public static void main(String[] args) {
        // 實(shí)例化一個(gè)初始容量為5的 ArrayList 集合
        List list = new ArrayList<String>(6);
        // 向指定索引位置插入數(shù)據(jù)
        list.add(1, "hello");// 代碼行號(hào)：17
        // 獲取指定索引位置的數(shù)據(jù)
        System.out.println(list.get(1));
    }

List list = new ArrayList<String>(5);

	/**
     * The array buffer into which the elements of the ArrayList are stored.
     * The capacity of the ArrayList is the length of this array buffer. Any
     * empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
     * will be expanded to DEFAULT_CAPACITY when the first element is added.
     */
    transient Object[] elementData; // non-private to simplify nested class access
    /**
     * Shared empty array instance used for empty instances.
     */
    private static final Object[] EMPTY_ELEMENTDATA = {};

    /**
     * Constructs an empty list with the specified initial capacity.
     * 根據(jù)指定的初始化容量構(gòu)造一個(gè)空的 list 集合
     * @param  initialCapacity  the initial capacity of the list 初始化的容量
     * @throws IllegalArgumentException if the specified initial capacity
     *         is negative 如果指定的容量為負(fù)數(shù)則拋出異常
     */
    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);
        }
    }

    /**
     * Inserts the specified element at the specified position in this
     * list. Shifts the element currently at that position (if any) and
     * any subsequent elements to the right (adds one to their indices).
     * 向 ArrayList 中指定的位置插入指定的元素，如果當(dāng)前位置已經(jīng)有元素，則會(huì)將該位置之后的所有元素統(tǒng)一往后移一位。
     * @param index index at which the specified element is to be inserted 待插入的索引位置
     * @param element element to be inserted 待插入的元素
     * @throws IndexOutOfBoundsException {@inheritDoc} 下標(biāo)越界異常
     */
    public void add(int index, E element) {
        rangeCheckForAdd(index); // 越界檢查

        ensureCapacityInternal(size + 1);  // Increments modCount!! 是否擴(kuò)容的判斷
        System.arraycopy(elementData, index, elementData, index + 1,
                         size - index); // 數(shù)組拷貝
        elementData[index] = element; // 將待添加的元素放入指定的位置
        size++; // 集合的實(shí)際大小累加
    }
     /**
     * The size of the ArrayList (the number of elements it contains).
     *  ArrayList 的成員變量，保存了已有元素的數(shù)量
     * @serial
     */
    private int size;
    
    /**
     * A version of rangeCheck used by add and addAll.
     */
    private void rangeCheckForAdd(int index) {
        if (index > size || index < 0)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

	// 判斷是否需要擴(kuò)容
    private void ensureCapacityInternal(int minCapacity) {
        ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
    }

    private void ensureExplicitCapacity(int minCapacity) {
        modCount++; // 修改次數(shù)累加

        // overflow-conscious code
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }
    /**
     * Increases the capacity to ensure that it can hold at least the
     * number of elements specified by the minimum capacity argument.
     * 增加集合的容量以確保容納至少
     * @param minCapacity the desired minimum capacity
     */
    private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        int newCapacity = oldCapacity + (oldCapacity >> 1);// 將原容量擴(kuò)容至原來(lái)的1.5倍，以本例來(lái)說(shuō)就是擴(kuò)容至：6+3=9
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity; //取 newCapacity 和 minCapacity 的最大值賦值給 newCapacity，考慮了溢出的情況
        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);
    }
    /**
     * The maximum size of array to allocate. 定義了數(shù)組允許分配的最大長(zhǎng)度
     * Some VMs reserve some header words in an array. 一些虛擬機(jī)在數(shù)列中會(huì)保留一些頭部信息（需要預(yù)留一定容量）
     * Attempts to allocate larger arrays may result in 嘗試取分配更長(zhǎng)的數(shù)組可能會(huì)導(dǎo)致內(nèi)存溢出
     * OutOfMemoryError: Requested array size exceeds VM limit ：申請(qǐng)的數(shù)組長(zhǎng)度超過(guò)了虛擬機(jī)的限制
     */
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
    
    private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) // overflow 溢出檢查
            throw new OutOfMemoryError();
        return (minCapacity > MAX_ARRAY_SIZE) ? // 如果申請(qǐng)的最小容量比數(shù)組的容量上限還大則容量設(shè)置為：
            Integer.MAX_VALUE : // Integer.MAX_VALUE，否則設(shè)置為：數(shù)組容量上限（MAX_ARRAY_SIZE）
            MAX_ARRAY_SIZE;
    }

