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【JDK源码分析系列】ArrayList源码分析

【0】ArrayList 整体架构图示

【1】ArrayList源码MCL视图

ArrayList的MCL视图如下图所示，包括内部类的MCL视图；

【2】ArrayList源码分析相关知识点总结

    /**     * 如果用transient声明一个实例变量，当对象存储时，它的值不需要维持；     * 即用transient关键字标记的成员变量不参与序列化过程；     *      * 此处使用transient关键的原因在于：     * ArrayList在序列化的时候会调用writeObject，反序列化时调用readObject 也就是自定义序列化     * 因为ArrayList数组elementData中有未使用的空间 ,如果没有使用的空间也序列化,势必会影响性能     */    transient Object[] elementData;

一：快速失败（fail—fast）    在用迭代器遍历一个集合对象时，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。    原理：迭代器在遍历时直接访问集合中的内容，并且在遍历过程中使用一个 modCount 变量。集合在被遍历期间如果内容发生变化，就会改变modCount的值。    每当迭代器使用hashNext()/next()遍历下一个元素之前，都会检测modCount变量是否为expectedmodCount值，是的话就返回遍历；否则抛出异常，终止遍历。    注意：这里异常的抛出条件是检测到 modCount！=expectedmodCount 这个条件。如果集合发生变化时修改modCount值刚好又设置为了expectedmodCount值，则异常不会抛出。因此，不能依赖于这个异常是否抛出而进行并发操作的编程，这个异常只建议用于检测并发修改的bug。    场景：java.util包下的集合类都是快速失败的，不能在多线程下发生并发修改（迭代过程中被修改）。	二：安全失败（fail—safe）    采用安全失败机制的集合容器，在遍历时不是直接在集合内容上访问的，而是先复制原有集合内容，在拷贝的集合上进行遍历。    原理：由于迭代时是对原集合的拷贝进行遍历，所以在遍历过程中对原集合所作的修改并不能被迭代器检测到，所以不会触发Concurrent Modification Exception。    缺点：基于拷贝内容的优点是避免了Concurrent Modification Exception，但同样地，迭代器并不能访问到修改后的内容，即：迭代器遍历的是开始遍历那一刻拿到的集合拷贝，在遍历期间原集合发生的修改迭代器是不知道的。    场景：java.util.concurrent包下的容器都是安全失败，可以在多线程下并发使用，并发修改。

ArrayList集合实现RandomAccess接口有何作用RandomAccess接口是一个标志接口只要List集合实现这个接口，就能支持快速随机访问RandomAccess接口这个空架子的存在，是为了能够更好地判断集合是否ArrayList或者LinkedList，从而能够更好选择更优的遍历方式，提高性能

System.arraycopy()方法的深拷贝与浅拷贝1. 当数组为一维数组，且元素为基本类型或String类型时，属于深复制，即原数组与新数组的元素不会相互影响2. 当数组为多维数组，或一维数组中的元素为引用类型时，属于浅复制，原数组与新数组的元素引用指向同一个对象

【3】ArrayList源码注解

package java.util;import java.util.function.Consumer;import java.util.function.Predicate;import java.util.function.UnaryOperator;/** * Resizable-array implementation of the <tt>List</tt> interface.  Implements * all optional list operations, and permits all elements, including * <tt>null</tt>.  In addition to implementing the <tt>List</tt> interface, * this class provides methods to manipulate the size of the array that is * used internally to store the list.  (This class is roughly equivalent to * <tt>Vector</tt>, except that it is unsynchronized.) * * <p>The <tt>size</tt>, <tt>isEmpty</tt>, <tt>get</tt>, <tt>set</tt>, * <tt>iterator</tt>, and <tt>listIterator</tt> operations run in constant * time.  The <tt>add</tt> operation runs in <i>amortized constant time</i>, * that is, adding n elements requires O(n) time.  All of the other operations * run in linear time (roughly speaking).  The constant factor is low compared * to that for the <tt>LinkedList</tt> implementation. * * <p>Each <tt>ArrayList</tt> instance has a <i>capacity</i>.  The capacity is * the size of the array used to store the elements in the list.  It is always * at least as large as the list size.  As elements are added to an ArrayList, * its capacity grows automatically.  The details of the growth policy are not * specified beyond the fact that adding an element has constant amortized * time cost. * * <p>An application can increase the capacity of an <tt>ArrayList</tt> instance * before adding a large number of elements using the <tt>ensureCapacity</tt> * operation.  This may reduce the amount of incremental reallocation. * * <p><strong>Note that this implementation is not synchronized.</strong> * If multiple threads access an <tt>ArrayList</tt> instance concurrently, * and at least one of the threads modifies the list structurally, it * <i>must</i> be synchronized externally.  (A structural modification is * any operation that adds or deletes one or more elements, or explicitly * resizes the backing array; merely setting the value of an element is not * a structural modification.)  This is typically accomplished by * synchronizing on some object that naturally encapsulates the list. * * If no such object exists, the list should be "wrapped" using the * {@link Collections#synchronizedList Collections.synchronizedList} * method.  This is best done at creation time, to prevent accidental * unsynchronized access to the list:<pre> *   List list = Collections.synchronizedList(new ArrayList(...));</pre> * * <p><a name="fail-fast"> * The iterators returned by this class's {@link #iterator() iterator} and * {@link #listIterator(int) listIterator} methods are <em>fail-fast</em>:</a> * if the list is structurally modified at any time after the iterator is * created, in any way except through the iterator's own * {@link ListIterator#remove() remove} or * {@link ListIterator#add(Object) add} methods, the iterator will throw a * {@link ConcurrentModificationException}.  Thus, in the face of * concurrent modification, the iterator fails quickly and cleanly, rather * than risking arbitrary, non-deterministic behavior at an undetermined * time in the future. * * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed * as it is, generally speaking, impossible to make any hard guarantees in the * presence of unsynchronized concurrent modification.  Fail-fast iterators * throw {@code ConcurrentModificationException} on a best-effort basis. * Therefore, it would be wrong to write a program that depended on this * exception for its correctness:  <i>the fail-fast behavior of iterators * should be used only to detect bugs.</i> * * <p>This class is a member of the * <a href="{@docRoot}/../technotes/guides/collections/index.html"> * Java Collections Framework</a>. * * @author  Josh Bloch * @author  Neal Gafter * @see     Collection * @see     List * @see     LinkedList * @see     Vector * @since   1.2 */// 时间复杂度// 新增或删除方法对数组元素的操作， 只需要根据数组索引， 直接新增和删除， 所以时间复杂度是 O(1)// 线程安全// 只有当 ArrayList 作为共享变量时， 才会有线程安全问题， 当 ArrayList 是方法内的局部变量时是没有线程安全的问题的// ArrayList 有线程安全问题的本质 : // ArrayList 自身的 elementData、size、modConut 在进行各种操作时， 都没有加锁， // 而且这些变量的类型并非是可见 (volatile) 的，// 所以如果多个线程对这些变量进行操作时可能会有值被覆盖的情况// 类注释的关键点 :// 1. 允许 put null 值，会自动扩容// 2. size、isEmpty、get、set、add 等方法时间复杂度都是 O(1)// 3. 非线程安全的，多线程情况下，推荐使用线程安全类 Collections#synchronizedList// 4. 增强 for 循环，或者使用迭代器迭代过程中，如果数组大小被改变，会快速失败，抛出异常public class ArrayList<E> extends AbstractList<E>        implements List<E>, RandomAccess, Cloneable, java.io.Serializable{    private static final long serialVersionUID = 8683452581122892189L;    /**     * Default initial capacity.     */    private static final int DEFAULT_CAPACITY = 10;    /**     * Shared empty array instance used for empty instances.     */    private static final Object[] EMPTY_ELEMENTDATA = {};    /**     * 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 == EMPTY_ELEMENTDATA will be expanded to     * DEFAULT_CAPACITY when the first element is added.     */    /**     * 如果用transient声明一个实例变量，当对象存储时，它的值不需要维持；     * 即用transient关键字标记的成员变量不参与序列化过程；     *      * 此处使用transient关键的原因在于：     * ArrayList在序列化的时候会调用writeObject，反序列化时调用readObject 也就是自定义序列化     * 因为ArrayList数组elementData中有未使用的空间 ,如果没有使用的空间也序列化,势必会影响性能     */    transient Object[] elementData; // non-private to simplify nested class access    /**     * The size of the ArrayList (the number of elements it contains).     *     * @serial     */    private int size;    /**     * Constructs an empty list with the specified initial capacity.     *     * @param  initialCapacity  the initial capacity of the list     * @throws IllegalArgumentException if the specified initial capacity     *         is negative     */    /**     * ArrayList构造方法     * 此方法初始化一个initialCapacity大小的Object类型数组并赋值给成员变量elementData     */    public ArrayList(int initialCapacity) {        super();        if (initialCapacity < 0)            throw new IllegalArgumentException("Illegal Capacity: "+                                               initialCapacity);        this.elementData = new Object[initialCapacity];    }    /**     * Constructs an empty list with an initial capacity of ten.     */    /**     * 无参初始化     *      * ArrayList构造方法     * 此方法构造一个空的Object数组     */    // ArrayList 无参构造器初始化时，默认大小是空数组，并不是大家常说的 10，    // 10 是在第一次 add 的时候扩容的数组值    public ArrayList() {        super();        this.elementData = EMPTY_ELEMENTDATA;    }    /**     * Constructs a list containing the elements of the specified     * collection, in the order they are returned by the collection's     * iterator.     *     * @param c the collection whose elements are to be placed into this list     * @throws NullPointerException if the specified collection is null     */    /**     * <? extends E> 是 Upper Bound（上限） 的通配符；     * 此处表明集合中元素的上限为E，即集合中的元素类型只能是E或E的子类     *      *  <? super E> 是 Lower Bound（下限） 的通配符；     * 此处用来限制元素的类型下限为E，即集合中的元素类型只能是E或E的父类     *      * c.toArray()：调用Collection接口中的toArray()方法，将集合转化为Array     */    // 指定初始数据初始化    public ArrayList(Collection<? extends E> c) {        // elementData 是保存数组的容器，默认为 null        elementData = c.toArray();        if ((size = elementData.length) != 0) {            // c.toArray might (incorrectly) not return Object[] (see 6260652)            // 这是 Java 的一个 bug，意思是当给定集合内的元素不是 Object 类型时，            // 我们会转化成 Object 的类型            // BUG 触发条件 : ArrayList 初始化之后（ArrayList 元素非 Object 类型），            // 再次调用 toArray 方法，得到 Object 数组，并且往 Object 数组赋值时，            // 才会触发此 bug            /**             * 若elementData的class类型不是Object[]类型，             * 则将elementData的class类型转换为Object[]类型，保留原始数组的数据             * 此处的处理目的是确保elementData的类型为Object[]             */            if (elementData.getClass() != Object[].class)                elementData = Arrays.copyOf(elementData, size, Object[].class);        } else {            // replace with empty array.            this.elementData = EMPTY_ELEMENTDATA;        }    }    /**     * Trims the capacity of this <tt>ArrayList</tt> instance to be the     * list's current size.  An application can use this operation to minimize     * the storage of an <tt>ArrayList</tt> instance.     */    /**     * 该方法的目的：调整ArrayList的大小     * 当size小于elementData.length，则取elementData前size个元素重新对elementData赋值     */    public void trimToSize() {        modCount++;        if (size < elementData.length) {            elementData = Arrays.copyOf(elementData, size);        }    }    /**     * Increases the capacity of this <tt>ArrayList</tt> instance, if     * necessary, to ensure that it can hold at least the number of elements     * specified by the minimum capacity argument.     *     * @param   minCapacity   the desired minimum capacity     */    /**     * 该方法对当前ArrayList容量进行扩容以确保至少可以保存当前所有的元素；     * 当elementData为EMPTY_ELEMENTDATA时，minExpand为DEFAULT_CAPACITY，否则minExpand为0；     * minCapacity大于minExpand时，调用ensureExplicitCapacity方法确定更精确的容量；     */    public void ensureCapacity(int minCapacity) {        int minExpand = (elementData != EMPTY_ELEMENTDATA)            // any size if real element table            ? 0            // larger than default for empty table. It's already supposed to be            // at default size.            : DEFAULT_CAPACITY;        if (minCapacity > minExpand) {            ensureExplicitCapacity(minCapacity);        }    }    /**     * 该方法对当前ArrayList容量进行扩容以确保至少可以保存当前所有的元素；     * 该方法为ArrayList内部使用的扩容方法；     */    private void ensureCapacityInternal(int minCapacity) {        // 如果初始化数组大小时， 有给定初始值， 以给定的大小为准， 不走 if 逻辑        if (elementData == EMPTY_ELEMENTDATA) {            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);        }        // 确保容积足够        ensureExplicitCapacity(minCapacity);    }    /**     * minCapacity比elementData容量大时，对elementData扩容     */    private void ensureExplicitCapacity(int minCapacity) {        // 记录数组被修改        modCount++;        // 如果我们期望的最小容量大于目前数组的长度，那么就扩容        // overflow-conscious code        if (minCapacity - elementData.length > 0)            grow(minCapacity);    }    /**     * The maximum size of array to allocate.     * Some VMs reserve some header words in an array.     * Attempts to allocate larger arrays may result in     * OutOfMemoryError: Requested array size exceeds VM limit     */    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;    /**     * Increases the capacity to ensure that it can hold at least the     * number of elements specified by the minimum capacity argument.     *     * @param minCapacity the desired minimum capacity     */    /**     * 扩容并把现有数据拷贝到新的数组里面去     * 扩容策略为将原有的elementData的容量扩充1.5倍     * elementData的容量的最大值为Integer.MAX_VALUE     */    private void grow(int minCapacity) {        // overflow-conscious code        int oldCapacity = elementData.length;        // oldCapacity >> 1 是把 oldCapacity 除以 2 的意思        int newCapacity = oldCapacity + (oldCapacity >> 1);        // 如果扩容后的值 < 我们的期望值， 扩容后的值就等于我们的期望值        if (newCapacity - minCapacity < 0)            newCapacity = minCapacity;        // 如果扩容后的值 > jvm 所能分配的数组的最大值，那么就用 Integer 的最大值        if (newCapacity - MAX_ARRAY_SIZE > 0)            newCapacity = hugeCapacity(minCapacity);        // minCapacity is usually close to size, so this is a win:        // 通过复制进行扩容        // 一下该行代码描述的本质是数组之间的拷贝， 扩容是会先新建一个符合我们预期容量的新数组， 然后把老数组的数据拷贝过去，        // 我们通过 System.arraycopy 方法进行拷贝，此方法是 native 的方法        // /**        //  * @param src     被拷贝的数组        //  * @param srcPos  从数组那里开始        //  * @param dest    目标数组        //  * @param destPos 从目标数组那个索引位置开始拷贝        //  * @param length  拷贝的长度         //  * 此方法是没有返回值的，通过 dest 的引用进行传值        //  */        // public static native void arraycopy(Object src, int srcPos,        //                                     Object dest, int destPos,        //                                     int length);        //        // System.arraycopy(elementData, 0, newElementData, 0,Math.min(elementData.length,newCapacity));        //        elementData = Arrays.copyOf(elementData, newCapacity);    }    /**     * hugeCapacity为静态方法     * 静态方法一般用于对静态属性进行操作     * 此处对MAX_ARRAY_SIZE，Integer.MAX_VALUE静态变量操作     */    private static int hugeCapacity(int minCapacity) {        if (minCapacity < 0) // overflow            throw new OutOfMemoryError();        return (minCapacity > MAX_ARRAY_SIZE) ?            