摘要：要注意的是并不是線程安全的，因此一般建議在單線程中使用。實現(xiàn)原理繼承關(guān)系繼承實現(xiàn)接口關(guān)鍵屬性數(shù)據(jù)的數(shù)組實際數(shù)據(jù)的數(shù)量底層使用數(shù)組保存所有元素如果用聲明一個實例變量，當(dāng)對象存儲時，它的值不需要維持。

ArrayList可以簡單的看作是動態(tài)數(shù)組，相對于普通的數(shù)組它可以動態(tài)的增加容量或者減少容量。要注意的是ArrayList并不是線程安全的，因此一般建議在單線程中使用ArrayList。

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

ArrayList繼承AbstractList實現(xiàn)List, RandomAccess, Cloneable, java.io.Serializable接口

    /**
     * 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.
     */
    // 數(shù)據(jù)的數(shù)組
    transient Object[] elementData; // non-private to simplify nested class access

    /**
     * The size of the ArrayList (the number of elements it contains).
     *
     * @serial
     */
    // 實際數(shù)據(jù)的數(shù)量
    private int size;

底層使用數(shù)組保存所有元素
transient 如果用transient聲明一個實例變量，當(dāng)對象存儲時，它的值不需要維持。換句話來說就是，用transient關(guān)鍵字標(biāo)記的成員變量不參與序列化過程

    /**
     * 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 = {};
    /**
     * Constructs an empty list with an initial capacity of ten.
     */
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }

默認(rèn)情況下初始化空數(shù)組（長度為0的數(shù)組）

    /**
     * Shared empty array instance used for empty instances.
     */
    private static final Object[] EMPTY_ELEMENTDATA = {};
    /**
     * 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) {
            this.elementData = new Object[initialCapacity];
        } else if (initialCapacity == 0) {
            this.elementData = EMPTY_ELEMENTDATA;
        } else {
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        }
    }

指定數(shù)組的初始容量
當(dāng)指定的初始容量大于0，初始化指定大小的數(shù)組
當(dāng)指定的初始容量等于0，初始化空數(shù)組
當(dāng)指定的初始容量小于0，拋出IllegalArgumentException異常

    /**
     * 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;
        }
    }

初始化指定集合的數(shù)組
當(dāng)指定集合不為空即長度不為0，則復(fù)制該集合，否則初始化一個空數(shù)組

    // Positional Access Operations
    // 返回index下標(biāo)的元素且強(qiáng)制轉(zhuǎn)化為E（List中的E）類型
    @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) {
        // 檢查index是否越界
        rangeCheck(index);
        // 返回index下標(biāo)的元素
        return elementData(index);
    }

    /**
     * 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) {
        // 檢查index是否大于等于size（數(shù)組的元素數(shù)量），因為數(shù)組下標(biāo)從0開始計算，所以也不能等于元素數(shù)量
        // 這里沒有檢查index < 0的情況，因為index < 0時數(shù)組會自動拋出異常，所以并未檢查index<0的情況
        if (index >= size)
            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;
    }

首先判斷index是否越界，這里并沒有判斷是否小于0，因為下標(biāo)小于0時數(shù)組會拋出異常。越界則拋出IndexOutOfBoundsException異常，反之返回數(shù)組對應(yīng)index位置的元素

    /**
     * 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;
    }

和get一樣先判斷index（下標(biāo)）是否越界，不越界則先獲取原來index位置上的元素，接著設(shè)置（覆蓋）index位置上的元素，然后返回原來的元素，反之拋出IndexOutOfBoundsException異常

    /**
     * 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) {
        // 檢查當(dāng)前容量是否還可以容納一個元素，不夠則擴(kuò)容
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        // 添加到數(shù)組末尾
        // 這個語句可以分解為
        // elementData[size] = e;
        // size += 1;
        elementData[size++] = e;
        return true;
    }

    /**
     * Default initial capacity.
     */
    private static final int DEFAULT_CAPACITY = 10;  // 默認(rèn)容量為10

    // 如果數(shù)據(jù)等于默認(rèn)數(shù)據(jù)，返回默認(rèn)容量和minCapacity（所需容量最小值）的最大值，反之返回所需容量最小值
    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++;  // 操作數(shù)+1

        // overflow-conscious code
        // 如果所需容量最小值大于實際數(shù)組的長度就擴(kuò)大實際數(shù)組容量
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }

