部分整体往前移一个单位
            将index那个元素覆盖掉
            System.arraycopy(elementData,index+1然后将最后一个元素置为null
        elementData[--size] =  clear to let GC do its work
    }

    
     * 移除所有元素（将所有元素置为null，容量不变）
      clear() {
        modCount++ clear to let GC do its work
        )
            elementData[i] = ;

        size = 0
     * 将集合C中的所有元素添加到列表中
     boolean addAll(Collection<?  c) {
        先将c转换为数组，然后再复制过来
        Object[] a =int numNew = a.length;
        ensureCapacityInternal(size + numNew);   Increments modCount
        System.arraycopy(a,numNew);
        size += numNew;
        return numNew != 0
     * 将集合C中的元素插入指定位置
     boolean addAll(int index,Collection<?  c) {
        rangeCheckForAdd(index);

        Object[] a = Increments modCount

        int numMoved = size - index;
         numNew,numMoved);

        System.arraycopy(a,0
     * 范围性移除，移除列表里序号从fromIndex到toIndex的所有元素，包含fromIndex，不包含toIndex
     protected void removeRange(int fromIndex,1)"> toIndex) {
        modCount++ toIndex;
        System.arraycopy(elementData,toIndex,fromIndex,numMoved);

         将其余位置置为null
        int newSize = size - (toIndex-fromIndex);
        int i = newSize; i < size; i++) {
            elementData[i] = ;
        }
        size = newSize;
    }

    
     * 检测index是否超过边界
     void rangeCheck(if (index >= IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    
     * 插入时边界检测
     void rangeCheckForAdd(if (index > size || index < 0
     * 生成 IndexOutOfBoundsException 的详细信息
     private String outOfBoundsMsg(return "Index: "+index+",Size: "+size;
    }

    
     * 移除列表中所有存在于集合c中的元素
     boolean removeAll(Collection<?> c) {
        Objects.requireNonNull(c);
        return batchRemove(c,1)">);
    }

    
     * 仅保留所有在集合C中的元素
     boolean retainAll(Collection<?>boolean batchRemove(Collection<?> c,1)"> complement) {
        final Object[] elementData = this.elementData;
        int r = 0,w = 0boolean modified = for (; r < size; r++if (c.contains(elementData[r]) == complement)
                    elementData[w++] = elementData[r];
        } finallyif (r != size) {
                System.arraycopy(elementData,r,w,1)"> r);
                w += size - r;
            }
            if (w != size) {
                 clear to let GC do its work
                int i = w; i < size; i++)
                    elementData[i] = ;
                modCount += size - w;
                size = w;
                modified = ;
            }
        }
         modified;
    }

    
     * 将列表实例保存到输出流（即序列化）
      writeObject(java.io.ObjectOutputStream s)
            throws java.io.IOException{
        int expectedModCount = modCount;
        s.defaultWriteObject();

        s.writeInt(size);

        int i=0; i<size; i++) {
            s.writeObject(elementData[i]);
        }

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

    
     * 从输入流中读取列表实例
      readObject(java.io.ObjectInputStream s)
             java.io.IOException,ClassNotFoundException {
        elementData = EMPTY_ELEMENTDATA;

        s.defaultReadObject();

        s.readInt();  ignored

        if (size > 0int capacity = calculateCapacity(elementData,size);
            SharedSecrets.getJavaOISAccess().checkArray(s,Object[]. elementData;
             Read in all elements in the proper order.
            ) {
                a[i] = s.readObject();
            }
        }
    }

    
     * 返回一个从指定序号元素开始的ListIterator
     public ListIterator<E> listIterator(if (index < 0 || index >new IndexOutOfBoundsException("Index: "+index);
         ListItr(index);
    }

    
     * 返回一个ListIterator
     public ListIterator<E> listIterator() {
        new ListItr(0
     * 返回一个Iterator
     public Iterator<E> iterator() {
         Itr();
    }

    
     * 优化版本的Iterator
     * todo 迭代器中modCount的作用
     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
         modCount;

