ArrayList源码分析

public class ArrayList<E> extends AbstractList<E>  implements List<E>, RandomAccess, Cloneable, java.io.Serializable{   
    private static final long serialVersionUID = 8683452581122892189L;
    /**
     * Default initial capacity.(默认容量大小)
     */
    private static final int DEFAULT_CAPACITY = 10;

    /**
     * Shared empty array instance used for empty instances.（空的数组）
     */
    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.
     * （默认的空数组）
   */
   private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {}; /**
　　　* （用于存放当前ArrayList数据的数组，transient代表不参与序列化）
     * 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).
     *（当前ArrayList的元素个数）
     * @serial
     */
    private int size;
}

　　/**
     * Constructs an empty list with the specified initial capacity.
     * 构造一个具有指定初始容量的空列表（空列表指数组只有大小，还没有元素）
     * @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);
        }
    }

    /**
　　　* 构造一个初始容量为10的空列表（ArrayList的初始容量是10）
     * Constructs an empty list with an initial capacity of ten.
     */
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }

    /**
　　  * 构造一个包含指定集合元素的列表，其顺序由集合的元素返回迭代器。
     * 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();
        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;
        }
    }

public static <T,U> T[] copyOf(U[] original, int newLength, Class<? extends T[]> newType) {
        @SuppressWarnings("unchecked")
　　　　 //根据class的类型来决定是new 还是反射去构造一个泛型数组
        T[] copy = ((Object)newType == (Object)Object[].class)
            ? (T[]) new Object[newLength]
            : (T[]) Array.newInstance(newType.getComponentType(), newLength);
　　　　 //利用native函数，批量赋值元素至新数组中。
        System.arraycopy(original, 0, copy, 0,
                         Math.min(original.length, newLength));
        return copy;
    }

public boolean add(E e) {
　　　　　//确定数组容量是否足够
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }
//确定数组容量是否足够（minCapacity：目前需要的数组容量）
private void ensureCapacityInternal(int minCapacity) {
　　ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
}

private static int calculateCapacity(Object[] elementData, int minCapacity) {
    if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
　　　　　//如果list的数组元素为空，则判断目前需要的数组容量是否大于默认的10，大于返回minCapacity，小于则返回默认的大小10
        return Math.max(DEFAULT_CAPACITY, minCapacity);
    }
    return minCapacity;
}

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

    // overflow-conscious code
    if (minCapacity - elementData.length > 0)
　　//如果需要的容量已经大于目前的数组长度，则进行扩容。这里和HashMap不一样，hashMap会有负载因子
        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
 */
//最大数组大小需要保留一些header words，所以减去8.
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

/**
 * 对list进行扩容
 * 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;
　　//新的容量为=旧的数组长度+旧的数组长度/2 （>>1为向右位移以为，即除以2）
    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:
　　//扩容方式为：复制数组数据到新的数组
    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;
}

     /**     
　　　* 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) {
　　　　　//判断数组越界，index不能大于等于数组长度
        rangeCheck(index);

        modCount++;
        E oldValue = elementData(index);
　　　　　//删除操作第一步：把当前数组的index+1位置后的数组，复制到当前数组，从index位置开始。
        int numMoved = size - index - 1;
        if (numMoved > 0)
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
　　　　 //删除操作第二步：把数组最后的元素置null，GC掉
        elementData[--size] = null; // clear to let GC do its work

        return oldValue;
    }

 /**
     * 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
     * <tt>i</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>
     * (if such an element exists).  Returns <tt>true</tt> 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 <tt>true</tt> if this list contained the specified element
     */
    public boolean remove(Object o) {
        if (o == null) {
　　　　　　//删除null元素
            for (int index = 0; index < size; index++)
                if (elementData[index] == null) {
　　　　　　　　　　　　//从0开始，第一个匹配的会被删除，删除完毕后return
                    fastRemove(index);
                    return true;
                }
        } else {
　　　　　　//删除非空元素
            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)
　　　　　　　//删除方式还是和第一个删除方法一样：把当前数组的index+1位置后的数组，复制到当前数组，从index位置开始。
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
　　　　　//置空最后的元素
        elementData[--size] = null; // clear to let GC do its work
    }

public boolean removeAll(Collection<?> c) {
        Objects.requireNonNull(c);
        return batchRemove(c, false);
}

/** 
 * 批量移除 * complement（true：求交集，即RetainAll;false：求差集，即remove）

 *
*/
private boolean batchRemove(Collection<?> c, boolean complement) {
        final Object[] elementData = this.elementData;
        int r = 0, w = 0;
        boolean modified = false;
        try {
　　　　　　　　//遍历list
            for (; r < size; r++)
                if (c.contains(elementData[r]) == complement)
　　　　　　　　　　　　//目前是removeall，complement为false，则保留不在c集合里面的元素
                    elementData[w++] = elementData[r];
        } finally {
            // Preserve behavioral compatibility with AbstractCollection,
            // even if c.contains() throws.
　　　　　　　//出现异常会导致 r !=size , 则将出现异常处后面的数据全部复制覆盖到数组里
            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;
    }