集合框架ArrayList 源码分析（一）

今天来分析一下Arraylist的一些源码解读，看一些源码以及学习一下设计者的思想我认为是很有必要的。

 

下面先分享一下官方文档，这也就是作者（设计者）所描述的一些注意点和思想，现在先简单介绍一下：后续会把一些自己的梳理和总结都会放到上面，希望大家可以共同进步，下面步入正题：

/**
 * 这是一个实现了list接口的可变长度的数组。实现所有可选列表操作，并允许存放所有元素，包括null。
 * Resizable-array implementation of the List interface.  Implements
 * all optional list operations, and permits all elements, including
 * null.
 * 除了实现list的接口所有方法，这个类提供了一些方法来操作内部用于存储列表数组的大小，
 * 此数组用来存储list的数据
 * In addition to implementing the List interface,
 * this class provides methods to manipulate the size of the array that is
 * used internally to store the list.
 * 这个类大致是相当于Vector，除了它是线程不安全的。
 * (This class is roughly equivalent to
 * Vector, except that it is unsynchronized.)
 * 

 * 

 * 
The size, isEmpty, get, set,
 * iterator, and listIterator
 * 使用的是一个恒定时间（一个方法具有恒定的执行时间的，也就是代码不会因为问题规模n的变化
 * 而发生变化，时间复杂度为o(1)）。
 * operations run in constant time.
 * add操作花费恒定分摊时间，即插入n个元素需要o(n)的时间。粗略的来说所有其他操作都以
 * 线性时间运行（即这些操作与元素的个数成线性关系，操作的时间复杂度o(n)）,这些操作与linkedlist相比
 * 常数因子较低。
 * The add operation runs in amortized constant time,
 * 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 LinkedList implementation.
 * 

 * 

 * 每个arraylist的实例都有一个容量
 * 
Each ArrayList instance has a capacity.
 * 容量是用于存储列表中元素数组的大小的
 * The capacity is the size of the array used to store the elements in the list.
 * 它至少和list的大小一样大
 * It is always at least as large as the list size.
 * 将元素添加到集合中
 * As elements are added to an ArrayList,
 * 它的容量会自动增加
 * its capacity grows automatically.
 * 除了要求添加一个元素的效率为“恒定分摊时间”，对于具体实现的细节没有特别的要求。
 * The details of the growth policy are not specified beyond the fact that adding an element has constant amortize time cost.
 * 

 * 在大批量插入元素前，使用ensureCapacity（）方法来增加集合的容量。这或许能够减少扩容增加量的大小。
 * An application can increase the capacity of an ArrayList instance
 * before adding a large number of elements using the ensureCapacity
 * operation.  This may reduce the amount of incremental reallocation.
 * 

 * 注意这个实现类是非同步的。如果有多个线程同时操作一个ArrayList的实例。
 * 然后，至少有一个线程修改了list的结构，就必须在外部保证它的线程同步。
 * (结构修改指的是增加或者删除一个或多个映射；如果仅仅是更改已经存在的
 * key和value值，不算做结构修改）。这通常需要在一个被封装好的list对象上，使用同步进行操作。
 * 
Note that this implementation is not synchronized.
 * If multiple threads access an ArrayList instance concurrently,
 * and at least one of the threads modifies the list structurally, it
 * must 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.
 * 

 * 

 * 

 * 

 * 如果没有这样的对象存在，那么就需要使用Collections.synchronizedList方法来包装这个list对象，
 * 而且最好是在创建对象的时候就进行包装，这是为了预防对这个list对象进行一些线程不同步的操作。
 * 举个例子：List list = Collections.synchronizedList(new ArrayList(...));
 * 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:
 *   List list = Collections.synchronizedList(new ArrayList(...));
 * 

 * 

 * 

 * 解释一下fail-fast机制
 * fail-fast，它是Java集合中的一种错误检测机制。某个线程在对collection进行迭代时，
 * 不允许其他线程对该collection进行结构上的修改。
 * 否则程序就会抛出ConcurrentModificationException 异常。快速终止操作
 * 

