概要

上一章，我們學習了Collection的架構。這一章開始，我們對Collection的具體實現類進行講解；首先，講解List，而List中ArrayList又最為常用。因此，本章我們講解ArrayList。先對ArrayList有個整體認識，再學習它的源碼，最後再通過例子來學習如何使用它。內容包括：
第1部分 ArrayList簡介
第2部分 ArrayList數據結構
第3部分 ArrayList源碼解析(基於JDK1.8)
第4部分 ArrayList遍歷方式

第1部分 ArrayList介紹

ArrayList簡介

ArrayList 是一個數組隊列，相當於 動態數組。與Java中的數組相比，它的容量能動態增長。它繼承於AbstractList，實現了List, RandomAccess, Cloneable, java.io.Serializable這些接口。

ArrayList 繼承了AbstractList，實現了List。它是一個數組隊列，提供了相關的添加、刪除、修改、遍歷等功能。
ArrayList 實現了RandmoAccess接口，即提供了隨機訪問功能。RandmoAccess是java中用來被List實現，為List提供快速訪問功能的。在ArrayList中，我們即可以通過元素的序號快速獲取元素對象；這就是快速隨機訪問。稍後，我們會比較List的“快速隨機訪問”和“通過Iterator叠代器訪問”的效率。

ArrayList 實現了Cloneable接口，即覆蓋了函數clone()，能被克隆。

ArrayList 實現java.io.Serializable接口，這意味著ArrayList支持序列化，能通過序列化去傳輸。

和Vector不同，ArrayList中的操作不是線程安全的！所以，建議在單線程中才使用ArrayList，而在多線程中可以選擇Vector或者CopyOnWriteArrayList。

ArrayList構造函數

    /**
     * 當指明初始化數組的大小時，直接將數組初始化為指定容量的數組。
     */
    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);
        }
    }

    /**
     * 當沒有指明數組容量時，初始化為空數組。當第一次添加元素時，會擴容為DEFAULT_CAPACITY，也就是容量為10.
     */
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }

第2部分 ArrayList數據結構

ArrayList的繼承關系

java.lang.Object
   ?     java.util.AbstractCollection<E>
         ?     java.util.AbstractList<E>
               ?     java.util.ArrayList<E>

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

ArrayList與Collection關系如下圖：

ArrayList包含了兩個重要的對象：elementData 和 size。

(01) elementData 是"Object[]類型的數組"，它保存了添加到ArrayList中的元素。實際上，elementData是個動態數組，我們能通過構造函數 ArrayList(int initialCapacity)來執行它的初始容量為initialCapacity；如果通過不含參數的構造函數ArrayList()來創建ArrayList，則elementData會初始化為空數組（上面構造函數源碼），當第一次添加元素時，會擴容至默認容量10。

(02) size 則是動態數組的實際大小。

第3部分 ArrayList源碼解析(基於JDK1.8)

為了更了解ArrayList的原理，下面對ArrayList源碼代碼作出分析。ArrayList是通過數組實現的，源碼比較容易理解。

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

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

    /**
     * Shared empty array instance used for empty instances.
     */
    private static final Object[] EMPTY_ELEMENTDATA = {};

    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

    /**
     * 數組用來存儲元素。當new ArrayList時沒有指明大小，那麽就會使用默認的空數組。
     * 當第一次add元素的時候，會將數組容量設為默認值DEFAULT_CAPACITY 10.
     */
    transient Object[] elementData; // non-private to simplify nested class access

    /**
     * 數組所包含的元素個數，註意和elementData.length區別開。
     *size<=length
     * @serial
     */
    private int size;

    /**
     * 當指明初始化數組的大小時，直接將數組初始化為指定容量的數組。
     */
    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);
        }
    }

    /**
     * 當沒有指明數組容量時，初始化為空數組。當第一次添加元素時，會擴容為DEFAULT_CAPACITY，也就是容量為10.
     */
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }


    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;
        }
    }

    /**
     * 將當前數組截成size大小的數組，也就是有元素部分留下，剩下的長度不要了。
     */
    public void trimToSize() {
        modCount++;
        if (size < elementData.length) {
            elementData = (size == 0)
                    ? EMPTY_ELEMENTDATA
                    : Arrays.copyOf(elementData, size);
        }
    }

