java框架集合List子接口之ArrayList源码剖析

// 默认初始容量
private static final int DEFAULT_CAPACITY = 10;
// 用于空实例的共享空数组实例。
private static final Object[] EMPTY_ELEMENTDATA = {};
// 用于默认大小的空实例的共享空数组实例
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
// 当前数据对象存放地方，当前对象不参与序列化
transient Object[] elementData
// 当前ArrayList的长度
private int size;
// 数组最大容量
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

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

    /**
     * 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;
        }
    }

 /**
     * Appends the specified element to the end of this list.
     *
     * @param e element to be appended to this list
     * @return <tt>true</tt> (as specified by {@link Collection#add})
     */
    public boolean add(E e) {
        // 判断是否需要扩容
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }
    private void ensureCapacityInternal(int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            // 从默认容量和当前ArrayList的size + 1选一个最大的最为扩容阈值
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }
        ensureExplicitCapacity(minCapacity);
    }
    private void ensureExplicitCapacity(int minCapacity) {
        // 将修改次数（modCount）自增1
        modCount++;
        // overflow-conscious code
        // 判断是否需要扩容 , 如果当前容量减去数组容量大于0 , 表示需要扩容
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }
        /**
     * 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
        // 旧的容量
        int oldCapacity = elementData.length;
        // 新容量等于旧容量 + 旧容量左移1位
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        if (newCapacity - minCapacity < 0)
            // 如果新容量 - 需要扩容阈值 < 0 那么新容量就使用扩容阈值
            newCapacity = minCapacity;
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            // 如果新容量 - 数组最大长度 > 0 , 那么就需要另外判断
            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 void add(int index, E element) {
        // 如果index大于当前ArrayList的size , 或者小于0 , 则抛异常
        rangeCheckForAdd(index);
        // 判断是否需要扩容
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        // 使用System.arraycopy 将需要插入的位置（index）后面的元素统统往后移动一位
        System.arraycopy(elementData, index, elementData, index + 1,
                         size - index);
        // 将新的数据内容存放到数组的指定位置（index）上
        elementData[index] = element;
        size++;
    }

    public E get(int index) {
        // 检查下标是否越界
        rangeCheck(index);
        return elementData(index);
    }
    E elementData(int index) {
        // 返回下标对应的元素
        return (E) elementData[index];
    }

    public boolean contains(Object o) {
        return indexOf(o) >= 0;
    }
    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 E remove(int index) {
        // 检查下标是否越界
        rangeCheck(index);
        // 修改次数 + 1
        modCount++;
        // 根据下标获取需要修改的值
        E oldValue = elementData(index);
        // 判断是否需要移动数组元素
        int numMoved = size - index - 1;
        if (numMoved > 0)
            // 使用System.arraycopy 将需要插入的位置（index）后面的元素统统往前移动一位
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
        // 不需要移动元素的话 , 把最后一位置为空
        elementData[--size] = null; // clear to let GC do its work
        return oldValue;
    }

    public boolean remove(Object o) {
        // 循环遍历所有对象，得到对象所在索引位置，然后调用fastRemove方法，执行remove操作
        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 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
    }

   public void clear() {    
        modCount++;
        // clear to let GC do its work
        for (int i = 0; i < size; i++)
            elementData[i] = null;
        size = 0;
    }

public List<E> subList(int fromIndex, int toIndex) {
        subListRangeCheck(fromIndex, toIndex, size);
        return new SubList(this, 0, fromIndex, toIndex);
    }

public Iterator<E> iterator() {
        return new Itr();
    }

    @SuppressWarnings("unchecked")
    public E next() {
        // 可能会出现上述问题
        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];
    }
    final void checkForComodification() {
        if (modCount != expectedModCount)
            throw new ConcurrentModificationException();
    }

    public static <T,U> T[] copyOf(U[] original, int newLength, Class<? extends T[]> newType) {
        @SuppressWarnings("unchecked")
        T[] copy = ((Object)newType == (Object)Object[].class)
            ? (T[]) new Object[newLength]
            : (T[]) Array.newInstance(newType.getComponentType(), newLength);
        System.arraycopy(original, 0, copy, 0,
                         Math.min(original.length, newLength));
        return copy;
    }