        System.arraycopy(elementData, index, elementData, index + 1,
                         size - index);
//[#System] 調(diào)用了系統(tǒng)級(jí)的數(shù)組拷貝方法
	/**
	 * @param      src      the source array. 源數(shù)組
     * @param      srcPos   starting position in the source array. 源數(shù)組的起始下標(biāo)
     * @param      dest     the destination array. 目標(biāo)數(shù)組
     * @param      destPos  starting position in the destination data. 目標(biāo)數(shù)組的起始下標(biāo)
     * @param      length   the number of array elements to be copied. 需要拷貝的元素?cái)?shù)量
     * */
    public static native void arraycopy(Object src,  int  srcPos,
                                        Object dest, int destPos,
                                        int length);

    /**
     * Returns the element at the specified position in this list.
     * 返回集合指定位置的元素
     * @param  index index of the element to return 要返回的元素下標(biāo)
     * @return the element at the specified position in this list 指定下標(biāo)的元素
     * @throws IndexOutOfBoundsException {@inheritDoc} 下標(biāo)越界異常
     */
    public E get(int index) {
        rangeCheck(index); // 索引越界檢查

        return elementData(index); // 按下標(biāo)獲取元素
    }
    /**
     * Checks if the given index is in range.  If not, throws an appropriate
     * runtime exception.  This method does *not* check if the index is
     * negative: It is always used immediately prior to an array access,
     * which throws an ArrayIndexOutOfBoundsException if index is negative.
     */
    private void rangeCheck(int index) {
        if (index >= size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    /**
     * Removes the element at the specified position in this list.
     * Shifts any subsequent elements to the left (subtracts one from their
     * indices).
     * 刪除特定位置的集合元素，將該元素之后的所有元素往前挪一位。
     * @param index the index of the element to be removed 待刪除的元素下標(biāo)
     * @return the element that was removed from the list 返回已刪除的元素
     * @throws IndexOutOfBoundsException {@inheritDoc} 下標(biāo)越界異常
     */
    public E remove(int index) {
        rangeCheck(index); // 下標(biāo)越界檢查

        modCount++; // 修改次數(shù)累加
        E oldValue = elementData(index);

        int numMoved = size - index - 1; // 計(jì)算需要移動(dòng)的元素?cái)?shù)量，指的就是當(dāng)前刪除位置之后的元素?cái)?shù)量
        if (numMoved > 0)
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved); // 重新進(jìn)行數(shù)據(jù)拷貝
        elementData[--size] = null; // clear to let GC do its work 將數(shù)組末尾空缺出來(lái)的位置引用置為null，便于GC

        return oldValue;
    }

    /**
     * Removes the first occurrence of the specified element from this list,
     * 如果集合中指定的元素存在的話，刪除首次出現(xiàn)的那個(gè)指定元素。
     * if it is present.  If the list does not contain the element, it is
     * 如果指定元素不存在，則不會(huì)有什么影響。
     * unchanged.  More formally, removes the element with the lowest index
     * <tt>i</tt> such that 
     * 一般情況下，會(huì)刪除下標(biāo)最小的那個(gè)元素
     * <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>
     * (if such an element exists).  Returns <tt>true</tt> if this list
     * contained the specified element (or equivalently, if this list
     * changed as a result of the call).
     * 
     * @param o element to be removed from this list, if present 待刪除的元素
     * @return <tt>true</tt> if this list contained the specified element 如果元素存在則返回true，反之為false
     */
    public boolean remove(Object o) {
        if (o == null) { // 如果待刪除的元素為 null，則直接遍歷數(shù)組元素和 null 進(jìn)行匹配
            for (int index = 0; index < size; index++)
                if (elementData[index] == null) {
                    fastRemove(index); // 執(zhí)行刪除操作
                    return true;
                }
        } else {
            for (int index = 0; index < size; index++)
                if (o.equals(elementData[index])) { // 遍歷匹配所有元素
                    fastRemove(index);// 執(zhí)行刪除操作
                    return true;
                }
        }
        return false;
    }
    /*
     * Private remove method that skips bounds checking and does not
     * return the value removed.
     */
    private void fastRemove(int index) {
        modCount++; // 修改次數(shù)累加
        int numMoved = size - index - 1;// 計(jì)算需要移動(dòng)的元素?cái)?shù)量，指的就是當(dāng)前刪除位置之后的元素?cái)?shù)量
        if (numMoved > 0)
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);// 重新進(jìn)行數(shù)據(jù)拷貝
        elementData[--size] = null; // clear to let GC do its work 將數(shù)組末尾空缺出來(lái)的位置引用置為null，便于GC
    }

