ArrayList 源代码

有代码可以知道：ArrayList的扩容机制为：newCap = oldCap + ( oldCap >> 1)  也就是原来的1.5倍

/* * Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved. * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. * * * * * * * * * * * * * * * * * * * * */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 *//*List 接口的大小可变数组的实现。实现了所有可选列表操作，并允许包括 null 在内的所有元素。除了实现 List 接口外，此类还提供一些方法来操作内部用来存储列表的数组的大小。（此类大致上等同于 Vector 类，除了此类是不同步的。）size、isEmpty、get、set、iterator 和 listIterator 操作都以固定时间运行。add 操作以分摊的固定时间运行，也就是说，添加 n 个元素需要 O(n) 时间。其他所有操作都以线性时间运行（大体上讲）。与用于LinkedList 实现的常数因子相比，此实现的常数因子较低。每个 ArrayList 实例都有一个容量。该容量是指用来存储列表元素的数组的大小。它总是至少等于列表的大小。随着向 ArrayList 中不断添加元素，其容量也自动增长。并未指定增长策略的细节，因为这不只是添加元素会带来分摊固定时间开销那样简单。在添加大量元素前，应用程序可以使用 ensureCapacity 操作来增加 ArrayList 实例的容量。这可以减少递增式再分配的数量。注意，此实现不是同步的。如果多个线程同时访问一个 ArrayList 实例，而其中至少一个线程从结构上修改了列表，那么它必须 保持外部同步。（结构上的修改是指任何添加或删除一个或多个元素的操作，或者显式调整底层数组的大小；仅仅设置元素的值不是结构上的修改。）这一般通过对自然封装该列表的对象进行同步操作来完成。如果不存在这样的对象，则应该使用 Collections.synchronizedList 方法将该列表“包装”起来。这最好在创建时完成，以防止意外对列表进行不同步的访问：List list = Collections.synchronizedList(new ArrayList(...)); 此类的 iterator 和 listIterator 方法返回的迭代器是快速失败的：在创建迭代器之后，除非通过迭代器自身的 remove 或 add 方法从结构上对列表进行修改，否则在任何时间以任何方式对列表进行修改，迭代器都会抛出 ConcurrentModificationException。因此，面对并发的修改，迭代器很快就会完全失败，而不是冒着在将来某个不确定时间发生任意不确定行为的风险。注意，迭代器的快速失败行为无法得到保证，因为一般来说，不可能对是否出现不同步并发修改做出任何硬性保证。快速失败迭代器会尽最大努力抛出 ConcurrentModificationException。因此，为提高这类迭代器的正确性而编写一个依赖于此异常的程序是错误的做法：迭代器的快速失败行为应该仅用于检测 bug。此类是 Java Collections Framework 的成员 */public class ArrayList<E> extends AbstractList<E>        implements List<E>, RandomAccess, Cloneable, java.io.Serializable{    private static final long serialVersionUID = 8683452581122892189L;    /**     * Default initial capacity.     */    //默认容器容量为10    private static final int DEFAULT_CAPACITY = 10;    /**     * Shared empty array instance used for empty instances.     */    //空的数组    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.     */    //数组缓冲    transient Object[] elementData; // non-private to simplify nested class access    /**     * The size of the ArrayList (the number of elements it contains).     *     * @serial     */    //容器存储的数量    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     */    //构造函数，传入默认容量    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.     */    //构造函数    public ArrayList() {        super();        //默认为空数组        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     */    //构造函数，传入容器，将容器的元素赋到缓冲中    public ArrayList(Collection<? extends E> c) {        elementData = c.toArray();        size = elementData.length;        // c.toArray might (incorrectly) not return Object[] (see 6260652)        //c.toArray可能不是返回Object[]类型        if (elementData.getClass() != Object[].class)            elementData = Arrays.copyOf(elementData, size, Object[].class);    }    /**     * 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.     */    //将此 ArrayList 实例的容量调整为列表的当前大小    public void trimToSize() {        //每次修改modCount++        modCount++;        if (size < elementData.length) {            //实际有存储数据的容量小于数组的容量            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     */    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);        }    }    private void ensureCapacityInternal(int minCapacity) {        if (elementData == EMPTY_ELEMENTDATA) {            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);        }        ensureExplicitCapacity(minCapacity);    }    private void ensureExplicitCapacity(int minCapacity) {        modCount++;        // overflow-conscious code        //真正需要扩展        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     */    private void grow(int minCapacity) {        // overflow-conscious code        int oldCapacity = elementData.length;        //扩容newCapacity = (int) ( 1.5 * oldCapacity )        int newCapacity = oldCapacity + (oldCapacity >> 1);        if (newCapacity - minCapacity < 0)            //最小容量大于新扩展的容量            newCapacity = minCapacity;        if (newCapacity - MAX_ARRAY_SIZE > 0)            newCapacity = hugeCapacity(minCapacity);        // minCapacity is usually close to size, so this is a win:        elementData = Arrays.copyOf(elementData, newCapacity);    }    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     */    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     */    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 ? e==null : 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     */    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 ? get(i)==null : o.equals(get(i)))</tt>,     * or -1 if there is no such index.     */    public int indexOf(Object o) {        if (o == null) {            //null时用==比较            for (int i = 0; i < size; i++)                if (elementData[i]==null)                    return i;        } else {            //不是null得用equals比较            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 ? get(i)==null : o.equals(get(i)))</tt>,     * or -1 if there is no such index.     */    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     */    public Object clone() {        try {            //克隆ArrayList            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     */    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     */    @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());        //如果a容量比size大        System.arraycopy(elementData, 0, a, 0, size);        if (a.length > size)            a[size] = null;        return a;    }    // Positional Access Operations    @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}     */    public E get(int index) {        //检测下标是否越界        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}     */    //替换元素    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})     */    public boolean add(E e) {        //每次添加都需要将modCount++        ensureCapacityInternal(size + 1);  // Increments modCount!!        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}     */    public void add(int index, E element) {        rangeCheckForAdd(index);        ensureCapacityInternal(size + 1);  // Increments modCount!!        //后面元素往后移        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}     */    public E remove(int index) {        rangeCheck(index);        modCount++;        E oldValue = elementData(index);        //需要移动的个数        int numMoved = size - index - 1;        if (numMoved > 0)            System.arraycopy(elementData, index+1, elementData, index,                             numMoved);        //设为null让垃圾回收器回收        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 ? 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     * @return <tt>true</tt> if this list contained the specified element     */    public boolean remove(Object o) {        if (o == null) {            for (int index = 0; index < size; index++)                if (elementData[index] == null) {                    fastRemove(index);                    return true;                }        } else {            for (int index = 0; index < size; index++)                if (o.equals(elementData[index])) {                    fastRemove(index);                    return true;                }        }        return false;    }    /*     * Private remove method that skips bounds checking and does not     * return the value removed.     */    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    }    /**     * Removes all of the elements from this list.  The list will     * be empty after this call returns.     */    public void clear() {        modCount++;        //设为null用于垃圾回收        // 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     */    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     */    public boolean addAll(int index, Collection<? extends E> c) {        rangeCheckForAdd(index);        Object[] a = c.toArray();        int numNew = a.length;        ensureCapacityInternal(size + numNew);  // Increments modCount        //需要移动        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})     */    protected void removeRange(int fromIndex, int toIndex) {        modCount++;        int numMoved = size - toIndex;        System.arraycopy(elementData, toIndex, elementData, fromIndex,                         numMoved);        //设为null 垃圾回收        // 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.     */    private void rangeCheck(int index) {        if (index >= size)            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));    }    /**     * A version of rangeCheck used by add and addAll.     */    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.     */    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)     */    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)     */    public boolean retainAll(Collection<?> c) {        Objects.requireNonNull(c);        return batchRemove(c, true);    }    private boolean batchRemove(Collection<?> c, boolean complement) {        //complement == true 表示 retainAll        //complement == false 表示 removeAll        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.            if (r != size) {                //没有遍历一遍elementData,将剩下的复制过来                System.arraycopy(elementData, r,                                 elementData, w,                                 size - r);                w += size - r;            }            if (w != size) {                //将删除的设为null,垃圾回收                // 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()        //写容量size        s.writeInt(size);        // Write out all elements in the proper order.        //写元素        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.            //读取每个元素            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}     */    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)     */    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     */    //ArrayList自己特有的迭代器    private class Itr implements Iterator<E> {        //光标        int cursor;       // index of next element to return        //最后返回的元素的下标        int lastRet = -1; // index of last element returned; -1 if no such        int expectedModCount = modCount;        public boolean hasNext() {            return cursor != size;        }        @SuppressWarnings("unchecked")        public E next() {            checkForComodification();            int i = cursor;            if (i >= size)                throw new NoSuchElementException();            Object[] elementData = ArrayList.this.elementData;            if (i >= elementData.length)                //有并发修改                throw new ConcurrentModificationException();            cursor = i + 1;            return (E) elementData[lastRet = i];        }        public void remove() {            if (lastRet < 0)                throw new IllegalStateException();            checkForComodification();            try {                ArrayList.this.remove(lastRet);                cursor = lastRet;                lastRet = -1;                expectedModCount = modCount;            } catch (IndexOutOfBoundsException ex) {                throw new ConcurrentModificationException();            }        }        @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) {                //没有元素了                return;            }            final Object[] elementData = ArrayList.this.elementData;            if (i >= elementData.length) {                throw new ConcurrentModificationException();            }            while (i != size && modCount == expectedModCount) {                consumer.accept((E) elementData[i++]);            }            // update once at end of iteration to reduce heap write traffic            cursor = i;            lastRet = i - 1;            checkForComodification();        }        final void checkForComodification() {            if (modCount != expectedModCount)                throw new ConcurrentModificationException();        }    }    /**     * An optimized version of AbstractList.ListItr     */    //ArrayList特有的ListIterator    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;        }        @SuppressWarnings("unchecked")        public E previous() {            checkForComodification();            int i = cursor - 1;            if (i < 0)                //i越界了                throw new NoSuchElementException();            Object[] elementData = ArrayList.this.elementData;            //检测i            if (i >= elementData.length)                throw new ConcurrentModificationException();            cursor = i;            return (E) elementData[lastRet = i];        }        public void set(E e) {            if (lastRet < 0)                throw new IllegalStateException();            checkForComodification();            try {                ArrayList.this.set(lastRet, e);            } catch (IndexOutOfBoundsException ex) {                throw new ConcurrentModificationException();            }        }        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}     */    public List<E> subList(int fromIndex, int toIndex) {        subListRangeCheck(fromIndex, toIndex, size);        return new SubList(this, 0, fromIndex, toIndex);    }    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 + ")");    }    private class SubList extends AbstractList<E> implements RandomAccess {        private final AbstractList<E> parent;        private final int parentOffset;        private final int offset;        int size;        SubList(AbstractList<E> parent,                int offset, int fromIndex, int toIndex) {            this.parent = parent;            this.parentOffset = fromIndex;            this.offset = offset + fromIndex;            this.size = toIndex - fromIndex;            this.modCount = ArrayList.this.modCount;        }        public E set(int index, E e) {            rangeCheck(index);            checkForComodification();            E oldValue = ArrayList.this.elementData(offset + index);            ArrayList.this.elementData[offset + index] = e;            return oldValue;        }        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);        }    }    @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     */    @Override    public Spliterator<E> spliterator() {        return new ArrayListSpliterator<>(this, 0, -1, 0);    }    /** 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.         */        private final ArrayList<E> list;        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() { // 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;        }        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);        }        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();        }        public long estimateSize() {            return (long) (getFence() - index);        }        public int characteristics() {            return Spliterator.ORDERED | Spliterator.SIZED | Spliterator.SUBSIZED;        }    }    @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;        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)) {                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;        if (anyToRemove) {            final int newSize = size - removeCount;            for (int i=0, j=0; (i < size) && (j < newSize); i++, j++) {                i = removeSet.nextClearBit(i);                elementData[j] = elementData[i];            }            for (int k=newSize; k < size; k++) {                elementData[k] = null;  // Let gc do its work            }            this.size = newSize;            if (modCount != expectedModCount) {                throw new ConcurrentModificationException();            }            modCount++;        }        return anyToRemove;    }    @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++;    }}