 * 该类的集合视图方法返回的迭代器是fail-fast机制的:在迭代器被创建后,如果list对象被结构化修改后,
 * 无论在何时,使用何种方法(除了迭代器本身的remove方法和add方法)来修改它,都会抛出ConcurrentModificationException.
 * 因此,面对并发修改操作时,迭代器会迅速且清晰地报错.而不是冒着在不确定的时间做不确定的操作的风险.
 * 

 * 

 * The iterators returned by this class's {@link #iterator() iterator} and
 * {@link #listIterator(int) listIterator} methods are fail-fast:
 * 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.
 * 

 * 

 * 

 * 注意,迭代器的fail-fast行为是不能保证的.一般来说,保证非同步的同步操作是不太可能的.
 * 在最优基础上,Fail-fast迭代器会抛出ConcurrentModificationException.
 * 因此,写一个为了自身正确性而依赖于这个异常的程序是不对的.
 * 迭代器的fail-fast行为应该只是用来检测bug而已.
 * 
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:  the fail-fast behavior of iterators
 * should be used only to detect bugs.
 * 

 * 
This class is a member of the
 * 
 * Java Collections Framework.
 *
 * @author Josh Bloch
 * @author Neal Gafter
 * @see Collection
 * @see List
 * @see LinkedList
 * @see Vector
 * @since 1.2
 */

先来说一下迭代器的fast-fail：目前先简单介绍一下，这里可能对于一些没看过源码的朋友，可能会有一些不是特别容易理解，这里可以简单理解为这是为多线程情况下导致线程不安全，可能会修改数据结构时进行的一种判断机制，用于检查数据结构是否发生改变 举个例子可能就明白了：那就是如果一个线程现在正在进行获取数据，遍历集合，另一个线程正在往里面进行添加数据，

这个时候，modcount就会发生变化，在另一个线程在遍历数组的时候，会记录一个expectedModCount 的初始值记录下来，每一次进行遍历元素的时候，都会进行比较，如果两个值进行比较不相同了，就会抛出一个ConcurrentModificationException异常。下面会详细介绍一下fast-fail机制的一些原理。

/*protected transient int modCount = 0;
   这是父类AbstractList的一个属性 ：用于记录列表结构被修改的次数。每次列表结构被修改都会modCount++
         为什么要记录此数据呢？
        在线程不安全的集合中，正如上面所说：迭代器采用了fail-fast机制。
        而fail-fast机制触发原理就是比对expectedModCount 和 modCount 是否相等，不相等就报ConcurrentModificationException异常
        此处不理解没关系,后面会讲迭代器方法的源码时，就会明白了*/

下面可以看下我在一些方法和实现的接口上进行的注释以及标注的内容，会比较容易理解一些，一些方法让我删减了一些

public class ArrayList extends AbstractList
        implements List, RandomAccess, Cloneable, java.io.Serializable {
    private static final long serialVersionUID = 8683452581122892189L;
    //1.RandomAccess：（标记接口）代表支持随机访问
    // 2.Cloneable：（标记接口）代表 Object.clone() 方法可以合法地对该类实例进行按字段复制。（
    //3.没有实现 Cloneable 接口的实例上调用 Object 的 clone 方法，则会导致抛出 CloneNotSupportedException 异常）
    //4.java.io.Serializable（标记接口）
    /**
     * 默认初始化的容量
     * Default initial capacity.
     */
    private static final int DEFAULT_CAPACITY = 10;

    /**
     * 指定该ArrayList容量为0时，返回该空数组
     * Shared empty array instance used for empty instances.
     */
    private static final Object[] EMPTY_ELEMENTDATA = {};

    /**
     * 用于默认大小的空实例的共享空数组实例。
     * 这个空数组的实例用来给无参构造使用。当调用无参构造方法，返回的是该数组。
     * 将此与EMPTY_ELEMENTDATA区分开来，以便了解在添加第一个元素时要增加多少容量。
     * Shared empty array instance used for default sized empty instances. We
     * distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when
     * first element is added.
     */
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

