ArrayList源碼及解析

package java.util;

import java.util.function.Consumer;
import java.util.function.Predicate;
import java.util.function.UnaryOperator;

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

    //默認容量
    private static final int DEFAULT_CAPACITY = 10;

    //靜態的一個屬性，全部實例共享屬性，當初始化容量爲0的時候，就使用這個屬性做爲實例底層數組
    private static final Object[] EMPTY_ELEMENTDATA = {};

    /*根據註釋，這個大概意思就是構造一個空的對象數組，用來與EMPTY_ELEMENTDATA 這個數組進行對比
    來肯定當第一次向ArrayList中添加數據時，應該若是進行擴容，就是增長多大的容量。*/
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

    //實際上真正保存數據的數組，今後出能夠看出ArrayList使用Object數組來保存數據
    transient Object[] elementData; // non-private to simplify nested class access

    //實際包含元素的個數
    private int size;

    /*
    傳遞一個初始化容量的構造函數，會判斷傳遞的參數與0的關係
    若是大於0，會在ArrayList內部構建一個長度爲initalCapacity的數組
    若是等於0，會將上述的靜態EMPTY_ELEMENTDATA屬性賦值給elementData，也不會產生新的數組。若是小於0，則拋出異常
     */
    public ArrayList(int initialCapacity) {
        if (initialCapacity > 0) {
            this.elementData = new Object[initialCapacity];//注意此處並無將initialCapacity賦值給size
        } else if (initialCapacity == 0) {
            this.elementData = EMPTY_ELEMENTDATA;
        } else {
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        }
    }

    /*
    無參的構造函數，在該構造函數中，會將上述的靜態的DEFAULTCAPACITY_EMPTY_ELEMENTDATA屬性，賦值給elementData屬性
    也即咱們用這種方法構造ArraList的時候，並不會真正產生實例化的數組，而是引用一個靜態的空數組
     */
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }

    /*
    傳遞一個集合給ArrayList，它首先會將集合轉換成數組賦值給elementData
    以後判斷數組長度，若是等於0，則將elementData賦值爲EMPTY_ELEMENTDATA
    若是不等於0，還須要判斷接受過來的數組(如今是elementData)是不是Object[]類型的
    若是不是的化，將它轉換成Object[]類型(根據註釋，toArray方法有可能獲得的不是Object[]類型)
     */
    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[].class);
        } else {
            // replace with empty array.
            this.elementData = EMPTY_ELEMENTDATA;
        }
    }

    /*
    本質上是將數組的尾部刪除掉造成新數組
    新數組的length與size一致，節約空間
     */
    public void trimToSize() {
        modCount++;
        if (size < elementData.length) {
            elementData = (size == 0)
              ? EMPTY_ELEMENTDATA
              : Arrays.copyOf(elementData, size);
        }
    }

    /*
    增長這個ArrayList實例的能力，若是有必要，以確保它至少能容納的最小容量參數指定元素個數。
    提供給外界的方法，是的使用者能夠經過這個方法本身去擴容
     */
    public void ensureCapacity(int minCapacity) {
        int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
            // any size if not default element table
            ? 0
            // larger than default for default empty table. It's already
            // supposed to be at default size.
            : DEFAULT_CAPACITY;//elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA意味着elementData可能不是一個length爲0的數組

        if (minCapacity > minExpand) {
            ensureExplicitCapacity(minCapacity);
        }
    }

    /*
    一個私有方法，確保minCapacity在容量範圍內
    若是elementData等於DEFAULTCAPACITY_EMPTY_ELEMENTDATA，則minCapacity會取DEFAULT_CAPACITY, minCapacity中比較大的那個
    也即若是minCapacity小於10，則取10，若是大於10，則去minCapacity
    隨後要執行ensureExplicitCapacity方法
     */
    private void ensureCapacityInternal(int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }

        ensureExplicitCapacity(minCapacity);
    }

    /*
    ensureExplicitCapacity要接受一個int類型的參數，意味着最少須要容量爲minCapacity
    首先會對modCount+1,modCount是AbstractList類中的一個成員變量,該值表示對List的修改次數，主要是爲了服務快速失敗功能的
    隨後若是minCapacity要大於現有數組elementData的長度的化，那麼就執行grow方法，grow是擴容的方法
     */
    private void ensureExplicitCapacity(int minCapacity) {
        modCount++;