    /**
     * The maximum size of array to allocate.
     * Some VMs reserve some header words in an array.
     * Attempts to allocate larger arrays may result in
     * OutOfMemoryError: Requested array size exceeds VM limit
     */
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;  // 數(shù)組最大容量為Integer最大值-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;
        // 新的容量為舊的容量的1.5倍
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        // 如果擴(kuò)充容量后還是不夠，則新的容量等于所需容量最小值（一般就是數(shù)組實際元素個數(shù)）
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        // 如果新的容量大于數(shù)組最大容量，再調(diào)用hugeCapacity計算新的容量
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        // minCapacity is usually close to size, so this is a win:
        // 復(fù)制原來的數(shù)據(jù)到新的數(shù)組，數(shù)組容量為新的容量
        elementData = Arrays.copyOf(elementData, newCapacity);
    }

    private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) // overflow
            throw new OutOfMemoryError();
        // 大于數(shù)組最大容量返回Integer最大值，反之返回數(shù)組最大容量
        return (minCapacity > MAX_ARRAY_SIZE) ?
                Integer.MAX_VALUE :
                MAX_ARRAY_SIZE;
    }

添加一個元素到列表尾，當(dāng)列表容量不足時自動擴(kuò)容（通常是擴(kuò)容至原來的1.5倍），添加成功返回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) {
        // 檢查下標(biāo)是否越界
        rangeCheckForAdd(index);
        // 檢查當(dāng)前容量是否還可以在容納一個元素，不夠則擴(kuò)容
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        // 將elementData從index開始后面的元素往后移一位
        System.arraycopy(elementData, index, elementData, index + 1,
                size - index);
        elementData[index] = element;
        size++;
    }

    /**
     * A version of rangeCheck used by add and addAll.
     */
    private void rangeCheckForAdd(int index) {
        // 當(dāng)index等于size時相當(dāng)于添加元素到列表尾
        if (index > size || index < 0)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

將elementData數(shù)組從index開始后面的元素往后移一位，接著在index處放置元素
模擬添加數(shù)據(jù)(lierabbit)到index=4過程如下

    /**
     * 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;
        // 檢查當(dāng)前容量是否還可以在容納a數(shù)組的元素，不夠則擴(kuò)容
        ensureCapacityInternal(size + numNew);  // Increments modCount
        // 將a數(shù)組里的元素添加到elementData末尾
        System.arraycopy(a, 0, elementData, size, numNew);
        size += numNew;
        // a數(shù)組不為空（長度不為0）時返回true，反之false
        return numNew != 0;
    }

將要添加的集合變?yōu)閿?shù)組，然后將其復(fù)制到elementData數(shù)組末尾

    /**
     * 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) {
        // 檢查下標(biāo)是否越界
        rangeCheckForAdd(index);

        Object[] a = c.toArray();
        int numNew = a.length;
        // 檢查當(dāng)前容量是否還可以在容納a數(shù)組的元素，不夠則擴(kuò)容
        ensureCapacityInternal(size + numNew);  // Increments modCount

        // 需要往后移動幾個位置
        int numMoved = size - index;
        if (numMoved > 0)
            // 從index開始，往后的元素向后移動numMoved個位置
            System.arraycopy(elementData, index, elementData, index + numNew,
                    numMoved);
        // 將a數(shù)組里的所有元素復(fù)制到elementData從index到index + numNew -1的位置上
        System.arraycopy(a, 0, elementData, index, numNew);
        size += numNew;
        // a數(shù)組不為空（長度不為0）時返回true，反之false
        return numNew != 0;
    }

將要添加的集合變?yōu)閿?shù)組，然后把elementData數(shù)組從index開始，往后的元素向后移動numMoved個位置，接著將要添加的數(shù)組里的所有元素復(fù)制到elementData從index到index + numNew -1的位置上

    /**
     * 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++;  // 操作數(shù)+1
        // 如果數(shù)組實際元素數(shù)量小于數(shù)組長度
        if (size < elementData.length) {
            // 如果數(shù)組實際元素數(shù)量等于0則數(shù)組被賦值為空數(shù)組，反之創(chuàng)建一個新的元素數(shù)量等于數(shù)組長度的數(shù)組
            elementData = (size == 0)
                    ? EMPTY_ELEMENTDATA
                    : Arrays.copyOf(elementData, size);
        }
    }

當(dāng)數(shù)據(jù)穩(wěn)定了之后可以使用這個方法來減少內(nèi)存的使用

    /**
     * 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) {
        //元素可以為null，如果為null返回null的下標(biāo)
        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;
        }
        // 沒有找到對應(yīng)的元素返回-1
        return -1;
    }