    /**
     * Removes from this list all of its elements that are contained in the
     * specified collection.
     * 按給定的特定元素集合去刪除當(dāng)前集合的匹配元素。
     * @param c collection containing elements to be removed from this list 包含待刪除元素的刪除
     * @return {@code true} if this list changed as a result of the call
     * @throws ClassCastException if the class of an element of this list
     *         is incompatible with the specified collection
     * (<a href="Collection.html#optional-restrictions" rel="external nofollow"  rel="external nofollow" >optional</a>)
     * @throws NullPointerException if this list contains a null element and the
     *         specified collection does not permit null elements
     * (<a href="Collection.html#optional-restrictions" rel="external nofollow"  rel="external nofollow" >optional</a>),
     *         or if the specified collection is null
     * @see Collection#contains(Object)
     */
    public boolean removeAll(Collection<?> c) {
        Objects.requireNonNull(c); // 對(duì) c 集合進(jìn)行空判斷
        return batchRemove(c, false); // 執(zhí)行批量刪除
    }
    // 批量刪除方法
    private boolean batchRemove(Collection<?> c, boolean complement) {
        final Object[] elementData = this.elementData;
        int r = 0, w = 0;
        boolean modified = false; // 操作結(jié)果標(biāo)識(shí)
        try {
        	// 通過(guò)遍歷原集合，將不符合刪除條件的 [r] 位置的元素替換掉 [w] 位置的元素，并將 [w] 累加。
            for (; r < size; r++) // 每次循環(huán) r++
                if (c.contains(elementData[r]) == complement) // 如果待刪除的集合不包含有原集合的元素
                    elementData[w++] = elementData[r]; // 則用原集合當(dāng)前下標(biāo)位置 [r] 的元素覆蓋掉下標(biāo)位置為 [w] 的元素
                    // 并將 [w] 累加。
        } finally {
            // Preserve behavioral compatibility with AbstractCollection,
            // even if c.contains() throws.
            // 只有在拋出異常時(shí)：r != size ，那么此時(shí)需要將未比對(duì)的元素拼接在已經(jīng)處理過(guò)的元素后面
            if (r != size) {
                System.arraycopy(elementData, r,
                                 elementData, w,
                                 size - r);
                w += size - r; // 重新設(shè)置 [w] 的值，因?yàn)樵谙乱徊綍?huì)將 [w] 之后的元素設(shè)置為null,此時(shí)的 [r] 為拋出異常位置
            }
            // 在極端情況下，如果待刪除集合和原集合的元素完全無(wú)交集，則 `w == size`，這種情況下無(wú)需對(duì)原集合進(jìn)行任何操作。
            if (w != size) {
                // clear to let GC do its work
                for (int i = w; i < size; i++)
                    elementData[i] = null; // 將 [w] 之后的元素全部賦值為 null。
                modCount += size - w; // 對(duì) modCount 進(jìn)行重新設(shè)置
                size = w;
                modified = true;
            }
        }
        return modified;
    }
    // 集合是否包含指定的元素
    public boolean contains(Object o) {
        return indexOf(o) >= 0; // 遍歷集合查找指定元素
    }