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


    /*
    數組所能開闢的最大長度
    由於有些虛擬機保留了一些header words在數組中
    嘗試要開闢更大的長度的數組，可能會出現OOM異常(在一些虛擬機實現中)
     */
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

    /*
    ArrayList的擴容，接收一個int類型參數，表示至少須要多少容量
     */
    private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;//獲得目前的容量
        //oldCapacity>>1表示除2取整數，該式子最終表示意思爲newCapacity大於爲oldCapacity的1.5倍數
        int newCapacity = oldCapacity + (oldCapacity >> 1);
           //判斷newCapacity是否溢出
        if (newCapacity - minCapacity < 0)
            //溢出：newCapacity等於minCapacity
            newCapacity = minCapacity;
        //判斷newCapacity是否超過了MAX_ARRAY_SIZE，超過了，則計算最大容量；具體緣由是由於不一樣虛擬機的實現不一樣
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        //執行Arrays.copyOf方法，傳遞原數組與新數組長度，由Arrays內部建立數組返回並接受給elementData
        elementData = Arrays.copyOf(elementData, newCapacity);
    }

    /*
    求出最大的容量值，首先判斷minCapacity是否已經溢出了，溢出了就直接拋出OOM
    不然就去判斷minCapacity 是否大於 MAX_ARRAY_SIZE 
       大於返回 Integer.MAX_VALUE ，不大於 返回MAX_ARRAY_SIZE
     */
    private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) // overflow
            throw new OutOfMemoryError();
        return (minCapacity > MAX_ARRAY_SIZE) ?
            Integer.MAX_VALUE :
            MAX_ARRAY_SIZE;
    }

    //獲得size，size是真正的保存的元素的數量
    public int size() {
        return size;
    }

    //判斷容器是否爲空(指是不包含元素)
    public boolean isEmpty() {
        return size == 0;
    }

    //判斷容器是否包含某個元素
    public boolean contains(Object o) {
        return indexOf(o) >= 0;
    }

    //indexOf是來得到o元素(包括null)在容器中的位置的,位置從0開始到size-1結束,若是返回-1表示不包含
    //對於重複的元素，只獲取第一個所在的位置
    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;
    }

    //與indexOf功能同樣，可是確實得到重複元素的最後一個位置
    public int lastIndexOf(Object o) {
        if (o == null) {
            for (int i = size-1; i >= 0; i--)
                if (elementData[i]==null)
                    return i;
        } else {
            for (int i = size-1; i >= 0; i--)
                if (o.equals(elementData[i]))
                    return i;
        }
        return -1;
    }

    //重寫了Object中的clone方法，用於賦值容器，淺複製
    public Object clone() {
        try {
            ArrayList<?> v = (ArrayList<?>) super.clone();
            v.elementData = Arrays.copyOf(elementData, size);
            v.modCount = 0;
            return v;
        } catch (CloneNotSupportedException e) {//看來大神也感受異常很差處理了...不可能發生異常的地方，卻仍是要處理...
            // this shouldn't happen, since we are Cloneable
            throw new InternalError(e);
        }
    }

    //獲得數組的副本
    public Object[] toArray() {
        return Arrays.copyOf(elementData, size);
    }

    /*
    給定一個指定數組，返回指定數組大小，類型的副本
     */
    @SuppressWarnings("unchecked")
    public <T> T[] toArray(T[] a) {
        if (a.length < size)
            // Make a new array of a's runtime type, but my contents:
            return (T[]) Arrays.copyOf(elementData, size, a.getClass());
        System.arraycopy(elementData, 0, a, 0, size);//此處是size == a.length
        if (a.length > size)
            a[size] = null;//若是a.length>size，則截取size的長度，可是若是a自己就是有數據的，可能會出現a[size+?]有數據，而a[size]爲null
        return a;
    }

    // Positional Access Operations
    //不須要檢查index的快速訪問元素，可是是包權限，只容許內部使用
    @SuppressWarnings("unchecked")
    E elementData(int index) {
        return (E) elementData[index];
    }

    /*
    判斷一下是否index是否越界
    而後經過快速訪問來返回元素
     */
    public E get(int index) {
        rangeCheck(index);

        return elementData(index);
    }

    /*
    判斷一下是否越界
    而後獲得處於index位置的原元素，隨後將index位置置入新元素
    返回原來的元素
    要求 index<size
     */
    public E set(int index, E element) {
        rangeCheck(index);