Integer.MAX_VALUE :            MAX_ARRAY_SIZE;    }    /**     * Returns the number of elements in this list.     *     * @return the number of elements in this list     */    /**     * 该方法用于返回当前ArrayList的容量     */    public int size() {        return size;    }    /**     * Returns <tt>true</tt> if this list contains no elements.     *     * @return <tt>true</tt> if this list contains no elements     */    /**     * 该方法用于判断当前ArrayList是否为空     */    public boolean isEmpty() {        return size == 0;    }    /**     * Returns <tt>true</tt> if this list contains the specified element.     * More formally, returns <tt>true</tt> if and only if this list contains     * at least one element <tt>e</tt> such that     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.     *     * @param o element whose presence in this list is to be tested     * @return <tt>true</tt> if this list contains the specified element     */    /**     * 该方法用于判断ArrayList中是否包含特定的元素     */    public boolean contains(Object o) {        return indexOf(o) >= 0;    }    /**     * Returns the index of the first occurrence of the specified element     * in this list, or -1 if this list does not contain the element.     * More formally, returns the lowest index <tt>i</tt> such that     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,     * or -1 if there is no such index.     */    /**     * 该方法用于查找元素在ArrayList中的索引     * 分元素为NULL与非NULL两种情况进行讨论     */    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;    }    /**     * Returns the index of the last occurrence of the specified element     * in this list, or -1 if this list does not contain the element.     * More formally, returns the highest index <tt>i</tt> such that     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,     * or -1 if there is no such index.     */    /**     * 该方法返回元素在ArrayList中的最后一个索引     */    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;    }    /**     * Returns a shallow copy of this <tt>ArrayList</tt> instance.  (The     * elements themselves are not copied.)     *     * @return a clone of this <tt>ArrayList</tt> instance     */    /**     * 父类中的clone方法是native的；     * native方法，Native Method是一个java调用非java代码的接口；     *      * 该方法功能：     * 1.调用Object中的clone()方法并强转为一个ArrayList对象；     * 2.拷贝当前ArrayList中的元素到拷贝的对象，并返回clone()方法新建的对象；     */    public Object clone() {        try {            ArrayList<?> v = (ArrayList<?>) super.clone();            v.elementData = Arrays.copyOf(elementData, size);            v.modCount = 0;            return v;        } catch (CloneNotSupportedException e) {            // this shouldn't happen, since we are Cloneable            throw new InternalError(e);        }    }    /**     * Returns an array containing all of the elements in this list     * in proper sequence (from first to last element).     *     * <p>The returned array will be "safe" in that no references to it are     * maintained by this list.  (In other words, this method must allocate     * a new array).  The caller is thus free to modify the returned array.     *     * <p>This method acts as bridge between array-based and collection-based     * APIs.     *     * @return an array containing all of the elements in this list in     *         proper sequence     */    /**     * 该方法将ArrayList转换为Object[]类型的数组     */    public Object[] toArray() {        return Arrays.copyOf(elementData, size);    }    /**     * Returns an array containing all of the elements in this list in proper     * sequence (from first to last element); the runtime type of the returned     * array is that of the specified array.  If the list fits in the     * specified array, it is returned therein.  Otherwise, a new array is     * allocated with the runtime type of the specified array and the size of     * this list.     *     * <p>If the list fits in the specified array with room to spare     * (i.e., the array has more elements than the list), the element in     * the array immediately following the end of the collection is set to     * <tt>null</tt>.  (This is useful in determining the length of the     * list <i>only</i> if the caller knows that the list does not contain     * any null elements.)     *     * @param a the array into which the elements of the list are to     *          be stored, if it is big enough; otherwise, a new array of the     *          same runtime type is allocated for this purpose.     * @return an array containing the elements of the list     * @throws ArrayStoreException if the runtime type of the specified array     *         is not a supertype of the runtime type of every element in     *         this list     * @throws NullPointerException if the specified array is null     */    /**     * 该方法将ArrayList转换为T[]类型的数组     * 当入参数组的容量大于当前ArrayList的容量时，T[]类型的数组多余的元素为NULL     *      * unchecked: 执行了未检查的转换时的警告，此处抑制该警告的产生；     */    @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);        if (a.length > size)            a[size] = null;        return a;    }    // Positional Access Operations    /**     * 该方法获取ArrayList指定索引的元素     */    @SuppressWarnings("unchecked")    E elementData(int index) {        return (E) elementData[index];    }    /**     * Returns the element at the specified position in this list.     *     * @param  index index of the element to return     * @return the element at the specified position in this list     * @throws IndexOutOfBoundsException {@inheritDoc}     */    /**     * 该方法获取ArrayList指定索引的元素     * 其中对索引作了范围检查     */    public E get(int index) {        rangeCheck(index);        return elementData(index);    }    /**     * Replaces the element at the specified position in this list with     * the specified element.     *     * @param index index of the element to replace     * @param element element to be stored at the specified position     * @return the element previously at the specified position     * @throws IndexOutOfBoundsException {@inheritDoc}     */    /**     * 该方法在ArrayList指定索引处设置入参元素     * 其中对索引作了范围检查     */    public E set(int index, E element) {        rangeCheck(index);        E oldValue = elementData(index);        elementData[index] = element;        return oldValue;    }    /**     * Appends the specified element to the end of this list.     *     * @param e element to be appended to this list     * @return <tt>true</tt> (as specified by {@link Collection#add})     */    /**     * 该方法向ArrayList中添加元素，该元素添加到ArrayList的尾部     * ensureCapacityInternal(size + 1)：确保ArrayList存在足够的存储容量；     */    public boolean add(E e) {        // 确保数组大小是否足够，不够执行扩容，size 为当前数组的大小        ensureCapacityInternal(size + 1);  // Increments modCount!!        // 直接赋值，线程不安全的        elementData[size++] = e;        return true;    }    /**     * 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).     *     * @param index index at which the specified element is to be inserted     * @param element element to be inserted     * @throws IndexOutOfBoundsException {@inheritDoc}     */    /**     * 该方法在ArrayList的指定位置添加指定的元素；     * ensureCapacityInternal(size + 1)：确保ArrayList存在足够的存储容量；     * 将index之后的元素后移一位，并在index处赋值；     */    public void add(int index, E element) {        rangeCheckForAdd(index);        ensureCapacityInternal(size + 1);  // Increments modCount!!        System.arraycopy(elementData, index, elementData, index + 1,                         size - index);        elementData[index] = element;        size++;    }    /**     * Removes the element at the specified position in this list.     * Shifts any subsequent elements to the left (subtracts one from their     * indices).     *     * @param index the index of the element to be removed     * @return the element that was removed from the list     * @throws IndexOutOfBoundsException {@inheritDoc}     */    /**     * 该方法在ArrayList的指定位置删除指定的元素；     * 将index之后元素前移一位，并将最后一位元素赋值为NULL；     * 同时改变size大小并返回删除的元素；     */    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);        elementData[--size] = null; // clear to let GC do its work        return oldValue;    }    /**     * Removes the first occurrence of the specified element from this list,     * if it is present.  If the list does not contain the element, it is     * unchanged.  More formally, removes the element with the lowest index     * <tt>i</tt> such that     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;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     * @return <tt>true</tt> if this list contained the specified element     */    /**     * 该方法删除ArrayList中指定的元素；     * 入参对象为NULL时，删除elementData中第一个为NULL的元素     * 入参对象不为NULL时，elementData存在入参对象变将其删除     * elementData不存在入参对象时返回false；     */    // 注意点 :    // 1. 新增的时候是没有对 null 进行校验的， 所以删除的时候也是允许删除 null 值    // 2. 找到值在数组中的索引位置， 是通过 equals 来判断的， 如果数组元素不是基本类型，     // 需要我们关注 equals 的具体实现    public boolean remove(Object o) {        // 如果要删除的值是 null，找到第一个值是 null 的删除        if (o == null) {            for (int index = 0; index < size; index++)                if (elementData[index] == null) {                    fastRemove(index);                    return true;                }        } else {            // 如果要删除的值不为 null，找到第一个和要删除的值相等的删除            for (int index = 0; index < size; index++)                // 这里是根据 equals 来判断值相等的， 相等后再根据索引位置进行删除                if (o.equals(elementData[index])) {                    fastRemove(index);                    return true;                }        }        return false;    }    /*     * Private remove method that skips bounds checking and does not     * return the value removed.     */    /**     * 该方法删除指定索引处的元素     * 将index之后元素前移一位，并将最后一位元素赋值为NULL，同时改变size大小；     */    private void fastRemove(int index) {        // 记录数组的结构要发生变动了        modCount++;        // numMoved 表示删除 index 位置的元素后，需要从 index 后移动多少个元素到前面去        // 减 1 的原因， 是因为 size 从 1 开始算起， index 从 0开始算起        int numMoved = size - index - 1;        if (numMoved > 0)            // 从 index + 1 位置开始被拷贝，拷贝的起始位置是 index，长度是 numMoved            System.arraycopy(elementData, index+1, elementData, index,                             numMoved);         // 数组最后一个位置赋值 null，帮助 GC        elementData[--size] = null; // clear to let GC do its work    }    /**     * Removes all of the elements from this list.  The list will     * be empty after this call returns.     */    /**     * 该方法遍历elementData，将其中的元素赋值为NULL，并将size清零     */    public void clear() {        modCount++;        // clear to let GC do its work        for (int i = 0; i < size; i++)            elementData[i] = null;        size = 0;    }    /**     * Appends all of the elements in the specified collection to the end of     * this list, in the order that they are returned by the     * specified collection's Iterator.  The behavior of this operation is     * undefined if the specified collection is modified while the operation     * is in progress.  (This implies that the behavior of this call is     * undefined if the specified collection is this list, and this     * list is nonempty.)     *     * @param c collection containing elements to be added to this list     * @return <tt>true</tt> if this list changed as a result of the call     * @throws NullPointerException if the specified collection is null     */    /**     * 该方法在ArrayList尾部添加集合对象；     * 先将集合对象转化为数组，在进行元素拷贝；     */    public boolean addAll(Collection<? extends E> c) {        Object[] a = c.toArray();        int numNew = a.length;        ensureCapacityInternal(size + numNew);  // Increments modCount        System.arraycopy(a, 0, elementData, size, numNew);        size += numNew;        return numNew != 0;    }    /**     * Inserts all of the elements in the specified collection into this     * list, starting at the specified position.  Shifts the element     * currently at that position (if any) and any subsequent elements to     * the right (increases their indices).  The new elements will appear     * in the list in the order that they are returned by the     * specified collection's iterator.     *     * @param index index at which to insert the first element from the     *              specified collection     * @param c collection containing elements to be added to this list     * @return <tt>true</tt> if this list changed as a result of the call     * @throws IndexOutOfBoundsException {@inheritDoc}     * @throws NullPointerException if the specified collection is null     */    /**     * 该方法在ArrayList指定的索引处添加集合对象     * 将索引后的元素统一后移入参元素长度，并在索引处做元素拷贝     */    public boolean addAll(int index, Collection<? extends E> c) {        rangeCheckForAdd(index);        Object[] a = c.toArray();        int numNew = a.length;        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;    }    /**     * Removes from this list all of the elements whose index is between     * {@code fromIndex}, inclusive, and {@code toIndex}, exclusive.     * Shifts any succeeding elements to the left (reduces their index).     * This call shortens the list by {@code (toIndex - fromIndex)} elements.     * (If {@code toIndex==fromIndex}, this operation has no effect.)     *     * @throws IndexOutOfBoundsException if {@code fromIndex} or     *         {@code toIndex} is out of range     *         ({@code fromIndex < 0 ||     *          fromIndex >= size() ||     *          toIndex > size() ||     *          toIndex < fromIndex})     */    /**     * 该方法从ArrayList中删除一定范围内的元素；     * 1.