本人在校小菜鸟，目前对源码得分析更着重于逻辑得分解剖析，更深层的架构设计可能我并不能一一都能分析出来。本文会用源码图片的方式制作图解，一览代码的设计过程，思路一定要清晰。更适合我这个小菜鸟以后复盘。

下面是本文借鉴的原博文作者和文章，站在巨人肩膀上，在此感谢。

作者 HelloWorld_EE :文章:https://blog.csdn.net/u010412719/article/details/51108357

一、属性

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable

ArrayList类主要是继承AbstractList类并实现了List接口，实现Cloneable和Serializable接口使得ArrayList具有克隆和序列化的功能。

private static final long serialVersionUID = 8683452581122892189L;

/**
 * 默认初始容量大小
 */
private static final int DEFAULT_CAPACITY = 10;

/**
 * Shared empty array instance used for empty instances.
 * 共享式的空数组实例，用于空实例
 */
private static final Object[] EMPTY_ELEMENTDATA = {};

/**
 * Shared empty array instance used for default sized empty instances.
 * 共享式的空数组实例，用于有默认大小的空实例（其实调用无参构造器就会使用到这个空数组实例）
 *  翻译：与 EMPTY_ELEMENTDATA 数组的区别在于当第一个元素被加入进来的时候它知道如何扩张；如何扩张这一点在 add 方法中会充分体现
 */
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

/**
 * 存储ArrayList元素的数组缓冲区。
 * ArrayList的容量是这个数组缓冲区的长度。
 * 添加第一个元素时，任何elementData==DEFAULTCAPACITY_empty_elementData的空ArrayList实例都将扩展为默认的DEFAULT_CAPACITY。
 */
transient Object[] elementData; // 非私有以简化嵌套类访问

/**
 * The size of the ArrayList (the number of elements it contains).
 * ArrayList 实例的元素个数
 * @serial
 */
private int size;

二、构造函数

2.1 无参构造函数

/**
 * Constructs an empty list with an initial capacity of ten.
 * 构造一个初始容量为10的，常量空数组。
 */
public ArrayList() {
    this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}

源码上介绍的功能为：构造一个初始容量为 10 的空列表。

即当我们不提供参数而new一个对象时，底层的数组就是直接用长度为10 的空常量数组DEFAULTCAPACITY_EMPTY_ELEMENTDATA进行实例化。

可能有的人会问，此时DEFAULTCAPACITY_EMPTY_ELEMENTDATA的长度我们还不知道呀，从哪里可以看到是构造了一个初始容量为10的空列表呢？这里我们先不解释，在下面的add(E e)函数源码的介绍中会给出答案。

2.2 带参构造函数

参数中指定容量

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);
    }
}

当参数小于0时，抛异常。

当参数等于0时，用空的常量数组对象EMPTY_ELEMENTDATA来初始化底层数组elementData。

当参数大于0时，new 一个指定容量的数组。默认初始化容量就是参数中指定的。

参数中指定Collection容器

public ArrayList(Collection<? extends E> c) {
    elementData = c.toArray();
    if ((size = elementData.length) != 0) {
        // c.toArray might (incorrectly) not return Object[] (see 6260652)
        if (elementData.getClass() != Object[].class)
            elementData = Arrays.copyOf(elementData, size, Object[].class);
    } else {
        // replace with empty array.
        this.elementData = EMPTY_ELEMENTDATA;
    }
}

将容器Collection转化为数组赋给数组elementData，还对Collection转化是否转化为了Object[]进行了检查判断。如果Collection为空，则就将空的常量数组对象EMPTY_ELEMENTDATA赋给了elementData；

三、add()方法与数组扩容

首先我们看一下源码中的方法定义：

public boolean add(E e) {
    ensureCapacityInternal(size + 1);  // Increments modCount!!
    elementData[size++] = e;
    return true;
}

 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++;
    }

在上面的add方法中，需要注意的是

ensureCapacityInternal(size + 1);