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

    /*
    集合中新增一個元素，首先要確保在承受能力範圍內
    以後將新加入進來的元素賦值到數組的第size的位置上
    隨後size+1
    新增的元素，插入到數組的末尾
     */
    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }

    /*
    插入一個元素element到指定index位置，原位置的元素依次向後移動一位
    改方法效率要低一些，若是並非特定必需要塞入哪一個位置的話，最好不要用
     */
    public void add(int index, E element) {
        //首先會去檢查一下index是否可使用
        rangeCheckForAdd(index);
        //確保數組可容納
        ensureCapacityInternal(size + 1);  // Increments modCount!! 會修改modCount的值，modCount+1
        //隨後調用System.arraycopy方法，將elementData的index位置元素依次向後移動，爲接下來的插入預留空間
        System.arraycopy(elementData, index, elementData, index + 1,
                         size - index);
        elementData[index] = element;//真正的插入操做
        size++;//size+1
    }

    /*
    刪除指定位置的元素，若是index>size的話，會出現數組越界
     */
    public E remove(int index) {
        rangeCheck(index);//index>size throw IndexOutOfBoundsException

        modCount++;
        E oldValue = elementData(index);//獲得原來elementData中的元素

        int numMoved = size - index - 1;//計算刪除以後須要移動元素的數量
        if (numMoved > 0)//移動元素
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);//移動的時候，就會覆蓋原來的元素
        //清除最後一個元素的引用，由於原來的元素以及被刪除了
        elementData[--size] = null; // clear to let GC do its work

        return oldValue;//返回被刪除的元素
    }

    /*
    刪除某一個元素，傳入要被刪除的元素
     */
    public boolean remove(Object o) {
        if (o == null) {//刪除null元素
            for (int index = 0; index < size; index++)//迭代ArrayList
                if (elementData[index] == null) {//若是在size以前的位置有存在空元素
                    fastRemove(index);//則快速刪除(所謂快速刪除，就是不去作越界檢查以及不返回結果，徹底給本類本身使用的private方法)
                    return true;
                }
        } else {//刪除非空元素，與刪除null元素邏輯相同
            for (int index = 0; index < size; index++)
                if (o.equals(elementData[index])) {//此處使用equals方法來進行比較，因此在使用remove(Object o)的時候，要考慮是否重寫了equals方法
                    fastRemove(index);//fastRemove也是會移動數組的，若是有刪除重複元素的時候，效率很低
                    return true;
                }
        }
        return false;
    }

    /*
    快速刪除
    不作index檢查，只容許內部使用
     */
    private void fastRemove(int index) {
        modCount++;
        int numMoved = size - index - 1;
        if (numMoved > 0)
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
        elementData[--size] = null; // clear to let GC do its work
    }

    /*
    清除數組，全部元素置爲null
     */
    public void clear() {
        modCount++;

        // clear to let GC do its work
        for (int i = 0; i < size; i++)
            elementData[i] = null;

        size = 0;
    }

    /*
    添加一次性add多個元素，接受參數爲集合類型
     */
    public boolean addAll(Collection<? extends E> c) {
        Object[] a = c.toArray();
        int numNew = a.length;//可能會產生空指針錯誤
        ensureCapacityInternal(size + numNew);  // Increments modCount
        //將a數組插入到elementData的size位置
        System.arraycopy(a, 0, elementData, size, numNew);
        size += numNew;
        return numNew != 0;
    }

    /*
    指定index位置插入多個元素，原來位置的元素依次向後移動
    index不能大於size,若是大於size會產生數組越界
     */
    public boolean addAll(int index, Collection<? extends E> c) {
        rangeCheckForAdd(index);

        Object[] a = c.toArray();
        int numNew = a.length;//可能會產生空指針錯誤
        ensureCapacityInternal(size + numNew);  // Increments modCount

        int numMoved = size - index;
        if (numMoved > 0)
            System.arraycopy(elementData, index, elementData, index + numNew,
                             numMoved);

        System.arraycopy(a, 0, elementData, index, numNew);
        size += numNew;
        return numNew != 0;
    }

    /*
    範圍刪除，刪除從fromIndex~toIndex,包含fromIndex，不包含toIndex
     */
    protected void removeRange(int fromIndex, int toIndex) {
        modCount++;
        int numMoved = size - toIndex;
        System.arraycopy(elementData, toIndex, elementData, fromIndex,
                         numMoved);