    /**
     * 存储ArrayList元素的数组缓冲区
     * ArrayList的容量(capacity)就是是此数组缓冲区的长度。
     * 声明为transient 不会被序列化
     * 非私有 是为了方便内部类调用
     * 

     * 

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

    /**
     * The size of the ArrayList (the number of elements it contains).
     *
     * @serial
     */
    private int size;

    /**
     * 构造具有指定初始容量的空列表
     * Constructs an empty list with the specified initial capacity.
     * initialCapacity列表的初始容量
     *
     * @param initialCapacity the initial capacity of the list
     *                        如果指定的值是负数则抛出异常
     * @throws IllegalArgumentException if the specified initial capacity
     *                                  is negative
     */
    public ArrayList(int initialCapacity) {
        if (initialCapacity > 0) {
            this.elementData = new Object[initialCapacity];
        } else if (initialCapacity == 0) {
            this.elementData = EMPTY_ELEMENTDATA;
        } else {
            throw new IllegalArgumentException("Illegal Capacity: " +
                    initialCapacity);
        }
    }

    /**
     * 构造一个初始容量为10的空列表
     * Constructs an empty list with an initial capacity of ten.
     */
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }

    /**
     * 构造一个包含指定集合元素的列表
     * 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
     */
    public ArrayList(Collection c) {
        elementData = c.toArray();
        if ((size = elementData.length) != 0) {
            // c.toArray might (incorrectly) not return Object[] (see 6260652)
            if (elementData.getClass() != Object[].class)
                elementData = Arrays.copyOf(elementData, size, Object[].class);
        } else {
            // replace with empty array.
            this.elementData = EMPTY_ELEMENTDATA;
        }
    }

    /**
     * Trims the capacity of this ArrayList instance to be the
     * list's current size.  An application can use this operation to minimize
     * the storage of an ArrayList instance.
     */
    public void trimToSize() {
        modCount++;
        if (size < elementData.length) {
            elementData = (size == 0)
                    ? EMPTY_ELEMENTDATA
                    : Arrays.copyOf(elementData, size);
        }
    }

    /**
     * Increases the capacity of this ArrayList 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 != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
                // any size if not default element table
                ? 0
                // larger than default for default empty table. It's already
                // supposed to be at default size.
                : DEFAULT_CAPACITY;

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

    private static int calculateCapacity(Object[] elementData, int minCapacity) {
        //因为如果是空的话，minCapacity=size+1；其实就是等于1，空的数组没有长度就存放不了，
        // 所以就将minCapacity变成10，也就是默认大小，到这里，还没有真正的初始化这个elementData的大小。
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            return Math.max(DEFAULT_CAPACITY, minCapacity);
        }
        return minCapacity;
    }

    private void ensureCapacityInternal(int minCapacity) {
        //用来得到一个数组的大小
        int num = calculateCapacity(elementData, minCapacity);
        //这个方法就是实现真正的判断，确认实际的容量，上面只是将minCapacity=10，这个方法就是真正的判断elementData是否够用
        ensureExplicitCapacity(num);
    }

    private void ensureExplicitCapacity(int minCapacity) {
        modCount++;

        // overflow-conscious code
        //判断是否通用，如何添加一个之后的数量大于当前数组的大小，则执行扩容操作
        //分为两种情况：1.第一次进行添加的时候，第一添加的时候，minCapacity是1，在上一个方法中，已经更改为了，已经默认返回数量10，，
        // 到这一步，还没有改变elementData的大小
        //第二种情况:elementData已经不是空数组了，那么在add的时候，minCapacity=size+1，也就是minCapacity代表着要和数组的大小进行比较，看minCapacity
        //和数组的长度进行比较，看数组的长度是否够用，如果够用直接返回添加就好了，如果不够用，需要执行扩容操作，不然增加的这个元素就会溢出。
        if (minCapacity - elementData.length > 0) {
            //扩容的关键方法所在
            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
        //扩容前的数组长度赋值给oldCapacity
        int oldCapacity = elementData.length;
        //newCapacity  后面的运算就是扩容前的数组长度1.5倍进行
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        //如果新的长度-默认容量<0;则把初始化的容量赋值给newCapacity
        if (newCapacity - minCapacity < 0) {
            newCapacity = minCapacity;
        }
        //如果扩容后的长度已经超过了最大的数组容量
        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) {
            throw new OutOfMemoryError();
        }// overflow
            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 true if this list contains no elements.
     *
     * @return true if this list contains no elements
     */
    public boolean isEmpty() {
        return size == 0;
    }