ArrayList中可以存放null元素，indexof是返回elementData數(shù)組中值相同的首個元素的下標(biāo)，indexof中比較方法是equals而equals是比較元素的值，因此必須對null多帶帶查找。如果未找到該元素則返回-1

    /**
     * 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) {
        // 檢查下標(biāo)是否越界
        rangeCheck(index);

        modCount++;  // 操作數(shù)+1
        E oldValue = elementData(index);  // 獲取index位置上的元素

        int numMoved = size - index - 1;  // 需要往前移動幾個位置
        if (numMoved > 0)
            // 從index + 1開始，往后的元素向前移動1個位置
            System.arraycopy(elementData, index+1, elementData, index,
                    numMoved);
        // 將數(shù)組末尾元素置空
        elementData[--size] = null; // clear to let GC do its work

        return oldValue;
    }

模擬刪除index=4（值為lierabbit）過程如下

    /**
     * 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) {
        // 元素可以為null，分開搜索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;
                }
        }
        // 沒有找到返回false
        return false;
    }

    /*
     * Private remove method that skips bounds checking and does not
     * return the value removed.
     */
    // 由于已經(jīng)找到元素，則元素必定存在，則index必定合理，所以不需要在檢查index是否越界
    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
    }

通過尋找o元素，可以獲得其下標(biāo)，再根據(jù)下標(biāo)刪除o元素

    /**
     * The number of times this list has been structurally modified.
     * Structural modifications are those that change the size of the
     * list, or otherwise perturb it in such a fashion that iterations in
     * progress may yield incorrect results.
     *
     * 
This field is used by the iterator and list iterator implementation
     * returned by the {@code iterator} and {@code listIterator} methods.
     * If the value of this field changes unexpectedly, the iterator (or list
     * iterator) will throw a {@code ConcurrentModificationException} in
     * response to the {@code next}, {@code remove}, {@code previous},
     * {@code set} or {@code add} operations.  This provides
     * fail-fast behavior, rather than non-deterministic behavior in
     * the face of concurrent modification during iteration.
     *
     * 
Use of this field by subclasses is optional. If a subclass
     * wishes to provide fail-fast iterators (and list iterators), then it
     * merely has to increment this field in its {@code add(int, E)} and
     * {@code remove(int)} methods (and any other methods that it overrides
     * that result in structural modifications to the list).  A single call to
     * {@code add(int, E)} or {@code remove(int)} must add no more than
     * one to this field, or the iterators (and list iterators) will throw
     * bogus {@code ConcurrentModificationExceptions}.  If an implementation
     * does not wish to provide fail-fast iterators, this field may be
     * ignored.
     */
    protected transient int modCount = 0;//操作數(shù)

    @Override
    public void forEach(Consumer action) {
        // 確保不為空
        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();
        }
    }

    /**
     * Checks that the specified object reference is not {@code null}. This
     * method is designed primarily for doing parameter validation in methods
     * and constructors, as demonstrated below:
     * 
     * public Foo(Bar bar) {
     *     this.bar = Objects.requireNonNull(bar);
     * }
     * 
     *
     * @param obj the object reference to check for nullity
     * @param  the type of the reference
     * @return {@code obj} if not {@code null}
     * @throws NullPointerException if {@code obj} is {@code null}
     */
    public static  T requireNonNull(T obj) {
        if (obj == null)
            throw new NullPointerException();
        return obj;
    }

這里可以看到modCount的用處，當(dāng)modCount發(fā)生改變后，立刻拋出ConcurrentModificationException異常。通過之前的分析可以知道當(dāng)列表內(nèi)容被修改時modCount會增加。也就是說如果在遍歷ArrayList的過程中有其他線程修改了ArrayList，那么將拋出ConcurrentModificationException異常

ArrayList是List接口的一個可變大小的數(shù)組的實現(xiàn)

ArrayList的內(nèi)部是使用一個Object對象數(shù)組來存儲元素的

初始化ArrayList的時候，可以指定初始化容量的大小，如果不指定，就會使用默認(rèn)大小，為10

當(dāng)添加一個新元素的時候，首先會檢查容量是否足夠添加這個元素，如果夠就直接添加，如果不夠就進(jìn)行擴(kuò)容，擴(kuò)容為原數(shù)組容量的1.5倍

當(dāng)在index處放置一個元素的時候，會將數(shù)組index處右邊的元素全部右移

當(dāng)在index處刪除一個元素的時候，會將數(shù)組index處右邊的元素全部左移

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