        // clear to let GC do its work
        int newSize = size - (toIndex-fromIndex);
        for (int i = newSize; i < size; i++) {
            elementData[i] = null;
        }
        size = newSize;
    }

    //index檢查判斷，專門封裝起來是由於不少地方使用
    private void rangeCheck(int index) {
        if (index >= size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    //專門爲add方法封裝的rangeCheck方法
    private void rangeCheckForAdd(int index) {
        if (index > size || index < 0)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    //爲IndexOutOfBoundsException提供信息的方法，告訴哪一個位置出現了數組越界
    private String outOfBoundsMsg(int index) {
        return "Index: "+index+", Size: "+size;
    }

    //一次性刪除多個元素
    public boolean removeAll(Collection<?> c) {
        Objects.requireNonNull(c);//判斷c是否爲空，爲空拋出異常
        return batchRemove(c, false);//批量刪除
    }

    //保留當前容器與c的並集，並返回
    public boolean retainAll(Collection<?> c) {
        Objects.requireNonNull(c);
        return batchRemove(c, true);
    }

    //批量刪除方法，complement爲true表示求交集，若是爲false表示在elementData中保留原有的非c的集合
    //也即true: a屬於elementData同時a屬於c; false: a屬於elementData同時a不屬於c
    private boolean batchRemove(Collection<?> c, boolean complement) {
        final Object[] elementData = this.elementData;
        int r = 0, w = 0;//一個讀的index,一個是寫的index
        boolean modified = false;
        try {
            for (; r < size; r++)
                if (c.contains(elementData[r]) == complement)
                    elementData[w++] = elementData[r];
        } finally {
            // Preserve behavioral compatibility with AbstractCollection,
            // even if c.contains() throws.
            if (r != size) {//只移動一次數組，比單獨remove效果要好
                System.arraycopy(elementData, r,
                                 elementData, w,
                                 size - r);
                w += size - r;
            }
            if (w != size) {//清理數組中不須要的引用
                // clear to let GC do its work
                for (int i = w; i < size; i++)
                    elementData[i] = null;
                modCount += size - w;//記錄修改次數
                size = w;//從新定義size
                modified = true;
            }
        }
        return modified;
    }

    //保存數組實例的狀態到一個流（即它序列化）
    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);

        // Write out all elements in the proper order.
        for (int i=0; i<size; i++) {
            s.writeObject(elementData[i]);
        }

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

    //從一個流中讀出數組實例的狀態
    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(); // ignored

        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();
            }
        }
    }

    //返回一個list迭代器，鏈表迭代器，能夠雙向迭代，而且還具備add方法，可是隻有在list類型中才可使用，別的集合類沒有
    //接受一個Index，肯定迭代器初始的位置
    public ListIterator<E> listIterator(int index) {
        if (index < 0 || index > size)//先判斷index是否合法
            throw new IndexOutOfBoundsException("Index: "+index);
        return new ListItr(index);
    }

    /**
     * Returns a list iterator over the elements in this list (in proper
     * sequence).
     *
     * <p>The returned list iterator is <a href="#fail-fast"><i>fail-fast</i></a>.
     *
     * @see #listIterator(int)
     */
    public ListIterator<E> listIterator() {
        return new ListItr(0);
    }

    /**
     * Returns an iterator over the elements in this list in proper sequence.
     *
     * <p>The returned iterator is <a href="#fail-fast"><i>fail-fast</i></a>.
     *
     * @return an iterator over the elements in this list in proper sequence
     */
    public Iterator<E> iterator() {
        return new Itr();
    }

    /**
     * An optimized version of AbstractList.Itr
     * AbstractList.Itr的優化版本迭代器
     */
    private class Itr implements Iterator<E> {
        int cursor;       // 下一個要被返回元素的下標
        int lastRet = -1; // 上一個被返回的元素的下標，若是沒有的話默認爲-1
        int expectedModCount = modCount;

        //判斷是否還有下一個元素
        public boolean hasNext() {
            return cursor != size;
        }

        //返回下一個元素，默認一開始的next是第一個元素
        @SuppressWarnings("unchecked")
        public E next() {
            checkForComodification();//快速失敗
            int i = cursor;
            if (i >= size)//會判斷一次位置是否合法，由於cursor只是盲目的+1
                throw new NoSuchElementException();
            Object[] elementData = ArrayList.this.elementData;
            if (i >= elementData.length)
                throw new ConcurrentModificationException();
            cursor = i + 1;//cursor設置爲下一個要被返回的元素下標
            return (E) elementData[lastRet = i];//將lastRet設置爲被返回的元素下標
        }