    /**
     * 調整容量。首先判斷，如果有必要，則將容量擴大至至少能放下minCapacity個元素。
     */
    public void ensureCapacity(int minCapacity) {
        /**
         * 這裏主要確保：如果數組為空，則至少需要擴容到DEFAULT_CAPACITY。
         * 如果不為空，擴大至至少能放下minCapacity個元素。
         */
        int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
                ? 0
                : DEFAULT_CAPACITY;

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

    private void ensureCapacityInternal(int minCapacity) {
        /**
         * 如果數組為空，則要擴大至Math.max(DEFAULT_CAPACITY, minCapacity)
         */
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }

        ensureExplicitCapacity(minCapacity);
    }

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

        // 當前要求的個數比當前數組的length要大，則擴容。
        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;

    /**
     * 擴容，保證最少可以存放minCapacity個元素。基本原則是擴容至數組長度的1.5倍
     */
    private void grow(int minCapacity) {
        // overflow-conscious code
        //當前數組長度（容量）
        int oldCapacity = elementData.length;
        //新容量是當前數組容量的1.5倍。
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        //如果1.5倍的新容量都比minCapacity小，那麽新容量就為minCapacity
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        /**
         * 新容量比最大數組容量還要大的時候，就要重新賦值新容量了。不能超過最大值。
         */
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        // minCapacity is usually close to size, so this is a win:
        elementData = Arrays.copyOf(elementData, newCapacity);
    }

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


    public int size() {
        return size;
    }


    public boolean isEmpty() {
        return size == 0;
    }


    public boolean contains(Object o) {
        return indexOf(o) >= 0;
    }

    /**
     * 找出元素的位置，可以看出ArrayList可以存放null
     */
    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;
    }


    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;
    }

    /**
     * 克隆的時候，數組得單獨克隆
     */
    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);
        }
    }


    public Object[] toArray() {
        return Arrays.copyOf(elementData, size);
    }


    @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;
    }

    // Positional Access Operations

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

    /**
     * 按索引得到元素只要判斷索引沒有越界，就直接返回數組對應元素
     */
    public E get(int index) {
        rangeCheck(index);

        return elementData(index);
    }

    /**
     * 將指定位置的元素換成新元素
     */
    public E set(int index, E element) {
        rangeCheck(index);

        E oldValue = elementData(index);
        elementData[index] = element;
        return oldValue;
    }

    /**
     * 添加元素，如果是第一次添加，就會擴容到DEFAULT_CAPACITY大小；
     * 不是第一次添加也會判斷是否需要擴容，基本規則是擴容到當前數組容量的1.5倍。
     * modCount會增加
     */
    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }

    /**
     *每次添加都要判斷是否需要擴容
     * 先將index後的元素後移一個，再插入
     * modCount++
     */
    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++;
    }

    /**
     * 刪除元素，modCount++
     * 將index後的元素往前移，並將最後一個元素置為Null
     */
    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;
    }

    /**
     * 刪除指定的元素。該元素可以為null。
     */
    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
    }

    /**
     * 將所有元素置為Null
     */
    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 <tt>true</tt> if this list changed as a result of the call
     * @throws NullPointerException if the specified collection is null
     */
    public boolean addAll(Collection<? extends E> c) {
        Object[] a = c.toArray();
        int numNew = a.length;
        ensureCapacityInternal(size + numNew);  // Increments modCount
        System.arraycopy(a, 0, elementData, size, numNew);
        size += numNew;
        return numNew != 0;
    }

    /**
     * Inserts all of the elements in the specified collection into this
     * list, starting at the specified position.  Shifts the element
     * currently at that position (if any) and any subsequent elements to
     * the right (increases their indices).  The new elements will appear
     * in the list in the order that they are returned by the
     * specified collection‘s iterator.
     *
     * @param index index at which to insert the first element from the
     *              specified collection
     * @param c collection containing elements to be added to this list
     * @return <tt>true</tt> if this list changed as a result of the call
     * @throws IndexOutOfBoundsException {@inheritDoc}
     * @throws NullPointerException if the specified collection is null
     */
    public boolean addAll(int index, Collection<? extends E> c) {
        rangeCheckForAdd(index);