这个方法的调用，每次都会在添加之前调用。那么这个方法是干什么的呢？

其实，这个方法的大致用意就是 “确定容量” ，因为我们正要添加一个元素嘛，为了保证底层数组不越界，那么在添加之前，必须保证目前数组的容量够用。

接下来就是数组自动扩容机制了。

其实我们知道，底层数组一旦创建就指定了初始大小，所谓的扩容，其实就是再开辟一个新的更大的数组，并拷贝原数组的数据。

那么，底层它是如何实现自动扩容的呢？扩容时应该扩大到多少才行？

带着这些问题，继续看下去吧。

private void ensureCapacityInternal(int minCapacity) {
    ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
}

第一步：确定容量的内部方法
调用 calculateCapacity() ,计算容量。会根据当前 elementData是一个如何构造的数组，来返回一个合适的容量。（详情请看下一步）
调用 ensureExplicitCapacity( ) 方法。

private static int calculateCapacity(Object[] elementData, int minCapacity) {
    if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
        return Math.max(DEFAULT_CAPACITY, minCapacity);
    }
    return minCapacity;
}

第二步：计算容量。
该方法的用意是：判断当前的 elementData数组实例，是如何创建的。来确定合适的扩容容量。
- 如果是以，defaultCapacity_Empty_ElementData 创建的实例，那么比较新容量 minCapacity 和默认容量 10 的大小。谁大谁就是新容量。

从源码可以看出，如果elementData==DEFAULTCAPACITY_EMPTY_ELEMENTDATA，则在默认容量10的基础上开辟空间。

这里的源码就解释了DEFAULTCAPACITY_EMPTY_ELEMENTDATA数组和EMPTY_ELEMENTDATA数组的区别之所在。

相似：这两个写法好像，都带有 EMPTY_ELEMENTDATA ,这点说明它俩在初始化时都是空数组，容量为0；

不同：DEFAULTCAPACITY_EMPTY_ELEMENTDATA在添加第一个元素时使用的扩容机制会和 10 比较大小，然后选择一个合适的新容量。

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

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

第三步：确定一个明确的容量
一是判断新容量是否超出了原来的范围
二是，超出的话，调用 grow(minCapacity);方法，进行容量的扩大。（该方法的详解请看下一步)

private void grow(int minCapacity) {
    // overflow-conscious code
    int oldCapacity = elementData.length;
    int newCapacity = oldCapacity + (oldCapacity >> 1);
    if (newCapacity - minCapacity < 0)
        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);
}

第四步：容量增长机制（重要！！）
首先，第一个 if 语句：将原数组扩大 1.5 倍，判断一下能不能满足需求
如果不能满足我们需要的空间 minCapacity ，则就扩大到 minCapacity好了。
第二个 if 语句：判断你的需求容量是否超出了可接受的最大值
如果超出，就是你想要的太大了，我不能无休止的满足你，调用 hugeCapacity(minCapacity); 返回 Integer.MAX_VALUE
最后， Arrays.copyOf ( ),可见扩容的本质是开辟新空间，并拷贝原数据

从源码中可以看到，此函数的功能就是一个数组的扩张。一般情况下是扩展到原来数组长度的1.5倍。
但是，由于扩张1.5倍可能和我们的需要不一致，即可能太小，也可能太大。

因此，就有了源码中的两个 if 条件的处理。即如果扩张1.5倍太小，则就用我们需要的空间大小minCapacity来进行扩张。如果扩张1.5倍太大或者是我们所需的空间minCapacity也太大，大到超出了可接受范围，则只能给你一个我能接受的最大数 Integer.MAX_VALUE来进行扩张。

private static int hugeCapacity(int minCapacity) {
    if (minCapacity < 0) // overflow
        throw new OutOfMemoryError();
    return (minCapacity > MAX_ARRAY_SIZE) ?
        Integer.MAX_VALUE :
        MAX_ARRAY_SIZE;
}