        //刪除上一個元素，也即最近被next()出來的元素
        public void remove() {
            if (lastRet < 0)
                throw new IllegalStateException();
            checkForComodification();

            try {
                ArrayList.this.remove(lastRet);//刪除的是下標爲lastRet元素
                cursor = lastRet;//回退
                lastRet = -1;//設置成爲-1，也即不能連續的刪除，該類不可以往回走，只能繼續前進，由於繼續刪除，會拋出IllegalStateException異常
                expectedModCount = modCount;
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }

        /*
        遍歷餘下的元素
         */
        @Override
        @SuppressWarnings("unchecked")
        public void forEachRemaining(Consumer<? super E> consumer) {
            Objects.requireNonNull(consumer);//判斷consumer不能爲null
            final int size = ArrayList.this.size;
            int i = cursor;//餘下的體如今這..
            if (i >= size) {
                return;
            }
            final Object[] elementData = ArrayList.this.elementData;
            if (i >= elementData.length) {
                throw new ConcurrentModificationException();
            }
            while (i != size && modCount == expectedModCount) {
                consumer.accept((E) elementData[i++]);//此處接受elementData元素，執行consumer中的方法，可能會去改變elementData元素
            }
            // update once at end of iteration to reduce heap write traffic
            cursor = i;
            lastRet = i - 1;
            checkForComodification();
        }

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

    /**
     * An optimized version of AbstractList.ListItr
     * 一個對AbstractList.ListItr的優化版本鏈表迭代器
     */
    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;
        }

        //返回上一個元素
        @SuppressWarnings("unchecked")
        public E previous() {
            checkForComodification();
            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];
        }

        //更新上一個位置的元素，將其置換成e
        public void set(E e) {
            if (lastRet < 0)
                throw new IllegalStateException();
            checkForComodification();

            try {
                ArrayList.this.set(lastRet, e);
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }

        //新增一個元素，處在上一個元素以後，下一個元素以前，會移動數組
        public void add(E e) {
            checkForComodification();

            try {
                int i = cursor;
                ArrayList.this.add(i, e);
                cursor = i + 1;
                lastRet = -1;
                expectedModCount = modCount;
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }
    }

    //獲得子列表 從fromIndex~toIndex位置
    public List<E> subList(int fromIndex, int toIndex) {
        subListRangeCheck(fromIndex, toIndex, size);
        return new SubList(this, 0, fromIndex, toIndex);
    }

    //判斷Index是否合法
    static void subListRangeCheck(int fromIndex, int toIndex, int size) {
        if (fromIndex < 0)
            throw new IndexOutOfBoundsException("fromIndex = " + fromIndex);
        if (toIndex > size)
            throw new IndexOutOfBoundsException("toIndex = " + toIndex);
        if (fromIndex > toIndex)
            throw new IllegalArgumentException("fromIndex(" + fromIndex +
                                               ") > toIndex(" + toIndex + ")");
    }

    //繼承與AbstractList的SubList類，其實這個類，只是去封裝了幾個屬性，實際上用的仍是原來ArrayList類的數組，外觀模式
    private class SubList extends AbstractList<E> implements RandomAccess {
        private final AbstractList<E> parent;
        private final int parentOffset;
        private final int offset;
        int size; 

        //參數：
        //parent 父類型
        //offset 父類型的偏移量
        //fromIndex 子列表的開始元素，位於父列表的位置
        //toIndex 子列表的結束元素，位於父列表的位置
        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;
        }

        public E get(int index) {
            rangeCheck(index);
            checkForComodification();
            return ArrayList.this.elementData(offset + index);
        }

        public int size() {
            checkForComodification();
            return this.size;
        }

        public void add(int index, E e) {
            rangeCheckForAdd(index);
            checkForComodification();
            parent.add(parentOffset + index, e);
            this.modCount = parent.modCount;
            this.size++;
        }

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

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

        public boolean addAll(Collection<? extends E> c) {
            return addAll(this.size, c);
        }

        public boolean addAll(int index, Collection<? extends E> c) {
            rangeCheckForAdd(index);
            int cSize = c.size();
            if (cSize==0)
                return false;

            checkForComodification();
            parent.addAll(parentOffset + index, c);
            this.modCount = parent.modCount;
            this.size += cSize;
            return true;
        }