	// 根據(jù)指定的過(guò)濾器判斷匹配的元素是否在集合內(nèi)：Predicate 接口主要用來(lái)判斷一個(gè)參數(shù)是否符合要求。
    public boolean removeIf(Predicate<? super E> filter) {
        Objects.requireNonNull(filter); // 指定的過(guò)濾器的非null判斷
        // figure out which elements are to be removed 找出要?jiǎng)h除的元素
        // any exception thrown from the filter predicate at this stage 在這階段拋出的任何異常都不會(huì)使得集合發(fā)生改變
        // will leave the collection unmodified
        int removeCount = 0; // 刪除數(shù)目
        final BitSet removeSet = new BitSet(size); // BitSet是一種特殊類型的數(shù)組來(lái)保存位值，其中數(shù)組大小會(huì)隨需要增加
        final int expectedModCount = modCount; // 記錄修改次數(shù)
        final int size = this.size;
        // 這個(gè)循環(huán)主要是為了記錄需要?jiǎng)h除的元素?cái)?shù)目
        for (int i=0; modCount == expectedModCount && i < size; i++) {
            @SuppressWarnings("unchecked")
            final E element = (E) elementData[i];
            if (filter.test(element)) {
                removeSet.set(i); // 通過(guò) removeSet 來(lái)記錄需要?jiǎng)h除的集合下標(biāo)
                removeCount++; // 刪除數(shù)目進(jìn)行累加
            }
        }
        if (modCount != expectedModCount) { // 正常情況下這兩個(gè)值應(yīng)該是相等的，不相等說(shuō)明有了并發(fā)修改，則拋出異常 
            throw new ConcurrentModificationException();
        }
        // shift surviving elements left over the spaces left by removed elements
        // 通過(guò)遍歷使用未刪除的元素替換已刪除元素，[i] 代表未刪除的元素下標(biāo)，[j] 代表被替換的元素下標(biāo)
        final boolean anyToRemove = removeCount > 0;
        if (anyToRemove) {
            final int newSize = size - removeCount; // 記錄刪除后的新的容量
            for (int i=0, j=0; (i < size) && (j < newSize); i++, j++) {
                i = removeSet.nextClearBit(i);  // 找出未刪除元素的下標(biāo) [i]
                elementData[j] = elementData[i]; // 使用未刪除的元素 [i] 替換對(duì)應(yīng)位置 [j] 的元素
            }
            for (int k=newSize; k < size; k++) { // 將下標(biāo)從 [k] 到之后的位置的元素賦值為null
                elementData[k] = null;  // Let gc do its work
            }
            this.size = newSize;
            if (modCount != expectedModCount) { // 出現(xiàn)并發(fā)修改時(shí)拋出異常
                throw new ConcurrentModificationException();
            }
            modCount++; // 修改次數(shù)累加
        }

        return anyToRemove;
    }

    /**
     * Removes from this list all of the elements whose index is between
     * {@code fromIndex}, inclusive, and {@code toIndex}, exclusive.
     * 刪除下標(biāo)區(qū)間 [fromIndex]至[toIndex]之間的元素，包含[fromIndex] 但是不包含 [toIndex] 。
     * Shifts any succeeding elements to the left (reduces their index).
     * 將刪除區(qū)間之后的集合元素統(tǒng)一左移動(dòng)。
     * This call shortens the list by {@code (toIndex - fromIndex)} elements.
     * (If {@code toIndex==fromIndex}, this operation has no effect.)
     * 如果[fromIndex]和[toIndex]相等，則對(duì)集合無(wú)影響。
     * @throws IndexOutOfBoundsException if {@code fromIndex} or
     *         {@code toIndex} is out of range
     *         ({@code fromIndex < 0 ||
     *          fromIndex >= size() ||
     *          toIndex > size() ||
     *          toIndex < fromIndex})
     */
    protected void removeRange(int fromIndex, int toIndex) {
        modCount++; // 修改次數(shù)累加
        int numMoved = size - toIndex; // 需要移動(dòng)的元素?cái)?shù)量，也就是待刪除區(qū)間的末尾之后的元素?cái)?shù)量。
        System.arraycopy(elementData, toIndex, elementData, fromIndex,
                         numMoved); // 用刪除區(qū)間的末尾之后的元素拷貝至刪除區(qū)間的元素進(jìn)行覆蓋。

        // clear to let GC do its work
        int newSize = size - (toIndex-fromIndex); // 計(jì)算新的集合的長(zhǎng)度，方便后續(xù)進(jìn)行多余數(shù)組元素的置空
        for (int i = newSize; i < size; i++) {
            elementData[i] = null; // 將后半部分的多余位置的元素賦值為 null，方便GC。
        }
        size = newSize; // 重新設(shè)置集合的大小
    }