该方法，用来返回一个巨大容量值，参数指定一个想要的容量，

如果容量过头了，只能提供 Integer.MAX_VALUE,

如果没有过头，则返回可接受的最大容量 MAX_ARRAY_SIZE

MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

如果干看代码逐句分析还是一团乱码，下面的一张图可以帮你快速回忆起整个逻辑过程

抽象总结：

add() 调用之前先检查容量
需要扩容时，检查默认1.5倍扩容能不能满足你，不能满足就按着你的需求扩张，但是你的需求不能超出 MAX_ARRAY_SIZE 。

补充：上面grow函数中最后一条语句elementData = Arrays.copyOf(elementData, newCapacity);的功能就是将原来的数组中的元素复制扩展到大小为newCapacity的新数组中，并返回这个新数组。Arrays.copyOf(elementData, newCapacity)的源码如下：

四、其他常用方法

get()

public E get(int index) {
    rangeCheck(index); //检查下标

    return elementData(index);
}

set(int index, E element)

修改ArrayList对象中某个位置元素的值的大小

public E set(int index, E element) {
    rangeCheck(index);

    E oldValue = elementData(index);//获取指定下标的原来数据
    elementData[index] = element;
    return oldValue;
}

E elementData(int index) {
    return (E) elementData[index];
}

remove (int index)

删除元素的方法有两个重载方法

public E remove(int index) ：删除指定索引的元素
public boolean remove(Object o) ：删除匹配指定值 o 的元素（只删除一个匹配元素）

public E remove(int index) {
    rangeCheck(index);

    modCount++;
    E oldValue = elementData(index);

    int numMoved = size - index - 1;
    if (numMoved > 0)
        System.arraycopy(elementData, index+1, elementData, index,
                         numMoved);
    elementData[--size] = null; // clear to let GC do its work

    return oldValue;
}

可见，该删除方法，删除指定索引的元素。底层使用拷贝机制。(即将数组元素从index+1位置开始到末尾拷贝到从index开始处)

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);//这是什么？
                return true;
            }
    }
    return false;
}

可见，该删除方法是删除第一个值与参数 o 值相等的元素

/*
 * 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
}

原以为fastRemove(int index)这个函数与remove(int index)会有很大的不同，看到源码之后，发现其实区别真心不大，唯一的区别是fastRemove函数没有对index进行有效性检查，以及没有返回移除的旧值，为什么不返回呢？这是因为remove(Object o)给出的就是要删除的值，因此不返回旧值是非常正常的

clear() ：清空

直接将数组中的所有元素设置为null即可，这样便于垃圾回收。

public void clear() {
    modCount++;

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

    size = 0;
}

五、ArrayList并发修改异常问题

ArrayList可以存储null值

原因分析

普通迭代器在迭代期间不允许修改元素
ArrayList每次修改（增加、删除）容器时，都是修改自身的modCount属性值 modCount++；
在生成迭代器时，迭代器会保存该modCount值，迭代器每次获取元素时，会比较自身的modCount与ArrayList的modCount是否相等，来判断容器是否已经被修改，如果被修改了则抛出异常（fast-fail机制）表示迭代器modCount和实际修改值不一致，则会出现：ConcurrentModificationException

数组添加、删除时，modCount都会增长
在这里插入图片描述

注释：在查看内部类 Itr
源码时，发现该迭代器竟然没有自己的add方法、但是却有个remove方法。如果使用迭代器自带的remove方法，在删除完集合元素后，有一段
expectedModCount = modCount;的代码同步了修改值，不晓得为什么不把add方法也整一个。

解决的方案

用 普通 for循环遍历，然后用集合对象做对应的操作即可,切记不可以用增强 for 循环
或者使用专有迭代器ListIterator

get() 方法在获取元素时，并没有检查集合修改次数是否有变动。
在这里插入图片描述

ListIterator 介绍

通过 List集合的 listIterator()方法得到，它是List集合特有的迭代器
允许从后往前遍历
允许在迭代期间修改列表，并获取列表中迭代器的当前位置

六、源码详细

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

    /**
     * 初始容量
     */
    private static final int DEFAULT_CAPACITY = 10;

    /**
     * 空的常量数组
     */
    private static final Object[] EMPTY_ELEMENTDATA = {};

    /**
     * 空的常量数组，添加第一个元素时自动扩容成10
     */
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