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

        public ListIterator<E> listIterator(final int index) {
            checkForComodification();
            rangeCheckForAdd(index);
            final int offset = this.offset;

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

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

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

                public boolean hasPrevious() {
                    return cursor != 0;
                }

                @SuppressWarnings("unchecked")
                public E previous() {
                    checkForComodification();
                    int i = cursor - 1;
                    if (i < 0)
                        throw new NoSuchElementException();
                    Object[] elementData = ArrayList.this.elementData;
                    if (offset + i >= elementData.length)
                        throw new ConcurrentModificationException();
                    cursor = i;
                    return (E) elementData[offset + (lastRet = i)];
                }

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

                public int nextIndex() {
                    return cursor;
                }

                public int previousIndex() {
                    return cursor - 1;
                }

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

                    try {
                        SubList.this.remove(lastRet);
                        cursor = lastRet;
                        lastRet = -1;
                        expectedModCount = ArrayList.this.modCount;
                    } catch (IndexOutOfBoundsException ex) {
                        throw new ConcurrentModificationException();
                    }
                }

                public void set(E e) {
                    if (lastRet < 0)
                        throw new IllegalStateException();
                    checkForComodification();

                    try {
                        ArrayList.this.set(offset + lastRet, e);
                    } catch (IndexOutOfBoundsException ex) {
                        throw new ConcurrentModificationException();
                    }
                }

                public void add(E e) {
                    checkForComodification();

                    try {
                        int i = cursor;
                        SubList.this.add(i, e);
                        cursor = i + 1;
                        lastRet = -1;
                        expectedModCount = ArrayList.this.modCount;
                    } catch (IndexOutOfBoundsException ex) {
                        throw new ConcurrentModificationException();
                    }
                }

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

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

        private void rangeCheck(int index) {
            if (index < 0 || index >= this.size)
                throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
        }

        private void rangeCheckForAdd(int index) {
            if (index < 0 || index > this.size)
                throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
        }

        private String outOfBoundsMsg(int index) {
            return "Index: "+index+", Size: "+this.size;
        }

        private void checkForComodification() {
            if (ArrayList.this.modCount != this.modCount)
                throw new ConcurrentModificationException();
        }

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

    //與forEachRemaining很像，一個是迭代全部，一個是迭代剩餘，都會去執行Consumer中定義的方法，可能會改變元素的值
    @Override
    public void forEach(Consumer<? super E> action) {
        Objects.requireNonNull(action);
        final int expectedModCount = modCount;
        @SuppressWarnings("unchecked")
        final E[] elementData = (E[]) this.elementData;
        final int size = this.size;
        for (int i=0; modCount == expectedModCount && i < size; i++) {
            action.accept(elementData[i]);
        }
        if (modCount != expectedModCount) {
            throw new ConcurrentModificationException();
        }
    }

    //返回spliterator,用於並行計算中，splitable iterator可分割迭代器
    @Override
    public Spliterator<E> spliterator() {
        return new ArrayListSpliterator<>(this, 0, -1, 0);
    }

    static final class ArrayListSpliterator<E> implements Spliterator<E> {

        private final ArrayList<E> list;//原數組
        private int index; // current index, modified on advance/split
        private int fence; // -1 until used; then one past last index
        private int expectedModCount; // initialized when fence set

        /** Create new spliterator covering the given  range */
        ArrayListSpliterator(ArrayList<E> list, int origin, int fence,
                             int expectedModCount) {
            this.list = list; // OK if null unless traversed
            this.index = origin;
            this.fence = fence;
            this.expectedModCount = expectedModCount;
        }

        private int getFence() { // 第一次使用時，初始化fence大小
            int hi; // (a specialized variant appears in method forEach)
            ArrayList<E> lst;
            if ((hi = fence) < 0) { //-1表示初始化的值
                if ((lst = list) == null)
                    hi = fence = 0;
                else {
                    expectedModCount = lst.modCount;
                    hi = fence = lst.size;
                }
            }
            return hi;
        }

        //這就是爲Spliterator專門設計的方法，區分與普通的Iterator，該方法會把當前元素劃分一部分出去建立一個新的Spliterator做爲返回，
        //兩個Spliterator變會並行執行，若是元素個數小到沒法劃分則返回null
        public ArrayListSpliterator<E> trySplit() {
            int hi = getFence(), lo = index, mid = (lo + hi) >>> 1;//因爲lo + hi都是整數，>>>至關於除2
            return (lo >= mid) ? null : // divide range in half unless too small
                new ArrayListSpliterator<E>(list, lo, index = mid,//注意index=min
                                            expectedModCount);
        }

