ArrayList

ArrayList简介：

1. 线程不安全---add方法等都是非原子性的，在扩容时先判断是否需要扩容，然后再添加，很容易发生错误
2. 容器底层采用数组存储，默认初始容量10，每次扩容为1.5倍
3. ArrayList随机元素时间复杂度O(1)，插入删除操作需大量移动元素，效率较低
4. 继承自AbstractList，实现了List，RandomAccess，Cloneable，和Serializable接口，具有List的特性，提供可随机访问，提供自身克隆以及序列化的一个容器类。
5. ArrayList可以存储null值
6. 为了节约内存，当新建容器为空时，会共享Object[] EMPTY_ELEMENTDATA = {}和 Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {}空数组
7. ArrayList的实现中大量地调用了Arrays.copyof()和System.arraycopy()方法，其实Arrays.copyof()内部也是调用System.arraycopy()。System.arraycopy()为Native方法

8. 两个ToArray方法

   Object[] toArray()方法。该方法有可能会抛出java.lang.ClassCastException异常

   解析：https://blog.csdn.net/u010257931/article/details/104356353
   T[] toArray(T[] a)方法。该方法可以直接将ArrayList转换得到的Array进行整体向下转型

9. ArrayList每次修改（增加(add)、删除(remove)、排序(sort)）容器时，都是修改自身的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）。而且modCount++本身是非原子操作的。

10. 有多个内部类，Itr（迭代器，花式迭代）、ListItr、SubList (里面的大部分方法调用都是ArrayList中的)、ArrayListSpliterator（要求内容不可变-不影响线程安全）                    https://blog.csdn.net/qq_33774822/article/details/82757071

11. IDEA会自动调用  trimToSize()方法，eclipse不会，

            List list = new ArrayList<>();
    
            for (int i=0; i<12; i++){
                list.add(i);
            }

    IDEA会显示list有12个元素，eclipse会显示有15个

//package java.util;
import java.util.*;
import java.util.function.Consumer;
import java.util.function.Predicate;
import java.util.function.UnaryOperator;

/**
 *
 * @author  Josh Bloch
 * @author  Neal Gafter
 * @see     Collection
 * @see     List
 * @see     LinkedList
 * @see     Vector
 * @since   1.2
 */

public class ArrayList extends AbstractList
        implements List, RandomAccess, Cloneable, java.io.Serializable
{

    // 序列版本号
    private static final long serialVersionUID = 8683452581122892189L;

    /**
     * Default initial capacity.
     * 初始容量10
     */
    private static final int DEFAULT_CAPACITY = 10;


//    为了节约内存，当新建容器为空时，会共享
//    Object[] EMPTY_ELEMENTDATA = {}   空数组
//    和
//     Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {}  默认容量的空数组
    // Object[] elementData   真正存放对象的数组

    /**
     * Shared empty array instance used for empty instances.
     * 指定大小的空的ArrayList共享此对象
     */
    private static final Object[] EMPTY_ELEMENTDATA = {};

    /**
     * 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 = {};

    /**
     * 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).
     * 容器大小
     * @serial
     */
    private int size;

//    来自父类和接口的成员变量：
// 操作计数
//protected transient int modCount = 0;
//    867行左右，迭代器，利用该变量证明list内容是否发生了改变
//    modCount的作用
//    https://blog.csdn.net/MonkeyITBoy/article/details/79020636
//    https://www.cnblogs.com/zuochengsi-9/p/7050351.html




    /**
     * 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
     */
    public ArrayList(int initialCapacity) {
        if (initialCapacity > 0) {
            //初始容量大于0
            //创建initialCapacity大小的数组
            this.elementData = new Object[initialCapacity];
        } else if (initialCapacity == 0) {
            //初始容量等于0
            //创建空数组
            this.elementData = EMPTY_ELEMENTDATA;
        } else {
            //初始容量小于0，抛出异常
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        }
    }

    /**
     * Constructs an empty list with an initial capacity of ten.
     函数直接将DEFAULTCAPACITY_EMPTY_ELEMENTDATA赋值给elementData
     *
     */
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }

    /**
     * Constructs a list containing the elements of the specified
     * collection, in the order they are returned by the collection's
     * iterator.
     *首先调用Collection的toArray()方法，返回的结果赋值给elementData,这里值得注意的是，有可能返回的结果不是Object[]（有可能是null），所以才会有if (elementData.getClass() != Object[].class)的判断,如果判断成立，调用Arrays.copyOf()来将collection中的数据复制到elementData中。其次，如果collection中没有数据，就将EMPTY_ELEMENTDATA赋值给elementData
     *
     * @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.
     * 将底层数组的容量调整为当前列表保存的实际元素的大小
     *
     */
    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) {
        //判断需要扩容的数组是否为空实例（空数组）
        // 如果为不为空，则变量等于0.
        // 为空则变量等于数组默认容量 10
        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);
        }
    }

//    如果EMPTY_ELEMENTDATA是由DEFAULTCAPACITY_EMPTY_ELEMENTDATA赋值而成，
//    那么就将扩容的数量minCapacity和DEFAULT_CAPACITY进行比较，然后取其中的最大值，
//    最大值再传参给ensureExplicitCapacity函数。
    private void ensureCapacityInternal(int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }

        ensureExplicitCapacity(minCapacity);
    }

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

//        如果最小容量大于当前长度，则调用grow扩容
        // overflow-conscious code
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }

    /**
     * The maximum size of array to allocate.
     * Some VMs reserve some header words in an array.
     * Attempts to allocate larger arrays may result in
     * OutOfMemoryError: Requested array size exceeds VM limit
     *
     * 保留了一些头信息
     */
    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;

//        容量增加为原来的1.5倍
        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);
    }


    private static int hugeCapacity(int minCapacity) {
        //        因为oldCapacity有可能为0，这样得到的newCapacity也会为0,
        if (minCapacity < 0) // overflow
            throw new OutOfMemoryError();

//        确保数组的容量不大于Integer的最大值
        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.
     * 数组中所找元素第一个出现的位置
     */
    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.
     * 最后一次出现的位置
     */
    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.)
     *
     * 浅拷贝
     * @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.
     *转数组
     * @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) {
        //检查脚标是否越界
        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}
     */
    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) {
        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!!
//        将index之后的数据往后移一位，最后将element添加到elementData的index，同时size增加
        System.arraycopy(elementData, index, elementData, index + 1,
                         size - index);
        elementData[index] = element;
        size++;
    }

}