    /**
     * Returns true if this list contains the specified element.
     * More formally, returns true if and only if this list contains
     * at least one element e such that
     * (o==null ? e==null : o.equals(e)).
     *
     * @param o element whose presence in this list is to be tested
     * @return true 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 i such that
     * (o==null ? get(i)==null : o.equals(get(i))),
     * or -1 if there is no such index.
     */
    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 i such that
     * (o==null ? get(i)==null : o.equals(get(i))),
     * 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 ArrayList instance.  (The
     * elements themselves are not copied.)
     *
     * @return a clone of this ArrayList instance
     */
    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).
     * 

     * 
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.
     * 

     * 
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.
     * 

     * 
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
     * null.  (This is useful in determining the length of the
     * list only 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[] 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

    @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) {
        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}
     */
    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 true (as specified by {@link Collection#add})
     */
    public boolean add(E e) {
        //确定内部容量是否够用，size是数组中数据的个数，因为要添加一个元素，所以size+1，
        //先判断size+1这个数组能否放得下，就在这个方法中去判断是否Object[].length是否够用。
        ensureCapacityInternal(size + 1);  // Increments modCount!!
      /*  elementData[size]=e;
        size++;*/
        //执行赋值操作，size并进行加1，增加数组的大小
        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!!
        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);

        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
     * i such that
     * (o==null ? get(i)==null : o.equals(get(i)))
     * (if such an element exists).  Returns true 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 true 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++;

        // 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 true if this list changed as a result of the call
     * @throws NullPointerException if the specified collection is null
     */
    public boolean addAll(Collection 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 true 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 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})
     */
    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.
     */
    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
     *                              (optional)
     * @throws NullPointerException if this list contains a null element and the
     *                              specified collection does not permit null elements
     *                              (optional),
     *                              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
     *                              (optional)
     * @throws NullPointerException if this list contains a null element and the
     *                              specified collection does not permit null elements
     *                              (optional),
     *                              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) {
        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) {
                System.arraycopy(elementData, r,
                        elementData, w,
                        size - r);
                w += size - r;
            }
            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 ArrayList instance to a stream (that
     * is, serialize it).
     *
     * @serialData The length of the array backing the ArrayList
     * instance is emitted (int), followed by all of its elements
     * (each an Object) 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 ArrayList 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
            int capacity = calculateCapacity(elementData, size);
            SharedSecrets.getJavaOISAccess().checkArray(s, Object[].class, 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.
     * 

     * 
The returned list iterator is fail-fast.
     *
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public ListIterator 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).
     * 

     * 
The returned list iterator is fail-fast.
     *
     * @see #listIterator(int)
     */
    public ListIterator listIterator() {
        return new ListItr(0);
    }

    /**
     * Returns an iterator over the elements in this list in proper sequence.
     * 

     * 
The returned iterator is fail-fast.
     *
     * @return an iterator over the elements in this list in proper sequence
     */
    public Iterator iterator() {
        return new Itr();
    }

    /**
     * An optimized version of AbstractList.Itr
     */
    private class Itr implements Iterator {
        int cursor;       // index of next element to return
        int lastRet = -1; // index of last element returned; -1 if no such
        int expectedModCount = modCount;

        Itr() {
        }

        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)
                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 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();
            }
            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
     */
    private class ListItr extends Itr implements ListIterator {
        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)
                throw new NoSuchElementException();
            Object[] elementData = ArrayList.this.elementData;
            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();
            }
        }
    }
}

后续我会继续梳理一些关于集合框架的一些知识和原理以及为什么是线程不安全的？