        //tryAdvance就是順序處理每一個元素，相似Iterator，若是還有元素要處理，則返回true，不然返回false
        public boolean tryAdvance(Consumer<? super E> action) {
            if (action == null)
                throw new NullPointerException();
            int hi = getFence(), i = index;
            if (i < hi) {
                index = i + 1;
                @SuppressWarnings("unchecked") E e = (E)list.elementData[i];
                action.accept(e);
                if (list.modCount != expectedModCount)
                    throw new ConcurrentModificationException();
                return true;
            }
            return false;
        }

        public void forEachRemaining(Consumer<? super E> action) {
            int i, hi, mc; // hoist accesses and checks from loop
            ArrayList<E> lst; Object[] a;
            if (action == null)
                throw new NullPointerException();
            if ((lst = list) != null && (a = lst.elementData) != null) {
                if ((hi = fence) < 0) {
                    mc = lst.modCount;
                    hi = lst.size;
                }
                else
                    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)
                        return;
                }
            }
            throw new ConcurrentModificationException();
        }

        //該方法用於估算還剩下多少個元素須要遍歷
        public long estimateSize() {
            return (long) (getFence() - index);
        }

        //其實就是表示該Spliterator有哪些特性，用於能夠更好控制和優化Spliterator的使用
        public int characteristics() {
            return Spliterator.ORDERED | Spliterator.SIZED | Spliterator.SUBSIZED;
        }
    }

    //刪除，增長過濾功能
    @Override
    public boolean removeIf(Predicate<? super E> filter) {
        Objects.requireNonNull(filter);//判斷過濾器是否爲空
        // 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 = 0;//要刪除元素的個數
        final BitSet removeSet = new BitSet(size);//使用BitSet類來保存要被刪除的Set，BitSet是使用位圖來保存數據，節省很大內存
        final int expectedModCount = modCount;//預期的modCount
        final int size = this.size;
        for (int i=0; modCount == expectedModCount && i < size; i++) {
            @SuppressWarnings("unchecked")
            final E element = (E) elementData[i];
            if (filter.test(element)) {//若是element匹配filter中的過濾條件的話，則會返回true
                removeSet.set(i);//使用位圖來保存要被刪除的index
                removeCount++;
            }
        }
        if (modCount != expectedModCount) {//快速失敗機制，在多線程狀況下，去報錯，引發程序員注意
            throw new ConcurrentModificationException();
        }

        // shift surviving elements left over the spaces left by removed elements
        final boolean anyToRemove = removeCount > 0;//用於記錄是否須要刪除
        if (anyToRemove) {
            //須要刪除...
            final int newSize = size - removeCount;//計算剩餘的長度，也即新數組的長度
            for (int i=0, j=0; (i < size) && (j < newSize); i++, j++) {//(i < size) && (j < newSize)會節約一些效率
                i = removeSet.nextClearBit(i);//獲得沒有被攔截的index
                elementData[j] = elementData[i];
            }
            for (int k=newSize; k < size; k++) {//清除數組後面的多餘引用，GC
                elementData[k] = null;  // Let gc do its work
            }
            this.size = newSize;
            if (modCount != expectedModCount) {
                throw new ConcurrentModificationException();
            }
            modCount++;//用於記錄本條數據也改變了數組結構，從這個地方能夠看出來，快速失敗機制並不能徹底確保必定會提醒到程序員，只是有可能
        }

        return anyToRemove;
    }

    //替換全部
    @Override
    @SuppressWarnings("unchecked")
    public void replaceAll(UnaryOperator<E> operator) {
        Objects.requireNonNull(operator);
        final int expectedModCount = modCount;
        final int size = this.size;
        for (int i=0; modCount == expectedModCount && i < size; i++) {
            elementData[i] = operator.apply((E) elementData[i]);
        }
        if (modCount != expectedModCount) {
            throw new ConcurrentModificationException();
        }
        modCount++;
    }

    @Override
    @SuppressWarnings("unchecked")
    public void sort(Comparator<? super E> c) {
        final int expectedModCount = modCount;
        Arrays.sort((E[]) elementData, 0, size, c);
        if (modCount != expectedModCount) {
            throw new ConcurrentModificationException();
        }
        modCount++;
    }
}