对elementData做toIndex到fromIndex的元素拷贝；     * 2.将原elementData因拷贝而空出的元素赋值为NULL，调整size；     */    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;    }    /**     * 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.     */    /**     * 该方法索引范围的正确性，存在索引溢出则抛出IndexOutOfBoundsException；     */    private void rangeCheck(int index) {        if (index >= size)            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));    }    /**     * A version of rangeCheck used by add and addAll.     */    /**     * 该方法索引范围的正确性，存在索引溢出或index为负时则抛出IndexOutOfBoundsException；     */    private void rangeCheckForAdd(int index) {        if (index > size || index < 0)            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));    }    /**     * Constructs an IndexOutOfBoundsException detail message.     * Of the many possible refactorings of the error handling code,     * this "outlining" performs best with both server and client VMs.     */    /**     * 该方法构造outOfBoundsMsg字符串     */    private String outOfBoundsMsg(int index) {        return "Index: "+index+", Size: "+size;    }    /**     * Removes from this list all of its elements that are contained in the     * specified collection.     *     * @param c collection containing elements to be removed from this list     * @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">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">optional</a>),     *         or if the specified collection is null     * @see Collection#contains(Object)     */    /**     * Objects.requireNonNull(c):判断对象非NULL；     * 从此列表中移除包含在指定集合中的所有元素；     */    public boolean removeAll(Collection<?> c) {        Objects.requireNonNull(c);        return batchRemove(c, false);    }    /**     * Retains only the elements in this list that are contained in the     * specified collection.  In other words, removes from this list all     * of its elements that are not contained in the specified collection.     *     * @param c collection containing elements to be retained in this list     * @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">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">optional</a>),     *         or if the specified collection is null     * @see Collection#contains(Object)     */    /**     * Objects.requireNonNull(c):判断对象非NULL；     * 从此列表中保留包含在指定集合中的所有元素；     */    public boolean retainAll(Collection<?> c) {        Objects.requireNonNull(c);        return batchRemove(c, true);    }    /**     * c.contains(elementData[r]):判断集合中是否包含指定的元素；true，包含；false，不包含；     * elementData[w++] = elementData[r];：将待保留的元素前移；     * elementData[i] = null;：将元素前移后多余的空间清空赋值NULL，调整size；     */    private boolean batchRemove(Collection<?> c, boolean complement) {        final Object[] elementData = this.elementData;        int r = 0, w = 0;        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.            /**             * 确保异常抛出前的部分可以完成期望的操作             * r!=size原因：c.contains(elementData[r])可能会抛出异常             */            if (r != size) {                System.arraycopy(elementData, r,                                 elementData, w,                                 size - r);                w += size - r;            }            /**             * w!=size时，即使try块抛出异常，也能正确处理异常抛出前的操作，             * w始终为待保存的前段部分，数组不会因此乱序；             */            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;    }    /**     * Save the state of the <tt>ArrayList</tt> instance to a stream (that     * is, serialize it).     *     * @serialData The length of the array backing the <tt>ArrayList</tt>     *             instance is emitted (int), followed by all of its elements     *             (each an <tt>Object</tt>) in the proper order.     */    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();        }    }    /**     * Reconstitute the <tt>ArrayList</tt> instance from a stream (that is,     * deserialize it).     */    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();            }        }    }    /**     * Returns a list iterator over the elements in this list (in proper     * sequence), starting at the specified position in the list.     * The specified index indicates the first element that would be     * returned by an initial call to {@link ListIterator#next next}.     * An initial call to {@link ListIterator#previous previous} would     * return the element with the specified index minus one.     *     * <p>The returned list iterator is <a href="#fail-fast"><i>fail-fast</i></a>.     *     * @throws IndexOutOfBoundsException {@inheritDoc}     */    /**     * 该方法判断index返回的正确性，若index正确则根据index新建List迭代器     */    public ListIterator<E> listIterator(int index) {        if (index < 0 || index > size)            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)     */    /**     * 该方法新建一个List迭代器，索引值默认为0；     */    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     */    /**     * 该方法新建一个迭代器；     */    public Iterator<E> iterator() {        return new Itr();    }    /**     * An optimized version of AbstractList.Itr     */    /**     * ArrayList类的内部类，该类实现了Iterator接口；     * 该内部类为基类     */    // 迭代器三大方法 :    // hasNext 还有没有值可以迭代    // next 如果有值可以迭代， 迭代的值是多少    // remove 删除当前迭代的值    private class Itr implements Iterator<E> {        // 迭代过程中，下一个元素的位置，默认从 0 开始        int cursor;       // index of next element to return        // 新增场景 : 表示上一次迭代过程中， 索引的位置； 删除场景为 -1        int lastRet = -1; // index of last element returned; -1 if no such        // expectedModCount 表示迭代过程中， 期望的版本号； modCount 表示数组实际的版本号        int expectedModCount = modCount;        // 当下一个元素的索引不为size时，表明存在下一个元素；        public boolean hasNext() {            //cursor 表示下一个元素的位置， size 表示实际大小，             //如果两者相等， 说明已经没有元素可以迭代了， 如果不等， 说明还可以迭代            return cursor != size;        }        /**         * checkForComodification()检查fast-fail机制；         * 确保cursor，下个元素索引在ArrayList容量内并且该索引处存在元素值；         * 取ArrayList中下个元素值并返回；         */        // 主要功能 :        // 1. 检验能不能继续迭代        // 2. 找到迭代的值并为下一次迭代做准备 (cursor + 1)        @SuppressWarnings("unchecked")        public E next() {            // 迭代过程中， 判断版本号有无被修改， 有被修改，             // 抛 ConcurrentModificationException 异常            checkForComodification();            // 本次迭代过程中， 元素的索引位置            int i = cursor;            if (i >= size)                throw new NoSuchElementException();            Object[] elementData = ArrayList.this.elementData;            if (i >= elementData.length)                throw new ConcurrentModificationException();            // 下一次迭代时， 元素的位置， 为下一次迭代做准备            cursor = i + 1;            // 返回元素值            return (E) elementData[lastRet = i];        }        /**         * 该方法作用：删除列表迭代器刚刚访问的元素；         * if (lastRet < 0)：确保元素是刚刚访问的；         *          * checkForComodification()检查fast-fail机制；         *          * 方法内部通过ArrayList.this.remove(lastRet)对刚刚访问的元素进行删除操作；         */        // 注意点 :        // 1. lastRet = -1 的操作目的， 是防止重复删除操作        // 2. 