    /**
     * 存放元素的缓冲数组，当有元素添加时超出容量可以自动扩容
     */
    transient Object[] elementData; // non-private to simplify nested class access

    /**
     * 元素个数
     */
    private int size;

    /**
     * 构造器，参数中指定容量大小
     */
    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);
        }
    }

    /**
     * 无参构造器，注意：初始的是一个特殊的空数组
     */
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }

    /**
     * 构造器，使用其他集合元素集拷贝一份新的数组
     */
    public ArrayList(Collection<? extends E> c) {
        elementData = c.toArray();
        if ((size = elementData.length) != 0) {
            // c.toArray might (incorrectly) not return Object[] (see 6260652)
            if (elementData.getClass() != Object[].class)
                elementData = Arrays.copyOf(elementData, size, Object[].class);
        } else {
            // replace with empty array.
            this.elementData = EMPTY_ELEMENTDATA;
        }
    }

    /**
	 * 因为容量基本会大于实际元素的数量。内存紧张时，可以调用该方法调整容量为元素实际数量。
	 * 如果确定不会有元素添加进来时也可以调用该方法来节约空间
	 */
    public void trimToSize() {
        modCount++;
        if (size < elementData.length) {
            elementData = (size == 0)
              ? EMPTY_ELEMENTDATA
              : Arrays.copyOf(elementData, size);
            //若size大于0，则获取当前数组的实际元素个数，重新拷贝复制给elementData
        }
    }

    /**
     *  如有必要，增加这个<tt>ArrayList</tt>实例的容量，
     * 以确保它至少可以容纳minimum capacity参数指定的元素数
     *
     * @param   期望的最小容量
     */
    public void ensureCapacity(int minCapacity) {
        int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
            // 如果不是默认元素表，则为任意大小
            ? 0
            // 如果是，使用默认为10的容量扩张
            : DEFAULT_CAPACITY;
         // 若参数大于预设的容量，再使用该参数进一步设置数组容量
        if (minCapacity > minExpand) {
            ensureExplicitCapacity(minCapacity);
        }
    }

    private static int calculateCapacity(Object[] elementData, int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            return Math.max(DEFAULT_CAPACITY, minCapacity);
        }
        return minCapacity;
    }

    private void ensureCapacityInternal(int minCapacity) {
        ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
    }

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

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

    /**
     * 数组的最大容量，可能会导致内存溢出
     */
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

    /**
     * 数组的扩容机制
     */
    private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        if (newCapacity - minCapacity < 0)
            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);
    }

    /**
    * 容量保护机制，防止内存溢出
    */
    private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) // overflow
            throw new OutOfMemoryError();
        return (minCapacity > MAX_ARRAY_SIZE) ?
            Integer.MAX_VALUE :
            MAX_ARRAY_SIZE;
    }

    /**
     *返回集合中的元素个数
     */
    public int size() {
        return size;
    }

    /**
     * 判断是否为空
     */
    public boolean isEmpty() {
        return size == 0;
    }

    /**
     * 是否包含某个元素
     */
    public boolean contains(Object o) {
        return indexOf(o) >= 0;
    }

    /**
     * 返回元素所在的下标（第一次出现时的）
     */
    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;
    }

    /**
     * 返回元素所在的下标（最后一次出现时的）
     */
    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;
    }

    /**
     * 获得当前集合数据的一份拷贝
     */
    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);
        }
    }

    /**
     * 集合转为数组
     */
    public Object[] toArray() {
        return Arrays.copyOf(elementData, size);
    }

    /**
     * 集合转为数组（指定泛型）
     */
    @SuppressWarnings("unchecked")
    public <T> T[] toArray(T[] a) {
        if (a.length < size)
            // Make a new array of a's runtime type, but my contents:
            return (T[]) Arrays.copyOf(elementData, size, a.getClass());
        System.arraycopy(elementData, 0, a, 0, size);
        if (a.length > size)
            a[size] = null;
        return a;
    }

    // 返回指定下标的元素

    @SuppressWarnings("unchecked")
    E elementData(int index) {
        return (E) elementData[index];
    }