        Itr() {}

         hasNext() {
            return cursor != size;
        }

        @SuppressWarnings("unchecked")
         E next() {
            checkForComodification();
            int i = cursor;
            if (i >= size)
                 NoSuchElementException();
            Object[] elementData = ArrayList..elementData;
             elementData.length)
                 ConcurrentModificationException();
            cursor = i + 1return (E) elementData[lastRet = i];
        }

         remove() {
            if (lastRet < 0 IllegalStateException();
            checkForComodification();

             {
                ArrayList..remove(lastRet);
                cursor = lastRet;
                lastRet = -1;
                expectedModCount = modCount;
            }  (IndexOutOfBoundsException ex) {
                 ConcurrentModificationException();
            }
        }

        @Override
        @SuppressWarnings("unchecked"void forEachRemaining(Consumer<? super E> consumer) {
            Objects.requireNonNull(consumer);
            int size = ArrayList..size;
            ;
            }
            final Object[] elementData = ArrayList. elementData.length) {
                 ConcurrentModificationException();
            }
            while (i != size && modCount == expectedModCount) {
                consumer.accept((E) elementData[i++]);
            }
             update once at end of iteration to reduce heap write traffic
            cursor = i;
            lastRet = i - 1;
            checkForComodification();
        }

         checkForComodification() {
             expectedModCount)
                
     * 优化版的ListIterator
     class ListItr extends Itr implements ListIterator<E> {
        ListItr( index) {
            ();
            cursor = index;
        }

         hasPrevIoUs() {
            return cursor != 0;
        }

         nextIndex() {
             cursor;
        }

         prevIoUsIndex() {
            return cursor - 1;
        }

        @SuppressWarnings("unchecked" E prevIoUs() {
            checkForComodification();
            int i = cursor - 1if (i < 0 ConcurrentModificationException();
            cursor = i;
             set(E e) {
            .set(lastRet,e);
            }  ConcurrentModificationException();
            }
        }

         add(E e) {
            checkForComodification();

             {
                 cursor;
                ArrayList..add(i,e);
                cursor = i + 1;
                lastRet = -1 ConcurrentModificationException();
            }
        }
    }

    
     * 获取从 fromIndex 到 toIndex 之间的子集合
     * 如果fromIndex == toIndex，则返回的空集合。对该子集合的操作，会影响原有集合。
     public List<E> subList( toIndex) {
        subListRangeCheck(fromIndex,1)">new SubList(this,toIndex);
    }

    
     * 检查传入索引的合法性
     void subListRangeCheck(int toIndex,1)"> size) {
        if (fromIndex < 0new IndexOutOfBoundsException("fromIndex = " + fromIndex);
        if (toIndex >new IndexOutOfBoundsException("toIndex = " + toIndex);
        if (fromIndex > toIndex)
            new IllegalArgumentException("fromIndex(" + fromIndex +
                    ") > toIndex(" + toIndex + ")"
     * 内部类——子列表
     class SubList extends AbstractList<E> implements RandomAccess {
        final AbstractList<E> parent;
         parentOffset;
         offset;
         size;

        SubList(AbstractList<E> parent,int offset,1)"> toIndex) {
            this.parent = parent;
            this.parentOffset = fromIndex;
            this.offset = offset +this.size = toIndex -this.modCount = ArrayList..modCount;
        }

        this.elementData(offset + index);
            ArrayList.this.elementData[offset + index] = e;
             oldValue;
        }

         index) {
            rangeCheck(index);
            checkForComodification();
            return ArrayList. index);
        }

         size() {
            checkForComodification();
            .size;
        }

        this.modCount = parent.modCount;
            this.size++ index) {
            rangeCheck(index);
            checkForComodification();
            E result = parent.remove(parentOffset + index);
            this.size-- result;
        }

         toIndex) {
            checkForComodification();
            parent.removeRange(parentOffset + fromIndex,parentOffset + toIndex);
            this.size -= toIndex - fromIndex;
        }

