ArrayList源碼閱讀記錄

ArrayList多是在開發中使用最多的集合類。ArrayList其實就是一個動態數組，它會根據元素的個數進行動態擴容。

下面是部分源碼，本次源碼基於jdk1.7：

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

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

    /**
     * 使用無參構造方法時ArrayList使用的空數組
     * Shared empty array instance used for empty instances.
     */
    private static final Object[] EMPTY_ELEMENTDATA = {};

    /**
     * ArrayList維護用於存放數據的數組
     * 
     */
    private transient Object[] elementData;

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

    /**
     * 構造一個空數組，自定義指定初始容量
     *
     * @param  initialCapacity  自定義初始容量
     * @throws IllegalArgumentException if the specified initial capacity
     *         is negative
     */
    public ArrayList(int initialCapacity) {
        super();
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        this.elementData = new Object[initialCapacity];
    }

    /**
     * 構造一個空數組，初始容量是10
     */
    public ArrayList() {
        super();
        this.elementData = EMPTY_ELEMENTDATA;
    }

    /**
     * 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<? extends E> c) {
        elementData = c.toArray();
        size = elementData.length;
        // c.toArray might (incorrectly) not return Object[] (see 6260652)
        if (elementData.getClass() != Object[].class)
            elementData = Arrays.copyOf(elementData, size, Object[].class);
    }

    /**
     * 私用方法用於保證數組容量，先判斷數組是否爲空，爲空則容量最少爲10
     */
    private void ensureCapacityInternal(int minCapacity) {
        if (elementData == EMPTY_ELEMENTDATA) {
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }

        ensureExplicitCapacity(minCapacity);
    }
    /**
     * 該方法被調用一次modCount內部修改計數器會加1，modCount計數器被定義在父類AbstractList中
     * modCount被內部實現Iterator和ListIterator的類使用
     * 該方法考慮了溢出，當須要的容量超過了當前數組的容量開始調用grow方法進行擴容
     */
    private void ensureExplicitCapacity(int minCapacity) {
        
        modCount++;

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

    /**
     * 最大可分配容量
     */
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

    /**
     * 該方法用於增長數組的容量，確保可容納指定的最小長度的元素個數
     * 按照數組元容量的1.5倍擴容，這個擴容量主要是綜合考慮了擴容次數和內存使用方便考慮的，
     * 若是擴容倍數大了浪費內存，小了會增長擴容次數增長開銷。
     * 該方法實際是計算好最終的擴容量而後交給Arrays.copyOf->System.arraycopy
     * Systeam的arraycopy是一個native的方法實現數組的拷貝生成新的數組
     * @param minCapacity the desired minimum capacity
     */
    private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        //等價於int newCapacity = oldCapacity + oldCapacity/2,即按照當前容量的1.5倍擴容;
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        //若是原數組1.5倍容量小於指定的最小容量，則使用指定的最小容量
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        //若是原數組1.5倍容量大於MAX_ARRAY_SIZE(ps:Integer.MAX_VALUE - 8),則使用MAX_ARRAY_SIZE作容量
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        // minCapacity is usually close to size, so this is a win:
        elementData = Arrays.copyOf(elementData, newCapacity);
    }
    /**
     * 確保最大容量不能超過MAX_ARRAY_SIZE，超過了直接強制使用MAX_ARRAY_SIZE做爲容量
     */
    private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) // overflow
            throw new OutOfMemoryError();
        return (minCapacity > MAX_ARRAY_SIZE) ?
            Integer.MAX_VALUE :
            MAX_ARRAY_SIZE;
    }
    /**
     * 
     * 添加指定的元素，添加元素前調用ensureCapacityInternal方法來保證容量正常
     * 固然每增長一個元素modCount會加1
     * @param e element to be appended to this list
     * @return <tt>true</tt> (as specified by {@link Collection#add})
     */
    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }

    /**
     * 它也增長了modCount，而後計算要移動的元素個數，從index日後的元素都往前移動一位，
     * 實際調用System.arraycopy方法移動元素。elementData[--size] = null;
     * 這行代碼將size減一，同時將最後一個位置設爲null方便被回收
     *
     * @param index the index of the element to be removed
     * @return the element that was removed from the list
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E remove(int index) {
        rangeCheck(index);

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

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

        return oldValue;
    }
}