删除元素成功， 数组当前 modCount 就会发生变化，         // 这里会把 expectedModCount 重新赋值， 下次迭代时两者的值就会一致了        public void remove() {            // 如果上一次操作时， 数组的位置已经小于 0 了， 说明数组已经被删除完了            if (lastRet < 0)                throw new IllegalStateException();            // 迭代过程中， 判断版本号有无被修改， 有被修改，             // 则抛 ConcurrentModificationException 异常            checkForComodification();            try {                ArrayList.this.remove(lastRet);                cursor = lastRet;                // -1 表示元素已经被删除， 这里也防止重复删除                lastRet = -1;                // 删除元素时 modCount 的值已经发生变化，在此赋值给 expectedModCount                // 这样下次迭代时，两者的值是一致的了                expectedModCount = modCount;            } catch (IndexOutOfBoundsException ex) {                throw new ConcurrentModificationException();            }        }        /**         * 该方法遍历ArrayList中的元素，并对其中的元素执行相应的操作；         */        @Override        @SuppressWarnings("unchecked")        public void forEachRemaining(Consumer<? super E> consumer) {            Objects.requireNonNull(consumer);            final int size = ArrayList.this.size;            int i = cursor;            if (i >= size) {                return;            }            final Object[] elementData = ArrayList.this.elementData;            if (i >= elementData.length) {                throw new ConcurrentModificationException();            }            /**             * 此处遍历elementData中的元素，并对元素执行consumer定义的操作             */            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();：重新对modCount做检查以确保不在             * foreach循环中进行过对modCount存在影响的操作；             */            checkForComodification();        }        /**         * 有两个线程（线程A，线程B），其中线程A负责遍历list、线程B修改list。          * 线程A在遍历list过程的某个时候（此时expectedModCount = modCount=N），线程启动，          * 同时线程B增加一个元素，这是modCount的值发生改变（modCount + 1 = N + 1）。          * 线程A继续遍历执行next方法时，          * 通告checkForComodification方法发现expectedModCount  = N，         * 而modCount = N + 1，两者不等，          * 这时就抛出ConcurrentModificationException 异常，从而产生fail-fast机制。         *          * modCount != expectedModCount时，抛出ConcurrentModificationException异常；         */        // 版本号比较        final void checkForComodification() {            if (modCount != expectedModCount)                throw new ConcurrentModificationException();        }    }    /**     * An optimized version of AbstractList.ListItr     */    /**     * 私有的内部类，该类继承Itr并实现了ListIterator接口     */    private class ListItr extends Itr implements ListIterator<E> {        //构造方法        ListItr(int index) {            super();            cursor = index;        }        //判断List是否存在前一个元素；        public boolean hasPrevious() {            return cursor != 0;        }        //获取List的下一个索引        public int nextIndex() {            return cursor;        }        //获取List中的前一个索引        public int previousIndex() {            return cursor - 1;        }        /**         * 获取List中的前一个元素         * checkForComodification();：检查fast-fail机制；         */        @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];        }        /**         * 该方法对刚刚访问的元素重新赋值；         * if (lastRet < 0)：确保元素是刚刚访问的；         * checkForComodification();：检查fast-fail机制；         */        public void set(E e) {            if (lastRet < 0)                throw new IllegalStateException();            checkForComodification();            try {                ArrayList.this.set(lastRet, e);            } catch (IndexOutOfBoundsException ex) {                throw new ConcurrentModificationException();            }        }        /**         * 该方法在迭代器的下一个索引处添加元素；         * checkForComodification();：检查fast-fail机制；         */        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();            }        }    }    /**     * Returns a view of the portion of this list between the specified     * {@code fromIndex}, inclusive, and {@code toIndex}, exclusive.  (If     * {@code fromIndex} and {@code toIndex} are equal, the returned list is     * empty.)  The returned list is backed by this list, so non-structural     * changes in the returned list are reflected in this list, and vice-versa.     * The returned list supports all of the optional list operations.     *     * <p>This method eliminates the need for explicit range operations (of     * the sort that commonly exist for arrays).  Any operation that expects     * a list can be used as a range operation by passing a subList view     * instead of a whole list.  For example, the following idiom     * removes a range of elements from a list:     * <pre>     *      list.subList(from, to).clear();     * </pre>     * Similar idioms may be constructed for {@link #indexOf(Object)} and     * {@link #lastIndexOf(Object)}, and all of the algorithms in the     * {@link Collections} class can be applied to a subList.     *     * <p>The semantics of the list returned by this method become undefined if     * the backing list (i.e., this list) is <i>structurally modified</i> in     * any way other than via the returned list.  (Structural modifications are     * those that change the size of this list, or otherwise perturb it in such     * a fashion that iterations in progress may yield incorrect results.)     *     * @throws IndexOutOfBoundsException {@inheritDoc}     * @throws IllegalArgumentException {@inheritDoc}     */    /**     * 该方法用于返回子List；     */    public List<E> subList(int fromIndex, int toIndex) {        subListRangeCheck(fromIndex, toIndex, size);        return new SubList(this, 0, fromIndex, toIndex);    }    /**     * 该方法检查子List的索引是否正确；     */    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 + ")");    }    /**     * 私有内部类，该类继承AbstractList并实现了RandomAccess接口     * 内部类可以使用外部类中的方法；     *      * RandomAccess接口说明：     * RandomAccess接口是一个标志接口     * 只要List集合实现这个接口，就能支持快速随机访问     * RandomAccess接口这个空架子的存在，是为了能够更好地判断集合是否ArrayList或者LinkedList，     * 从而能够更好选择更优的遍历方式，提高性能     */    private class SubList extends AbstractList<E> implements RandomAccess {        private final AbstractList<E> parent;        private final int parentOffset;        private final int offset;        int size;        /**         * SubList构造方法；         * SubList是在父类的基础上创建的，与父类共用一个内存空间；         */        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;        }        /**         * 该方法用于在SubSet指定的索引处，设置元素的值；         */        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;        }        /**         * 该方法用于获取SubSet指定的索引处元素的值；         */               public E get(int index) {            rangeCheck(index);            checkForComodification();            return ArrayList.this.elementData(offset + index);        }        /**         * 该方法返回SubList的大小；         * 此处的this指的是SubList类；         */        public int size() {            checkForComodification();            return this.size;        }        /**         * 该方法在SubList指定索引处添加元素；         * parent.add(parentOffset + index, e);：SubList与parent共用一段内存空间         */        public void add(int index, E e) {            rangeCheckForAdd(index);            checkForComodification();            parent.add(parentOffset + index, e);            this.modCount = parent.modCount;            this.size++;        }        /**         * 该方法删除SubList指定索引处的元素；         * parent.remove(parentOffset + index);：SubList与parent共用一段内存空间         */        public E remove(int index) {            rangeCheck(index);            checkForComodification();            E result = parent.remove(parentOffset + index);            this.modCount = parent.modCount;            this.size--;            return result;        }        /**         * 该方法删除SubList指定索引范围内的元素；         * parent.removeRange(parentOffset + fromIndex, parentOffset + toIndex);         * ：SubList与parent共用一段内存空间         */        protected void removeRange(int fromIndex, int toIndex) {            checkForComodification();            parent.removeRange(parentOffset + fromIndex,                               parentOffset + toIndex);            this.modCount = parent.modCount;            this.size -= toIndex - fromIndex;        }        /**         * 该方法在SubList的尾部添加集合元素；         */        public boolean addAll(Collection<? extends E> c) {            return addAll(this.size, c);        }        /**         * 该方法在SubList的指定索引处添加集合元素；         * parent.addAll(parentOffset + index, c);：SubList与parent共用一段内存空间         */        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;        }        /**         * 该方法返回一个迭代器对象；         */        public Iterator<E> iterator() {            return listIterator();        }        /**         * 该方法返回一个迭代器对象，迭代器在入参index的基础上创建；         */        public ListIterator<E> listIterator(final int index) {            checkForComodification();            rangeCheckForAdd(index);            //this：SubList            final int offset = this.offset;            /**             * ListIterator<E>(){}：匿名内部类             * return 返回了该匿名内部类的实例；             */            return new ListIterator<E>() {                int cursor = index;                int lastRet = -1;                int expectedModCount = ArrayList.this.modCount;                public boolean hasNext() {                    return cursor != SubList.this.size;                }                //迭代器获取SubList的下一个元素；                @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)];                }                //迭代器判断SubList是否存在前一个元素；                public boolean hasPrevious() {                    return cursor != 0;                }                //迭代器获取SubList的上一个元素；                @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)];                }                //迭代器遍历SubList的所有元素并执行相关的操作                @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();                    }                    /**                     * 此处遍历elementData中的元素，并对元素执行consumer定义的操作                     */                    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();                     * ：重新对modCount做检查以确保不在foreach循环中进行过对modCount存在影响的操作；                     */                    checkForComodification();                }                //迭代器下一个元素索引                public int nextIndex() {                    return cursor;                }                //迭代器上一个元素索引                public int previousIndex() {                    return cursor - 1;                }                //迭代器删除刚刚访问的元素                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();                    }                }                //迭代器用于对刚刚访问的元素设置值；                //ArrayList.this.set(offset + lastRet, e);                //：SubList中的set方法需要提供index入参；                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();                    }                }                //迭代器在迭代器处添加新的元素；                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();                    }                }                //迭代器检查fast-fail规则                final void checkForComodification() {                    if (expectedModCount != ArrayList.this.modCount)                        throw new ConcurrentModificationException();                }            };        }        //SubList类的subList方法，用于返回SubList的SubList，即返回子序列的子序列；        public List<E> subList(int fromIndex, int toIndex) {            subListRangeCheck(fromIndex, toIndex, size);            return new SubList(this, offset, fromIndex, toIndex);        }        //对SubList进行范围检查；        private void rangeCheck(int index) {            if (index < 0 || index >= this.size)                throw new IndexOutOfBoundsException(outOfBoundsMsg(index));        }        //对SubList进行范围检查，以便添加元素；        private void rangeCheckForAdd(int index) {            if (index < 0 || index > this.size)                throw new IndexOutOfBoundsException(outOfBoundsMsg(index));        }        //构建SubList的异常警告信息；        private String outOfBoundsMsg(int index) {            return "Index: "+index+", Size: "+this.size;        }        //SubList的fast-fail规则；        private void checkForComodification() {            if (ArrayList.this.modCount != this.modCount)                throw new ConcurrentModificationException();        }        /**         * 该方法创建ArrayListSpliterator实例         * Spliterator是一个可分割迭代器(splitable iterator)         * Spliterator就是为了并行遍历元素而设计的一个迭代器         */        public Spliterator<E> spliterator() {            checkForComodification();            return new ArrayListSpliterator<E>(ArrayList.this, offset,                                               offset + this.size, this.modCount);        }    }    /**     * 遍历SubList中的元素并对其执行相关的操作     * 操作不能改变modCount     */    @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();        }    }    /**     * Creates a <em><a href="Spliterator.html#binding">late-binding</a></em>     * and <em>fail-fast</em> {@link Spliterator} over the elements in this     * list.     *     * <p>The {@code Spliterator} reports {@link Spliterator#SIZED},     * {@link Spliterator#SUBSIZED}, and {@link Spliterator#ORDERED}.     * Overriding implementations should document the reporting of additional     * characteristic values.     *     * @return a {@code Spliterator} over the elements in this list     * @since 1.8     */    //返回ArrayListSpliterator迭代器，用于并发迭代处理    @Override    public Spliterator<E> spliterator() {        return new ArrayListSpliterator<>(this, 0, -1, 0);    }    /**     * static final：     * static修饰的属性强调它们只有一个；     * final修饰的属性表明是一个常数（创建后不能被修改）；     * static final修饰的属性表示一旦给值，就不可修改，并且可以通过类名访问；     *      * ArrayListSpliterator内部类，基于索引二分，迟加载的可分割迭代器，实现了Spliterator接口；     */    /** Index-based split-by-two, lazily initialized Spliterator */    static final class ArrayListSpliterator<E> implements Spliterator<E> {        /*         * If ArrayLists were immutable, or structurally immutable (no         * adds, removes, etc), we could implement their spliterators         * with Arrays.spliterator. Instead we detect as much         * interference during traversal as practical without         * sacrificing much performance. We rely primarily on         * modCounts. These are not guaranteed to detect concurrency         * violations, and are sometimes overly conservative about         * within-thread interference, but detect enough problems to         * be worthwhile in practice. To carry this out, we (1) lazily         * initialize fence and expectedModCount until the latest         * point that we need to commit to the state we are checking         * against; thus improving precision.  (This doesn't apply to         * SubLists, that create spliterators with current non-lazy         * values).  (2) We perform only a single         * ConcurrentModificationException check at the end of forEach         * (the most performance-sensitive method). When using forEach         * (as opposed to iterators), we can normally only detect         * interference after actions, not before. Further         * CME-triggering checks apply to all other possible         * violations of assumptions for example null or too-small         * elementData array given its size(), that could only have         * occurred due to interference.  This allows the inner loop         * of forEach to run without any further checks, and         * simplifies lambda-resolution. While this does entail a         * number of checks, note that in the common case of         * list.stream().forEach(a), no checks or other computation         * occur anywhere other than inside forEach itself.  The other         * less-often-used methods cannot take advantage of most of         * these streamlinings.         */        //私有常量，只能赋值一次，构造时赋值；         private final ArrayList<E> list;        //当前索引值        private int index; // current index, modified on advance/split        //当前List的最后一个索引值        private int fence; // -1 until used; then one past last index        //修改次数，用于fast-fail规则        private int expectedModCount; // initialized when fence set        /** Create new spliterator covering the given  range */        //构造方法        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;        }        //获取fence，初始化为list的size；        private int getFence() { // initialize fence to size on first use            int hi; // (a specialized variant appears in method forEach)            ArrayList<E> lst;            if ((hi = fence) < 0) {                if ((lst = list) == null)                    hi = fence = 0;                else {                    expectedModCount = lst.modCount;                    hi = fence = lst.size;                }            }            return hi;        }        /**         * 对任务分割，返回一个新的Spliterator迭代器         * ArrayListSpliterator：         * 采用二分的分割方式，返回lo--mid区间的Spliterator迭代器         */        public ArrayListSpliterator<E> trySplit() {            int hi = getFence(), lo = index, mid = (lo + hi) >>> 1;            return (lo >= mid) ? null : // divide range in half unless too small                new ArrayListSpliterator<E>(list, lo, index = mid,                                            expectedModCount);        }        /**         * 获取Spliterator迭代器区间，并对区间内的元素执行相应的操作；         */        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;        }        /**         * 对Spliterator迭代器区间中的元素执行相关操作；         */        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) {                //初始化时的情况，初始化时fence为list的size；                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);                    }                    //判断fast-fail规则；                    if (lst.modCount == mc)                        return;                }            }            throw new ConcurrentModificationException();        }        /**         * 用于估算还剩下多少个元素需要遍历         */        public long estimateSize() {            return (long) (getFence() - index);        }        /**         * 返回当前对象有哪些特征值         */                public int characteristics() {            return Spliterator.ORDERED | Spliterator.SIZED | Spliterator.SUBSIZED;        }    }    /**     * Predicate：判断输入的对象是否符合某个条件     */    @Override    public boolean removeIf(Predicate<? super E> 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 = 0;        /**         * Bitset类创建一种特殊类型的数组来保存位值         */        final BitSet removeSet = new BitSet(size);        final int expectedModCount = 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element满足相应的条件，则removeSet相应的位置位，removeCount加1；                removeSet.set(i);                removeCount++;            }        }        if (modCount != expectedModCount) {            throw new ConcurrentModificationException();        }        // shift surviving elements left over the spaces left by removed elements        final boolean anyToRemove = removeCount > 0;        //若存在需要删除的元素        if (anyToRemove) {            final int newSize = size - removeCount;            for (int i=0, j=0; (i < size) && (j < newSize); i++, j++) {                //返回下一个清除位的索引，即BitSet中未置位的索引对应需要保留元素                i = removeSet.nextClearBit(i);                elementData[j] = elementData[i];            }            //对删除元素后对应的索引位置处的元素清空，赋值为NULL；            for (int k=newSize; k < size; k++) {                elementData[k] = null;  // Let gc do its work            }            //更新List的size；            this.size = newSize;            //检查fast-fail规则            if (modCount != expectedModCount) {                throw new ConcurrentModificationException();            }            //该方法对list进行了改动，因此需要修改modCount；            modCount++;        }        return anyToRemove;    }    /**     * 该方法使用UnaryOperator定义的方法修改List中的元素值；     *      * UnaryOperator：一元运算，接受一个T类型参数，输出一个与入参一模一样的值      */    @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++) {            //遍历elementData，对elementData中的元素执行operator相关的操作，            //并重新赋值到list原来索引处；            elementData[i] = operator.apply((E) elementData[i]);        }        //检验fast-fail规则；        if (modCount != expectedModCount) {            throw new ConcurrentModificationException();        }        //改动了list，更新modCount；        modCount++;    }    /**     * 按照Comparator定义的规则对list进行排序；     *      * Comparator：     * 按指定的Comparator规则对List排序；     */    @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++;    }}

致谢

本博客为博主的学习实践总结，并参考了众多博主的博文，在此表示感谢，博主若有不足之处，请批评指正。