         c) {
            return addAll(.size,c);
        }

         c) {
            rangeCheckForAdd(index);
            int cSize = c.size();
            if (cSize==0;

            checkForComodification();
            parent.addAll(parentOffset +this.size += cSize;
             iterator() {
             listIterator();
        }

         index) {
            checkForComodification();
            rangeCheckForAdd(index);
            int offset = .offset;

            new ListIterator<E>() {
                int cursor = index;
                int lastRet = -1;
                int expectedModCount = ArrayList..modCount;

                 hasNext() {
                    return cursor != ArrayList.SubList..size;
                }

                @SuppressWarnings("unchecked" E next() {
                    checkForComodification();
                     cursor;
                    if (i >= ArrayList.SubList..size)
                         NoSuchElementException();
                    Object[] elementData = ArrayList..elementData;
                    if (offset + i >= elementData.length)
                         ConcurrentModificationException();
                    cursor = i + 1;
                    return (E) elementData[offset + (lastRet = i)];
                }

                 hasPrevIoUs() {
                    ;
                }

                @SuppressWarnings("unchecked" E prevIoUs() {
                    checkForComodification();
                    )
                         ConcurrentModificationException();
                    cursor = i;
                     i)];
                }

                @SuppressWarnings("unchecked" consumer) {
                    Objects.requireNonNull(consumer);
                    int size = ArrayList.SubList..size;
                     size) {
                        ;
                    }
                     elementData.length) {
                         ConcurrentModificationException();
                    }
                     expectedModCount) {
                        consumer.accept((E) elementData[offset + (i++)]);
                    }
                     update once at end of iteration to reduce heap write traffic
                    lastRet = cursor = i;
                    checkForComodification();
                }

                 nextIndex() {
                     cursor;
                }

                 prevIoUsIndex() {
                    ;
                }

                 remove() {
                     IllegalStateException();
                    checkForComodification();

                     {
                        ArrayList.SubList..remove(lastRet);
                        cursor = lastRet;
                        lastRet = -1;
                        expectedModCount = ArrayList..modCount;
                    }  (IndexOutOfBoundsException ex) {
                         ConcurrentModificationException();
                    }
                }

                 set(E e) {
                     {
                        ArrayList.this.set(offset + lastRet,e);
                    }  add(E e) {
                    checkForComodification();

                     {
                         cursor;
                        ArrayList.SubList.;
                        lastRet = -1 checkForComodification() {
                    if (expectedModCount != ArrayList..modCount)
                         ConcurrentModificationException();
                }
            };
        }

         toIndex) {
            subListRangeCheck(fromIndex,size);
            new ArrayList.SubList(if (index < 0 || index >= .size)
                 IndexOutOfBoundsException(outOfBoundsMsg(index));
        }

        if (index < 0 || index > if (ArrayList.this.modCount != .modCount)
                 ConcurrentModificationException();
        }

        public Spliterator<E> spliterator() {
            checkForComodification();
            new ArrayList.ArrayListSpliterator<E>(ArrayList.this.size,1)">.modCount);
        }
    }

    
     * 遍历
     
    @Override
    void forEach(Consumer<?  action) {
        Objects.requireNonNull(action);
         modCount;
        @SuppressWarnings("unchecked"final E[] elementData = (E[]) int size = .size;
        int i=0; modCount == expectedModCount && i < size; i++) {
            action.accept(elementData[i]);
        }
        
     * 创建一个分割器
      spliterator() {
        new ArrayList.ArrayListSpliterator<>( 基于索引的、二分的、懒加载的分割器class ArrayListSpliterator<E> implements Spliterator<E> {

        用于存放ArrayList对象
        final ArrayList<E> list;
        起始位置（包含），advance/split操作时会修改
        结束位置（不包含），-1 表示到最后一个元素
         fence;
        用于存放list的modCount，当fence被设值后初始化
         expectedModCount;