    /**
     * 返回指定下标的元素
     */
    public E get(int index) {
        rangeCheck(index);

        return elementData(index);
    }

    /**
     * 修改集合指定下标的元素，返回旧的值
     */
    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.
     */
    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }

    /**
     * index位置添加元素element，会检查添加的位置和容量
     * 原来的元素后移
     */
    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++;
    }

    /**
     * 删除指定下标的元素，并返回该元素
     */
    public E remove(int index) {
        rangeCheck(index);

        modCount++;
        E oldValue = elementData(index);

        int numMoved = size - index - 1;
        if (numMoved > 0)
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
        elementData[--size] = null; // clear to let GC do its work

        return oldValue;
    }

    /**
     * 删除指定元素
     */
    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;
    }

    /*
     * 私有方法，已经跳过了下标范围检查
     * 返回已删除元素
     */
    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
    }

    /**
     *删除所有元素
     */
    public void clear() {
        modCount++;

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

        size = 0;
    }

    /**
     * 从指定集合添加所有它的元素到当前
     */
    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;
    }

   /**
	 * 从第index位开始，将c全部拷贝到ArrayList
	 * @param index  在哪个索引开始插入
	 * @return true 如果该列表因添加而改变
	 */
    public boolean addAll(int index, Collection<? extends E> c) {
        rangeCheckForAdd(index);

        Object[] a = c.toArray();
        int numNew = a.length;
        ensureCapacityInternal(size + numNew);  // Increments modCount

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

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

    /**
     * 删除指定下标范围的元素
     */
    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;
    }

    /**
     * 下标范围是否越界检查
     */
    private void rangeCheck(int index) {
        if (index >= size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    /**
     * 下标范围检查 used by add and addAll.
     */
    private void rangeCheckForAdd(int index) {
        if (index > size || index < 0)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    /**
     * 返回下标越界的消息
     */
    private String outOfBoundsMsg(int index) {
        return "Index: "+index+", Size: "+size;
    }

    /**
	* 删除指定集合中的所有元素
     */
    public boolean removeAll(Collection<?> c) {
        Objects.requireNonNull(c);
        return batchRemove(c, false);
    }

    /**
     * 取两个集合的交集
     */
    public boolean retainAll(Collection<?> c) {
        Objects.requireNonNull(c);
        return batchRemove(c, true);
    }
    /**
	 * 根据complement值，将ArrayList中包含c中元素的元素删除或者保留
	 * 
	 * @param c
	 * @param complement
	 *            true时从数组保留指定集合中元素的值，为false时从数组删除指定集合中元素的值。
	 * @return 数组中重复的元素都会被删除(而不是仅删除一次或几次)，有任何删除操作都会返回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 <tt>ArrayList</tt> instance to a stream (that
     * is, serialize it).
     *
     * @serialData The length of the array backing the <tt>ArrayList</tt>
     *             instance is emitted (int), followed by all of its elements
     *             (each an <tt>Object</tt>) in the proper order.
     */
    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException{
        // Write out element count, and any hidden stuff
        int expectedModCount = modCount;
        s.defaultWriteObject();

        // Write out size as capacity for behavioural compatibility with clone()
        s.writeInt(size);

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

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

    /**
     * Reconstitute the <tt>ArrayList</tt> instance from a stream (that is,
     * deserialize it).
     */
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
        elementData = EMPTY_ELEMENTDATA;

        // Read in size, and any hidden stuff
        s.defaultReadObject();

        // Read in capacity
        s.readInt(); // ignored

        if (size > 0) {
            // be like clone(), allocate array based upon size not capacity
            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.
     *
     * <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
     */
    private class Itr implements Iterator<E> {
        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<? super E> consumer) {
            Objects.requireNonNull(consumer);
            final int size = ArrayList.this.size;
            int i = cursor;
            if (i >= size) {
                return;
            }
            final Object[] elementData = ArrayList.this.elementData;
            if (i >= elementData.length) {
                throw new ConcurrentModificationException();
            }
            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<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)
                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();
            }
        }
    }

    /**
     * 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++;
    }
}

【Java源码分析】ArrayList底层原理（带图解）