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

        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
     * (<a href="Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if this list contains a null element and the
     *         specified collection does not permit null elements
     * (<a href="Collection.html#optional-restrictions">optional</a>),
     *         or if the specified collection is null
     * @see Collection#contains(Object)
     */
    public boolean removeAll(Collection<?> c) {
        Objects.requireNonNull(c);
        return batchRemove(c, false);
    }

    /**
     * Retains only the elements in this list that are contained in the
     * specified collection.  In other words, removes from this list all
     * of its elements that are not contained in the specified collection.
     *
     * @param c collection containing elements to be retained in this list
     * @return {@code true} if this list changed as a result of the call
     * @throws ClassCastException if the class of an element of this list
     *         is incompatible with the specified collection
     * (<a href="Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if this list contains a null element and the
     *         specified collection does not permit null elements
     * (<a href="Collection.html#optional-restrictions">optional</a>),
     *         or if the specified collection is null
     * @see Collection#contains(Object)
     */
    public boolean retainAll(Collection<?> c) {
        Objects.requireNonNull(c);
        return batchRemove(c, true);
    }

    private boolean batchRemove(Collection<?> c, boolean complement) {
        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
            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();
    }

    /**
     * 叠代器中有expectedModCount
     */
    private class Itr implements Iterator<E> {
        int cursor;       // 下一個返回元素的下標，默認是0
        int lastRet = -1; // 上一個返回元素的下標。-1表示還沒有返回
        int expectedModCount = modCount;

        public boolean hasNext() {
            return cursor != size;
        }

        @SuppressWarnings("unchecked")
        public E next() {
            /**
             * 檢查expectedModCount與modCount是否相等，不相等表示已經被修改過，拋出異常
             */
            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];
        }

總結：
(01) ArrayList 實際上是通過一個數組去保存數據的。當我們構造ArrayList時；若使用默認構造函數，會先分配一個空數組，當第一次添加時，則擴容為默認容量大小10。
(02) 當ArrayList容量不足以容納全部元素時，ArrayList會重新設置容量：newCapacity = oldCapacity + (oldCapacity >> 1)。也就是為原數組容量的1.5倍。（如果超過最大容量，就設為最大容量）

(03) ArrayList的克隆函數，即是將全部元素克隆到一個數組中。
(04) ArrayList實現java.io.Serializable的方式。當寫入到輸出流時，先寫入“容量”，再依次寫入“每一個元素”；當讀出輸入流時，先讀取“容量”，再依次讀取“每一個元素”。

(05)源碼中有個modCount變量，每做一次修改，都會增加一個。在叠代器中有expectedModCount變量，變量時會判斷這兩個變量是否相同。如果不相同，表示在遍歷過程中，數組被修改過，拋出異常。fail-fast

第4部分 ArrayList遍歷方式

ArrayList支持3種遍歷方式

(01) 第一種，通過叠代器遍歷。即通過Iterator去遍歷。

Integer value = null;
Iterator iter = list.iterator();
while (iter.hasNext()) {
    value = (Integer)iter.next();
}

(02) 第二種，隨機訪問，通過索引值去遍歷。
由於ArrayList實現了RandomAccess接口，它支持通過索引值去隨機訪問元素。

Integer value = null;
int size = list.size();
for (int i=0; i<size; i++) {
    value = (Integer)list.get(i);        
}

(03) 第三種，for循環遍歷。如下：

Integer value = null;
for (Integer integ:list) {
    value = integ;
}

下面通過一個實例，比較這3種方式的效率，實例代碼(ArrayListRandomAccessTest.java)如下：

運行結果：

iteratorThroughRandomAccess：3 ms
iteratorThroughIterator：8 ms
iteratorThroughFor2：5 ms

由此可見，遍歷ArrayList時，使用隨機訪問(即，通過索引序號訪問)效率最高，而使用叠代器的效率最低！

參考：http://www.cnblogs.com/skywang12345/p/3308556.html，這篇文章是1.6，本文1.8

ArrayList源碼解讀（jdk1.8）