         创建一个范围性的分割器 
        ArrayListSpliterator(ArrayList<E> list,1)">int origin,1)"> fence,1)">this.list = list;  OK if null unless traversed
            this.index = origin;
            this.fence = fence;
            this.expectedModCount = expectedModCount;
        }

        在第一次使用时实例化结束位置
         getFence() {
             hi;
            ArrayList<E> lst;
            if ((hi = fence) < 0) {
                if ((lst = list) == )
                    hi = fence = 0 {
                    expectedModCount = lst.modCount;
                    hi = fence = lst.size;
                }
            }
             hi;
        }

        
         * 分割list，返回一个新分割出的spliterator实例，相当于二分法，这个方法会递归
         * 1.ArrayListSpliterator本质上还是对原list进行操作，只是通过index和fence来控制每次处理范围
         * 2.ArrayListSpliterator在遍历元素时，不能对list进行结构变更操作，否则抛错。
         */
        public ArrayList.ArrayListSpliterator<E> trySplit() {
            int hi = getFence(),lo = index,mid = (lo + hi) >>> 1return (lo >= mid) ? null :  divide range in half unless too small
                    new ArrayList.ArrayListSpliterator<E>(list,lo,index = mid,expectedModCount);
        }

        
         * 返回true 时，只表示可能还有元素未处理
         * 返回false 时，没有剩余元素需要处理
         boolean tryAdvance(Consumer<?  action) {
            if (action ==  NullPointerException();
             index;
            if (i < hi) {
                index = i + 1;
                @SuppressWarnings("unchecked") E e = (E)list.elementData[i];
                action.accept(e);
                if (list.modCount != expectedModCount)
                     ConcurrentModificationException();
                
         * 顺序遍历处理所有剩下的元素
         int i,hi,mc;  hoist accesses and checks from loop
            ArrayList<E> lst; Object[] a;
            if ((lst = list) != null && (a = lst.elementData) != ) {
                    mc = lst.modCount;
                    hi = lst.size;
                }
                
                    mc = expectedModCount;
                if ((i = index) >= 0 && (index = hi) <= a.length) {
                    for (; i < hi; ++i) {
                        @SuppressWarnings("unchecked") E e = (E) a[i];
                        action.accept(e);
                    }
                    if (lst.modCount == mc)
                        ;
                }
            }
            
         * 估算大小
         long estimateSize() {
            return (long) (getFence() -
         * 获取特征值
          characteristics() {
            return Spliterator.ORDERED | Spliterator.SIZED | Spliterator.SUBSIZED;
        }
    }

    
     * 条件过滤
     boolean removeIf(Predicate<?  filter) {
        Objects.requireNonNull(filter);
         figure out which elements are to be removed
         any exception thrown from the filter predicate at this stage
         will leave the collection unmodified
        int removeCount = 0final BitSet removeSet =  BitSet(size);
         modCount;
        ) {
            @SuppressWarnings("unchecked"final E element = (E) elementData[i];
             (filter.test(element)) {
                removeSet.set(i);
                removeCount++ shift surviving elements left over the spaces left by removed elements
        boolean anyToRemove = removeCount > 0 (anyToRemove) {
            int newSize = size - removeCount;
            int i=0,j=0; (i < size) && (j < newSize); i++,j++) {
                i = removeSet.nextClearBit(i);
                elementData[j] = elementData[i];
            }
            int k=newSize; k < size; k++) {
                elementData[k] = null;   Let gc do its work
            }
            this.size = newSize;
             expectedModCount) {
                 ConcurrentModificationException();
            }
            modCount++ anyToRemove;
    }

    
     * 全部替换
     
    @Override
    @SuppressWarnings("unchecked"void replaceAll(UnaryOperator<E> operator) {
        Objects.requireNonNull(operator);
        ) {
            elementData[i] = operator.apply((E) elementData[i]);
        }
         ConcurrentModificationException();
        }
        modCount++
     * 用外部比较器来排序
     void sort(Comparator<?  modCount;
        Arrays.sort((E[]) elementData,c);
        ;
    }
}